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A Stumble At Cruise

  A study of a stumble/hesitation under constant speed cruising

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A Stumble At Cruise - Resolved

Technical Items:



  An Introduction
  This page documents a two year long investigation, trying to understand why my SU equipped vehicle stumbled at cruise.

      Just because this page is under my MG section, don't for a minute believe this is the only marquee that can have this problem. And it's not just limited to our LBC's. Even the computer controlled vehicles of today can have similar problems.


     Acceleration, and all other modes of throttle were fine. Only under a steady state cruise speed, at any speed, the car would seem to hesitate or stumble. My first impressions were SU carb needles. Why? Well I had converted to K&N filters and was running a 2" Peco free flow header and rear box. Other than that, my 1973 MGBGT was basically stock as far as engine modifications were concerned.

    Over the initial two year period of ownership I have gone through quite an exercise, trying to resolve this issue. Some of the experiments, included swapping distributors and playing with the mechanical advance curves. I tested multiple vacuum advance curves (units), had played with ported and manifold vacuum, and finally went as far as to buy brand new SU AUD405 HS4 carbs. Not the newer 'Burlen Fuel Systems' units, but original Skinner Union carbs. (check out Midel Pty Ltd.  in Australia. They were nearly $100 cheaper including shipping, than trying to get a pair from Kansas here in the states!) Needles tested included the original AAE, then ADB, then AAA. Then I put in fixed needles and tested #6 and #7 needles. All my needles came from
Joe Curto, Mr. SU himself (718-762-SUSU  (7878) ), regarded as one of the best SU rebuilders in existence. I have no financial interest in these two sources. I mention them here directly, as I believe them to be two of the best resources on the planet for SU carburator parts and service.

      The thing that made this such a bugger to resolve was the impression that when I pulled out the choke during cruising, the problem disappeared. Hence all the needle and carb experimentation.

       After a couple of months now, I'm sure I have the problem resolved.
       Trying to find a way to describe everything I went through, everything that was tested and played with, and explain the actual problem, is the reason this page took so long to come to fruition. Trying to figure out how to present it all. So... I believe including the following email threads/conversations, is the easiest way to explain what happened.

   Reading ALL the emails I know is boring compared to all the pictures on all the other pages on this site. But if you read them all, I guarantee you will learn something you never knew before, whether directly related to this problem or not.

    The details of the problem and it's resolution will reveal themselves as you read the emails.

     I'd like to thank everyone (listed here or not) for all the input over the years and their wise cracks as well as their wisdom. Even if someone doesn't hit the nail on the head, it sometimes sparks a thought that resolves an issue. So no matter how wacky the world can seem, .... listen carefully.  You may hear your own voice screaming at you!


  The Email Trail
    Granted, the following may be a bit out of order, and not all of them are included here, but the series of email correspondences present here, I think, represent the common beliefs and routes to problem solving and thing to investigate that most people go through.  They chronicle what transpired while trying to resolve this. It's amazing how close people were, and yet how far off and clueless we ALL were.


OK... here we go... this problem has not changed through all the following repairs and upgrades.

Driving at steady constant speed I get a pulsing or surging/dropout (what ever you want to call it) You pull the choke out and it disappears with it seeming like a bit more power coming in. I've replaced fuel pumps, carbs, added fuel pressure regulators, changes carb piston springs, changed carb needles, air filter styles (stock -
free flow) exhaust system diameters, manifolds, head gaskets. re-curved and /or rebuilt distributors, changed dist. models, points and electronic conversions.
This problem appears on more than one marque I own. 

Anyone have any wild ideas? past experiences? Wonderments?

Paul Tegler  wizardz@toad.net        http://www.teglerizer.com

Exhaust (prob'ly gasket) leak, Paul??


Does it happen cold or hot?  Could be the gas line too close to the exhaust
header?  Heat shield?


Hello Paul

At 08:23 PM 1/14/2001 -0500, Ptegler wrote:

>>Driving at steady constant speed I get a pulsing or
>>surging/dropout (what ever you want to call it)
>>description <snipped>

>>This problem appears on more than one marque I own.
>>Anyone have any wild ideas?

That nervous "tic" in your throttle foot maybe...   :)

>>past experiences?

I have had that exact same symptom due to the EGR system in a couple cars I have owned (I'm talking carb'ed, not fuel injected).  On one car I replaced the EGR and it went away, on another I just "disabled" the EGR valve by plugging the vacuum line.


Yeah, what engine/year/carb/induction setups were you running on your
problem cars? Do they even have EGR systems?

