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??
Ed
Does it happen cold or hot? Could be the gas line
too close to the exhaust
header? Heat shield?
Mike
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.
>>Wonderments?
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
Paul,
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
Paul,
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
Paul,
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
thecarguru@aol.com
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
Paul,
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.
-Terry
Get email at your own domain with Yahoo! Mail.
http://personal.mail.yahoo.com/
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.
thecarguru@aol.com
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.
--John
-----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
Hi
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...
Scott
64 Herald 1200 Convy with Lucas Super Park system
Paul,
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.
>>
>>Paul,
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
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.
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.
Paul,
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
Paul,
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
Paul,
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)
thecarguru!@aol.com
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
Paul,
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
Paul;
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?
Cheers,
Hans Duinhoven
Paul:
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
https://www.teglerizer.com/mgstuff/advance_curves.htm
and also not too bad but a bit confusing to read)
http://www.mgbmga.com/tech/mgb21.htm
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)
http://www.ntsource.com/~barneymg
...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
www.nosimport.com
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
All,
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.
Cheers,
CR