Good Luck,

Atwell Haines
'79 Spitfire
Succasunna, NJ USA

It sounds like a classic weak mixture to me, especially if pulling the choke out stops the problem. From the late sixties there was a drive for more economy and better emmissions, and so cars where set to run leaner and leaner, so even with everythin on the car correct care has to be taken not to try and set it too lean, as then the car won't go properley and perversely more fuel is consumed. With the inevitable wear in particularly carb jets and needles fitting new standard items can result in more problems as worn items wear rich, and with any deviation from a standard spec richer needles are required as the bolt on goodies inevitably flow better and so weaken the mixture. Apart from some standard bolt on combinations and carb specs where a correct replacement  needle is well known a car will have to be set up on a rolling road, where the CO/CO2 balance can be analyzed under all conditions.

John Kipping

    If pulling the choke masks the symptoms, the first (and most likely)  place I would look is in one of the float bowls.  I've seen the problem many times; one  float valve hangs in the "up" (bowl filled) position. Fuel runs out and the engine sputters. Pull the choke, the mixture shifts rich enough to compensate. After  10 seconds to a minute or so, the needle unseats and the bowl refills. 
    Whenever I want to isolate the cause to a float, I install an inline shut off valve with a lever. Run a stiff wire to the lever.  When the engine starts sputtering, release the throttle, shut  off the ignition, then the fuel valve, coast to a stop, pop the hood and  remove the float covers.  If you find one dry bowl, replace the needle and seat assy. I've seen brand new grosse jets and old viton seats alike hang up. This  happens even more often if  the float axle is a bit worn, although I have seen the float all the way down and the needle slammed tight against the seat. Replace the needle and seat and the problem should go away (if that's what it is....) 
    The only other thing I recall that might cause those symptoms, along with the "cure" is a loss of vacuum between the dashpot piston and housing. esp if the two holes at the top of the throttle inlet were closed because an air filter gasket was turned upside down. This can be caused by icing, as well.

Mark Childers   Union Jack Spares & Repairs Virginia Beach Virginia

May sound dumb, but I had same problem with an Olds I had. Replaced
everything from the headlights back. Turned out to be the new plugs I put in short time before all the surging started. Might be worth a look. Maybe the choke thing masks it from all the raw fuel getting to the cylinders.

Tim Roy

Paul, looks like the only thing left is to change the driver, Hehehe.
90% of all fuel/carb problems are electrical.
My guess would have been points.  But I guess you covered that.

Larry Hoy

      Have you  tried checking for a vacum leak? Try spraying a small amount of starter fluid in different spots while the engine is running. If you get a rise in RPM when you do, most likey there is a leak. I found my intake manifold leaking, so I reseated/sealed it, Bingo! Good Luck Buddy!

Mitch D. Johnson

Needles were my first route (way back when) K&N's went to ABD than AAA... then bought new carbs. Even went to #7's after getting rid of the biased needle setup. Have tried lighter, and heavier carb piston springs ...still does it.

Paul Tegler  wizardz@toad.net        http://www.teglerizer.com

Try opening the plug gaps out to about .040 see if it changes

no EGR or air pump on either car. just the carbon canisters as breathers.

73 BGT 2" Peco header and exhaust system otherwise totally stock.
75 Spit. Totally stock except for the dual HS4's stock exhaust system.

Paul Tegler  wizardz@toad.net        http://www.teglerizer.com

I have a problem with a pulsing/loss of power but it's only in the range of 3k to 3500 rpms.

I get a sputter and power loss..Like the engine is going to die (and it probably will) from flooding. If I give it gas early enough I can power through past 3,500 (lingering in the 3 to 3.5k range seems to do it) or if I really start to lose power, I take my foot off the gas for a second (clutch out of gear and then back in) and give it gas a few seconds later, it clears up immediately. But if I creep back into the "zone" it will start to do the 'pulse' again, (It doesn't sputter or backfire, it just shimmy's a bit and you can feel the car's power fading.)

It does it no matter what carb or needle I use (hs4 or hif4, aau abt or whatever else I had needles of). It always happens in that 3k to 3,500 rpm range. I spoke with a mechanic (dave) at Motorhead and he said it
sounds like an ignition problem.

I tried new wires, plugs and points and the coil I have is a 40kv lucas sport. I have an adjustible elec. fuel pump also. I do NOT have an EGR valve.

I assume now that it's a vaccum advance problem w/dizzy. I keep meaning to do a vaccum pressure test on the intake to see what pressure it's pulling and also test the vac advance on the dizzy, but I keep putting it off until I get some more info on the dizzy (I'm using one from a midget right now).

It doesn't sound like you're having the exact problem as me, and you've already tried 'fixing' it.  But maybe the ignition deserves another look. I had a vacuum leak on my dizzy before and the car was really sluggish and sputtered at almost any speed (not really what I'm having now). It would rev fine (by hand) but if you put a load on it (got in and drove it) you get what I just mentioned.

Of course, since I haven't fixed the current pulse/power fade  on my car yet, it's probably a dead end. Sorry. That's probably not any help other than to know I have/had a similar problem.

Get email at your own domain with Yahoo! Mail.

Sounds like a lean mixture this could have many causes including:

1)  if Stromberg try the tempature compensator. 

2)  If late SUs with "pop-off" valves in the throttle plates it could be weak springs. 

3)  If you have increased exhaust flow it could be that your needles need to be richer. 

4)  Do you have any vacuum leaks?

5)  It could be EGR if you've got one but this would be worse at low revs.

Gary Kristjansson

my reply/comments intermingled below  ****
----- Original Message -----
From: "Gary Kristjansson" <kristjag@EM.AGR.CA>
To: "Ptegler" <ptegler@gouldfo.com>
Sent: Monday, January 15, 2001 7:58 AM
Subject: Re: Pulsing and surging

Sounds like a lean mixture this could have many causes including:

1)  if Stromberg try the tempature compensator. 
2)  If late SUs with "pop-off" valves in the throttle plates it could be weak springs. 
**** replaced with non-poppet valve disks. - new carbs- did  the same thing.
3)  If you have increased exhaust flow it could be that your needles need to be richer. 
**** running #7 with K&N's (up from orig ADB's to AAA's then to fixed #7's)
4)  Do you have any vacuum leaks?
**** I run a vacuum gauge in the cockpit. Let's me know early if
I have problems and how hard I'm pushing it. (great way to know if valves
or timing are having problems.)
5)  It could be EGR if you've got one but this would be worse at low revs.
**** no egr or air pump system.

Paul Tegler     ptegler@gouldfo.com    www.teglerizer.com

I'm running pertronix and a 40KV coil (old points and coil did this too. hence the elec conversation)  ...plugs are presently at .035 ...I'm wondering if I need to close them back down a bit to get a more reliable/repeatable spark

Paul Tegler     ptegler@gouldfo.com    www.teglerizer.com

Well, as you increase the gap two things occur.
a) takes higher voltage to spark
b) more fuel/air mixture will be between the electrodes of the plug.

Look at the gaps that american car manufacturers were using through the late 70's and 80's. Some as big as .080 because mixtures were so lean and emissions so critical. The main reason for electronic ignition is to have enough power to jump the larger plug gaps. .035 is fine for point ignition and slightly rich mixtures. If you are running performance ignition you should be more than able to fire .050 gap or better.  I've run .040 with point ignition with good results.
If you close the gaps, you reduce the voltage required to fire the plug, also reducing the duration of the spark if I remember correctly.
Inconsistent spark would more likely  produce complete misfire instead of uneven firing.
Try it out. .040 then .045 just as a test.
Will only cost you the time to do it. Smoothed many a midrange driveability
issue for me on several different types of vehicle.


If your original comment below is accurate, I'd start looking at things
outside the car, like where you buy fuel, what grade, additives, any outside influence you can think of that the cars have in common.  Maybe you've ruled out this stuff already, but no one else has suggested it ... just thought
I'd give it a shot!  Good luck.

-----Original Message-----
From: Ptegler
Date: Sunday, January 14, 2001 8:23 PM
Subject: [spitfire-enthusiast] Pulsing and surging

>This problem appears on more than one marque

John Koenig

best logic behind any suggestions I've had yet.   :-)
OK...here's one for you.
I've retained the carbon canister with the valve cover feeding the center top port.  Carb float bowl vents to one of the small ones, fuel tank to the other small one.
Do you think it possible that the vents are battling each other? That is to say... the float bowls may be being effected by either the vacuum/pressure in the crank (valve cover port) or the fuel tank vent?

Paul Tegler     ptegler@gouldfo.com    www.teglerizer.com

Since pulling the choke richens the mixture I would check first the float level. It may not be correct making it harder for the motor to get enough fuel. The other thing to look at would be a vacuum leak  which is causing a lean mixture, choking it adds more fuel, which smooth things out. Check the not obvious things like worn throttle shafts, a gasket that has slipped out during assembly, manifold cracks etc.

Good luck,
John D'Agostino

Well first off... a hardy thanks to all the responses. Wow! I've asked questions before... but have never gotten back so many responses before (let alone so quickly!)  Apparently this is a subject where I'm not alone or others have seen this too.
From all the responses I've come to consider the following as a possible problem, as it's one of the few common elements on both marques.

Even on the dual HS4 conversion on the Spit, I retained the breather system and carbon canister setup of the single Stromberg. This setup is almost identical on both the BGT and the Spit.

Both cars have the valve cover breather to carbon canister connection. The fuel tank runs to the same canister as does the float bowl vents.

What are your thoughts on something in this breather system fighting each other? That is to say... the float bowls being effected by vacuum from the valve cover or fuel tank vent.

If the fuel tank vacuum created as gas is extracted, or the crank vacuum pulling on the canister were to create a small vacuum in the float bowls, does it sound feasible that this would effect the gas level in the float bowls which would explain why pulling the choke to compensate for the level in the jets makes it run better.

Paul Tegler     ptegler@gouldfo.com    www.teglerizer.com

I dunno, but perhaps this pulsing is part of the Lucas Warp-Drive system.
Better check the matter-antimatter transducer. Or clean the plugs.

Just a suggestion.
And a smart remark...
64 Herald 1200 Convy with Lucas Super Park system


Have you considered that you might have some flooding going on in the carbs. Are the bottoms of the carbs wet with gasoline?  If they are you might have the infamous black specs that are making your float valve stick open and let gas flow unabated which could cause the pulsing/hesitation thing you're getting.  You can pull the bottom of  the float chamber off without removing the carbs.  Dump the contents in a clean white or yellow bowl and if you see tiny black specs (varnish particles) then that could be your culprit.  I can tell you remedies later if it is.

Pulling the choke could be reducing the problem by opening the butterfly and letting more air in which reduces your over rich mixture caused by the flooding.   By "correcting" your mixture using the choke that would improve the performance temporarily.

Have you pulled your spark plugs to see what color they are?  If they're black and sooty or even wet with gasoline you definitely have a flooding problem.

And I'd guess you buy your gas for both marques at the same place which may explain why you have the same problem on both cars.  But I could be wrong about that too!

Good luck.

Bud Rolofson

Spark plugs look great!  Light brown ash color. I've cruised at speed... killed the engine and drifted over to the shoulder of the road to see if the plugs were running lean. They were still light brown.

even the new carbs do this. two fuel filters in line ...tests show more than adequate flow rates (roughly 1gal/ 6 min) at 2.5 -3 psi.

At cursing speed the butterflies are not effected by pulling the choke out. If I pull it out that far I can usually fell it in the peddle.

Nope... various gas station... various brands.

Paul Tegler     ptegler@gouldfo.com    www.teglerizer.com

Hey Paul,

I have a '70 Mk3 with a 1296 engine and HS4's.  I have my valve cover connected directly to the carbs.  I run the vapor line from the fuel tank to the carbon canister but I do not have it hooked to the carbs.  The float bowls I have vented to atmosphere.  This set up has worked well for me.  When I had the full emissions stuff hooked up I used a PCV valve between the carbs and the valve cover and then a "tee" to the carbon canister.  This set up made it nearly impossible to get a good idle and I noticed it surging while driving.  Ted at TSI suggested I only hook the valve cover to the carbs and that I not use a PCV valve. So I would say try running your carbs without the emissions stuff and see if it helps.

Good Luck,

Bill Munts
Staff Engineer
Kohler Co. - Huntsville

P.S. I'm on digest so I just wrote to you directly.  Haven't figured out how to respond to the list while on digest...haven't really tried either.

Paul :

I believe you'll find that there is a vent in the bottom of the carbon cannister (the end away from the hose connections).  It _should_ be impossible for the cannister to communicate vacuum to the bowl and tank vent lines, but it's possible the cannister vent is blocked somehow.

The test is easy, just pull the tank and bowl vent lines off the cannister, make sure they can't tangle with anything, and try driving the car.  You should not be able to detect any difference.  If it makes a difference, you may have a defective cannister. (I once succeeded in filling one with mud, but not on a LBC.)

According to my Spitfire Haynes, the filter in the bottom of the
cannister should be changed every 15K miles.

Randall Young

Message text written by "Ptegler"
>>What are your thoughts on something in this breather system fighting each other? That is to say... the float bowls being effected by vacuum from the valve cover or fuel tank vent.

If the fuel tank vacuum created as gas is extracted, or the crank vacuum pulling on the canister were to create a small vacuum in the float bowls, does it sound feasible that this would effect the gas  level in the float bowls which would explain why pulling the choke to compensate for the level in the jets makes it run better.


The canister should have a vent on the bottom side to atmosphere that will accomodate any reasonable amount of air flow to/from any of these devices - unless the canister is plugged up.  If you have a working canister then there is no pressure differential to ambient.

Dave Massey

thanks Bill!  

Having been through (testing configs / rhymes and reasons)   more  than most would even understand...

this is the only thing that seems to make any sense to me. My '73, and '70 Midgets are setup close to what you describe as your config and do not do this. (not at all noticeably anyway)   The Midgets has a breather down on the timing chain cover (crank case vent) that feeds either a PCV valve directly to the intake manifold or via T-pipe directly into the carbs (model year variations) The valve cover on the 73 has no breather other than a small hole in the filler cap. The bowls vent to atmosphere.
Only the gas tank vents through the carbon canister. The '70 Midget has a valve cover to carbon canister pipe, as well as the above mentioned plumbing.


Presently.... On the Spit (1493cc)  the carbs (between piston and butterfly) go into the valve cover as well as T'ing off into the carbon canister. Tank and bowls go to the canister as well. On the BGT the valve cover goes to the carbon canister as do the float bowls and gas tank. The timing cover has a vent that goes directly to the ports on the carbs.


So... you can see I've had a lot of fun (yeah right) playing with this problem with so many different configs to play with.


Your suggestions sound like a winner.

 Thanks again.


Paul Tegler     ptegler@gouldfo.com    www.teglerizer.com

It sounds like you've really been through a battle.  I must say that converting to the HS4's has been a major pain, they just don't dial in as well as the HS2's.  I am sure that this is due to the fact that the HS4's are really too much carb for the 1296 engine, however, when accelerating I can really feel and hear the extra CFM come in at around 3,800 rpm.  My largest problem with the bigger carbs is getting a stable, under 1,000 rpm, idle.  I have had the most success with the vacuum advance disconnected on the dizzy.  My higher profile cam produces a lumpy idle by itself so the combination of the bigger carbs and lumpy cam make idle adjustment tricky.
Still a lot of fun though.

Bill  Munts

  The way I see the operation of the carbon cannister setup is this:   Engine running at road speed--carbs pull vacuum, some vacuum advances distributor. Some vacuum is routed to pick up fumes from the sump breather system on the left side of the engine. Air enters the engine via rocker cover via hose to carbon cannister.  Carbon cannister is open at the bottom to atmosphere, so any vacuum in CC would not pull a vacuum on top of the carbs' float bowls. And, if there were such a vacuum on the float bowls, it would raise the level of gas in the float bowl, slightly riching the mixture.  I don't think that this system is causing you the trouble. 
  You asked if the fuel tank created a vacuum as fuel was pumped out. Well, yes, there would be lowered atmospheric pressure in the tank if  the tank were not vented.  It wouldn't take much to collapse the tank, by the way.  You can check that vent line, which is one from the charcoal cannister to the fuel tank, by blowing backward through it toward the tank. Lungpower should suffice. If you can't blow through it, then it  or the separation cannister in the boot will be clogged.  At 20 miles per gallon, only three gallons of gas per hour will be pulled from the tank, so it doesn't need much airflow in to replace that amount.  I don't think that the rate of fuel out of the tank would affect the fuel in the float chambers, except to richen the mixtrure, but the amount would be infinitesmal.
  I believe that somewhere in the system there is a small air leak that
you are struggling to locate. You sure have been thorough, but I think
it's there, somewhere.

Bob Howard

  I couldn't find the RunOn valve in my wiring diagram in the 62-74 Bentley manual, though I'm sure it is there.   In the text, p. 3-8, it says "when the ignition is switched off the solenoid is energised through an oil pressure switch"     Next paragraph it says to "check the control valve line fuse", Turn the ignition switch to off, Disconnect control valve electrical lead at the oil pressure switch,  Touch disconnected lead to a good earth point on the vehicle. If the  control ecectrical circuit and valve are satisfactory, the valve will be heard to operate as the control lead is earthed.

Bob Howard

  If you suspect the carbon cannister, set aside an hour and attack it. MG says somewhere that if the charcoal gets saturated with gas it should be replaced.  Mine got wet once, which is how I was forced to figure out (finally) how the vapor recovery system works.  The charcoal was still granular and should have passed air OK.  But, I do know that charcoal briquettes soak up water over the winter, get clumpy and then don't burn well, so it's not unlikely that a replacement of the stuff from time to time, as MG recommends, is appropriate.
  Go buy about a quart of charcoal at the pet store and swipe a couple of scotchbrite pads from under your kitchen sink.
  Once cannister is off the car, invert it and unscrew the bottom. As you look at the filter wondering what to do next, the spring inside will launch the charcoal and the filter all over you and the backyard. 
  Refill with your new charcoal and hold the charcoal in place with the scotchbrite filter that replaces the deteriorated rubber one. Job is complete.
  By the way, when it's off the car, a few minutes of buffing time on the cannister will shine it up impressively.

Bob Howard

Easy enough to find out... disconnect em' - vent the bowls to a  pop/beer can with a length of fuel hose and set it behind the heater box 'n drive it.
Generally speaking if you apply pressure to the bowl vents you get RICH mix (blackened plugs) slight VACUUM on the bowls will then create LEAN mix (stark white plugs) I've seen more lean conditions due to people running vacuum to bowl vents (and some GM products that had solenoid actuated bowl venting sometimes became defective and placed direct manifold vacuum on the bowl vent... very hard to spot at 60 plus on the interstate ;)

the OTHER possible scenerio is that atmospheric pressure is no longer present on the bowl vents. This is due to carbon cannister having been filled with liquid fuel, dirt, turned upside down etc. which will plug off the orifices (or in the case of fuel saturation turns the charcoal into one solid mass that air cannot pass through) or even on some models the filter element gets completely plugged with dirt/oily dirt  etc. The cannister is x number of years old, and is both the carb bowl venting source and the fuel tank venting source. Plug that off and you get all sorts of weird leanout (cannot remove fuel efficiently from a completely closed gas tank, nor can you siphon it from a closed off float bowl) The valve cover vent is really a "hydrocarbon catchall" so that blowby and "after shutoff" fuel vapors are returned to the charcoal which will (hopefully) turn it back into some sort of liquid state.

For test purposes, I would disconnect ALL THREE lines from this device. Placing only the carb bowl vent line in a recepticle (this is in case of a stuck float or debris in a float valve that would cause a mighty big fire should the liquid fuel not be contained somehow.) One of our MG car club members used a coke can for this purpose as a stopgap til he could locate a charcoal cannister for his BGT. A stuck float had rendered the original defunct and (before he knew the value of it) had  tossed it out.
I have heard they can (somehow) be refilled with simple aquarium tank charcoal and made functional again (considering how much an entire replacement may cost.... wow)

The "no new parts, easy fix" ideas are always the best. Although Ed Kaler drives me nuts his "Keep it Simple Stupid" usually is "sound advice" Now I am off to figure out why an 89 Pontiac goes "code 44 lean mixture" after being driven for hours without incident it becomes undriveable when parked for ten minutes and restarted... sounds fun *NOT* I wish all cars were as simple as our LBC's (or in my case old LBC's, GM's,Fords,Datsuns,
Mopars...) Gary (with more cars than time)


It appears carbs will actually operate over a wide range of fuel height levels. Pulling the choke out richens the mixture and if this cures the problem then it is too weak. The problem is finding out if the weak mixture is constant (probably carb needles, could be air leaks) or varying. We never used to change the springs in the carbs, for twin 1 1/2 SUs with EITHER K&Ns OR a long branch manifold we fitted AATs (AAQ almost identical), with K & Ns AND a long branch we fitted ??? (can't remember! contact Canley Classics at john@johnkipping.demon.co.uk ) The actual identification letters for the needles do not follow any pattern, the booklet listing out all SU needle profiles is available from Burlen Fuel Systems in Salisbury, Wilts, UK.

John Kipping

     I think you may be on to something,I know the vaccuum from HS4's is strong enough to suck the oil from the valve cover and give those following you a smokey view. I would try to disconnect the float bowls  and fuel tank vent from the canister.Both these on my '76 are open to atmosphere. My vaccuum lines combine to a tee and go to the valve cover.This worked fine until I got an alloy valve cover (no baffle). Then I started to put up a smoke screen. I tried putting a fiber filter in the line and this plugged the vaccuum,making my engine build pressure,forcing oil from every orifice.I've gone back to the stock valve cover for now. Anyway,I think you could be pulling too much vaccuum on the float bowls and shutting down the fuel supply,or too much vaccuum on the fuel tank causing a fight with your fuel pump. Let me know what you find out.

Good Luck,

Bob Harris


 I recall having this problem a while back, and an MG mechanic fixed  it by drilling a small hole in the gas cap.  It seems that the vacuum  breather pipe from the charcoal canister to the gas tank was clogged or pinched and there was a vacuum created in the tank as the gas was being pumped out.  The gas was going out but no air was being drawn back into  the tank.  This caused fuel starvation at the carb end.  The hole in the gas cap was a quick easy fix.. I've since gone back and using compressed  air blew out the line from the charcoal canister to the gas tank, sealed up the gas cap hole, and my problem was solved.  Hope this helps.

 John Di Fede

Is there play on the butterfly spindles? Are there any other spots where
false air can run into the carbs?


Hans Duinhoven


I have a Datsun pickup which does the same thing and has for over 50,000 miles, never getting worse.  It probably has a more complcated carb than the B.  I've done everything but replace the carb.  In my instance, I think it's part of the leaning out of the fuel by the emission controls.  This could be an economizer valve not working, or perhaps the EGR system.

Blake           (Bullwinkle)

Well once again I hit all the lists...to thank everyone...
and to announce a resolution has been found.

I don't think anyone is going to believe this but after two years of this problem, tonight I fixed it.  No stumble/pulse and it'll now idle as low as 550 without dying!

I'm going to write this up as an extensive web page with inclusions of some of the 60+ email responses I've received with ideas as to correcting this problem.

for now.... the only hint I'll give.... Just because you buy a certain year/model LBC, have all the correct year/model spec ancillaries on the engine... have REBUILT EVERYTHING  to spec.... doesn't mean they're the right ancillaries or the right specs for the condition of your engine.

Gotta' love the neighborhood kids who like to watch me work and ask a ton of questions.  A 6 year olds' question... triggered the final fix.

Although pulling the choke out a bit at cruise corrected the problem, breathing, fuel, exhaust, spark... had nothing to do with it

Paul Tegler   ptegler@cablespeed.com  www.teglerizer.com

       ************* Big clue here folks! ****************

     ----- written to R.O. Lindsey 'off-list' ---------
Well if you've been following my stumble/surge threads... you're giving everyone a hint as to what was going on my '73 BGT.

Here's the deal. You don't repeat this..  yet   (to the lists) And I'll give you a clue on how to determine your answer correctly. I'm still writing up  details for my web pages.

To start with.... check
and also not too bad but a bit confusing to read)
for a complete list of curves, part numbers, specs.

On your dist. body, you'll find a flat area that has one of the numbers listed on the chart stamped into the body of the dist.  This identifies the mech. advance curve.
Around the neck of your vac adv. unit (right near the vac line attachment) will be tiny numbers stamped into it. The long number will match one in the chart as well. Make sure the right vac unit is on the  right dist. per the chart. The chart also lists the 'ported' or 'manifold' vac source for setup requirement for that unit.

I had rebuilt everything to '73 specs. Had a '73 Spec distributor with the correct vac unit for the dist. In my case... a 41491 dist with vac unit 54425359 running 10.15.5 vac connected directly to the manifold (See the teglerizer chart)

Apparently both the vac unit and dist model did not match the motor/cam config which WAS causing this stumble at cruise. I was using manifold vac (correct spec for the 18V '73 model engine I 'thought' I had.

Turns out my engine is a 68-70 model as far as cam is concerned and has a HC (high compression) head on it. I put in a spare 41288 dist I had laying around with the correct matching 54411985 vac unit (5.13.10) and hooked up the vac source to the port on the carb.

I initially reset timing to 10... then bumped it up to 14 BTDC at 600 rpm.

BINGO! I can actually set the idle as low as 550 rpm without dying. Starts even easier than before (even in cold weather) I was able to lean out the carbs a bit. and have more power at the low end (upper end seams un effected by the change over)

So .... to tell what you need (what runs best regardless of what you think, or what the books say it should be.... you need to determine what you have...and then test a few variations. I'm sure you can find a friend with a car that has some setup different then yours and experiment.
I didn't even change dist. caps. It took less than ten minutes to swap distributors, hook up the vac lines and retime it.

I was actually a little pissed-off that it was such a simple  fix to my problem. In the past I had replaced carbs (brand new ones) tried two different elec. ign. systems as well as points ran no vac/with vac played with mech. advance weights... all kinds of stupid stuff with no change in this problem.
The distributor changed everything. Even my lady friend says it feels like a different car.

I haven't sent this info off to the list.

Paul Tegler

At 10:46 AM 2/1/2001 -0600, Rick Lindsay wrote:
>   The original designs for vacuum advance used a port on the carburetor that was connected to a small opening in the throat of the carburetor. ....
As the throttle is opened, the vacuum rises (depresses?) and the vacuum advance module goes to work. ....
>   .... Newer designs have a vacuum port on the inlet manifold rather than the carb.  That port shows pretty strong vacuum even at idle.  My car has BOTH ports. Which method of applying advance is BETTER for my engine and my application? ....

The method ported from the manifold uses a different distributor, which can be thought of as "vacuum retard" distributor.  The vacuum from the manifold is high at idle and holds the distributor in a retarded position.  When you open the throttle the manifold vacuum drops, allowing the timing to advance.  You need to use whichever port is appropriate for your distributor.  Or conversely, for those who do not have both ports, you need to use whichever distrubutor is appropriate for your existing port.

Barney Gaylord
1958 MGA with an attitude (and no vac Mallory dizzy)

...have to add a bit of 'technology flip-flop' info that has taken place over the years. As Barney described...manifold vac goes with retard ported goes to adv.

But there were years/models that did use a vac adv. pot on the dist. while hooked to the manifold rather than ported to the carb throat. The idea 'at that time' was with max vacuum at idle... you'd get max advance. No engine load at idle with max advance meant a cleaner idle burn. As you opened the carb... (accel) the vac dropped off a bit, so it wouldn't ping. Then up at cruise... the high manifold vac would then again adv. for max burn time.

The reason this format is no longer used is one of component 'slop' and timing of cam/dist/spark etc. If just a wee bit off...the systems start fighting each other very inefficiently.

If anyone has been following my  stumble/surge problem.... this very subject was the root of the problem.

The aging systems were the 'stumble' I had. I've gone to an older model dist, (different vac adv. curve) and now use ported vac, and the problem is totally gone now. A side reward was also better power at the lower rpm ranges.  

So ...just because you  rebuilt everything to spec for your model year doesn't mean it's the best setup for your aging toy.

The cat's out of the bag Robert L. ...   :-)

...web pages to come.

Paul Tegler     ptegler@gouldfo.com    www.teglerizer.com

you got it right Peter

Paul Tegler     ptegler@gouldfo.com    www.teglerizer.com

----- Original Message -----
From: "Peter C." <nosimport@mailbag.com>
To: "Ptegler" <ptegler@gouldfo.com>; "Barney Gaylord" <barneymg@ntsource.com>
Sent: Thursday, February 01, 2001 4:02 PM
Subject: Re: Vacuum Advance

Barney & Paul,
Isn't the vacuum tube on the dizzy the way to tell if retard or advance?
If the tube aims away from the dizzy body, then it's for advance. If
toward the dizzy body then retard. In the case of TR6s, they had twin
vacuum chambers, one pointing each way.
Peter C

World Wide Auto Parts
2517 Seiferth Rd., Madison, WI  53716
(800) 362-1025     Fax (608) 223-9403


Yep... it all depends on the vac advance curve.
yes... there are different points at which vac
advance start and finish.

Paul Tegler     ptegler@gouldfo.com    www.teglerizer.com

----- Original Message -----
From: "Charley & Peggy Robinson" <ccrobins@ktc.com>
To: "Ptegler" <ptegler@gouldfo.com>
Cc: "Barney Gaylord" <barneymg@ntsource.com>; "R. O. Lindsay" <rolindsay@dgrc.com>; <mgs@autox.team.net>
Sent: Thursday, February 01, 2001 6:09 PM
Subject: Re: Vacuum Advance


  I wonder if the difference in how the vacuum diaphragm is used on a particular engine (or FWT) could explain why some people install a Weber conversion and get a stumble, while others have no problem with the installation.  Next time I make a club meeting I'm going to ask about that. 




 A possible Resolution
     So what was it?  Back up 5 emails and re-read carefully. Simply put, a combination of getting the proper mechanical advance curve, matched to the vacuum advance curve, cam pattern and vacuum source. Once these were all properly matched, the problem disappeared.


    A Final Resolution!
     Once and for all.... my stumble is totally gone now.  Actually it was a simple fix at that...and one I could kick myself over.   what 'rung a bell in my head' was a fellow at a car show that had duct tape all over his K&N's sealing off nearly 80% of the surface area. After asking him what the tape was for, he said it ran better in the cold weather that way. But this was the middle of the summer! Then it struck me... DUH!

    After a final test of going back to the stock needles for the twin SU's (which worked the best as far as mixture across the board was concerned...the stumble was still there.  I repeated my tests with the AAA's and modified #7's  (super rich) and the stumble was there as before.

    So I totally removed the K&N's and installed the stock air filters and boxes. NO STUMBLE! None, nada, zip, zero!  After a week of no stumble...with no other changes.... I put the K&N's back on. Sure 'nough....it stumbled.   Put the orig filters back on... and no stumble.  And to boot... my mpg has gone up nearly 7mpg! 

    What it amounts to, is the lack of proper air pressure differential across the jet bridge. The air box design works in tandum with the carb to 'set' the differnetial. This incorrect differntial lets the piston sit lower than it should, which results in more gas being pulled, do the the pressure imbalance. (like accelerating..damped by the oil, the dash pot rises more slowly, letting more gas in breifly until the piston position catches up. (The ZS carb is even MORE sensitive than the SU to this condition).  With the proper 'limiting' of the air flow, through the filter, the correct 'damper' is controlled, thus stopping 'pulsing' of fuel.

    Now the only test left perhaps, is to try the K&N's again, but with a full fledged box around them with a snorkle, to setup the proper pressures. But that somewhat defeats the purpose of the K&N's (except for perhaps the washable re-usability of them) compared to any other filter. 

     So I guess my fancy MGB logo plate and twin K&N's are now avail for sale, as I'll NOT be ever re-installing them!  All that... just for the stupidity of believing the hype the catalogs spew, trying to sell you stuff that is useless on the street.

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