At last, the cause of 308 backfiring...

Birdman · May 5, 2005

Hi All,
Many of us who own carbed 308s have at times wrestled with the reasons why the car sometimes backfires when shut down. This embarassing trait causes people to dive for cover at the gas station and often gets a snicker or two. It has been driving me nuts.

Last year when I embarked upon my quest to learn how to synch carbs, I managed to get my 308 to stop doing it by carefully synching the carbs so that the car would idle nice and slow. Unfortunately, that was only masking the problem, not fixing it. At low RPM it doesn't happen, but if I step on the gas a teeny bit and bring the idle up to 1100 RPM or so, it will still backfire. I resigned myself to the idea that the car just "does that", and there is nothing I can do about it. But that makes no sense. Did Ferrari sell brand new cars in 1977 that backfired when you shut them off? No.

I got to thinking and experimenting and I believe I have it licked.

You will notice if your car does this, that the backfire occurs well after the key has been shut off, and generally a good half second after the engine has completely stopped turning. There have been many theories on what is happening, but there is really only one that makes sense given a complete examination of the facts.

When you shut the car off, the ignition stops but the engine continues to turn a revolution or two before it stops as well. Without the sparks plugs doing anything, one or two mixtures in the combustion chambers get pumped out into the hot exhaust manifold where it ignites. Because the backfire occurs so long after the car is shut down, there is no way that an additional spark is causing this, as has been suggested.

My theory was that if the mixture is too rich, there is enough fuel to ignite. So I went out today, drive the car, got it good and hot and came home for some experiments. I set my idle at 1000 RPM. It backfired when I shut it off. Next I cranked down my idle mixture screws 1/4 turn to lean it out. Guess what, the backfore got weaker. I cranked them down some more and eventually I got rid of it completely. My idle speed increased as I leaned out the mixture, indicating that my mixture was too rich.

OK, but here's the problem. A lot of people are running very lean (stock) idle jets that were chosen for emissions, not optimum performance. With modern fuels as well as (often) less restrictive exhausts on these cars now, a lot of people have idle jets that are just plain not big enough. With a very lean mixture, the car pops and hiccups at idle, because combustion is tenuous. So people crank open the idle mixtures to try to make it stop popping. Since the idle mixture screw only affects the first progression hole in the Weber, it may be possible to get the idle mixture too rich, so the car idles well, but still runs poorly in the off-idle to 3000 RPM range where the around town driving is done and where the idle jets provide 90% of fuel delivery.

Last year I swapped out my too-lean .55 idles for .60 idles and it made a world of difference in how the car runs, but it didn't fix my backfire at shutdown. After tweaking the mixtures for a while, I found a happy medium where the car doesn't seem to backfire. It's tricky....too lean and it pops and hiccups, too rich and it backfires when you shut it off.

Anyway, if your car backfires when you shut it off, close the idle mixture screws a bit. If that makes it run badly just off idle (hiccuping and hesitation), your idle jets are too small. You can increase the size of the idle jets to get the car to drive well around town, but still use the mixture screw to cut down the mixture for idling so it doesn't backfire.

Of course, having the carbs in synch to begin with is very important. If they are in synch, the car will idle smoother, and it is possible to set the idle a tad lower. Lowering the idle speed ALWAYS helps the backfire issue even if it's way too rich, because the engine is turning slower when you shut it off, so less fuel gets dumped into the exhaust. This is the reason why using the clutch with the car in gear to slow the engine just as you shut the key off works. As soon as the ignition is off, with the load of being in gear with the brakes on, the engine stops immediately and doesn't turn long enough to dump fuel into the exhaust.

Hope this helps.

Birdman

ham308 · May 5, 2005

Birdman, thanks for the tip for getting the engine to stop quickly using the clutch. As a fellow sufferer of the almighty back-fire on shutting off the engine (nothing like drawing attention to youself), it drives me crackers as well.

After various attempts to set the idle mixture to get stable and correct running for the emissions test, I've found it's not easy to get it right for all conditions. My engine still tends to peter-out (slowly die) when it is very hot.

I think one of the main problems is, as you say, that the idle 'mixture' screw only works on the first progression hole, so it's important if possible to adjust the mixture at very low speed so the next hole isn't working.

I was wondering if it would help to set up the idle with the air by-pass screws open say one turn so that the butterfly valve position is no longer as critical to the whole process.

Birdman, (or anybody else) what do you think?

Richard

thecarreaper · May 5, 2005

Birdman said:

Hi All,
Many of us who own carbed 308s have at times wrestled with the reasons why the car sometimes backfires when shut down. This embarassing trait causes people to dive for cover at the gas station and often gets a snicker or two. It has been driving me nuts.

Last year when I embarked upon my quest to learn how to synch carbs, I managed to get my 308 to stop doing it by carefully synching the carbs so that the car would idle nice and slow. Unfortunately, that was only masking the problem, not fixing it. At low RPM it doesn't happen, but if I step on the gas a teeny bit and bring the idle up to 1100 RPM or so, it will still backfire. I resigned myself to the idea that the car just "does that", and there is nothing I can do about it. But that makes no sense. Did Ferrari sell brand new cars in 1977 that backfired when you shut them off? No.

I got to thinking and experimenting and I believe I have it licked.

You will notice if your car does this, that the backfire occurs well after the key has been shut off, and generally a good half second after the engine has completely stopped turning. There have been many theories on what is happening, but there is really only one that makes sense given a complete examination of the facts.

When you shut the car off, the ignition stops but the engine continues to turn a revolution or two before it stops as well. Without the sparks plugs doing anything, one or two mixtures in the combustion chambers get pumped out into the hot exhaust manifold where it ignites. Because the backfire occurs so long after the car is shut down, there is no way that an additional spark is causing this, as has been suggested.

My theory was that if the mixture is too rich, there is enough fuel to ignite. So I went out today, drive the car, got it good and hot and came home for some experiments. I set my idle at 1000 RPM. It backfired when I shut it off. Next I cranked down my idle mixture screws 1/4 turn to lean it out. Guess what, the backfore got weaker. I cranked them down some more and eventually I got rid of it completely. My idle speed increased as I leaned out the mixture, indicating that my mixture was too rich.

OK, but here's the problem. A lot of people are running very lean (stock) idle jets that were chosen for emissions, not optimum performance. With modern fuels as well as (often) less restrictive exhausts on these cars now, a lot of people have idle jets that are just plain not big enough. With a very lean mixture, the car pops and hiccups at idle, because combustion is tenuous. So people crank open the idle mixtures to try to make it stop popping. Since the idle mixture screw only affects the first progression hole in the Weber, it may be possible to get the idle mixture too rich, so the car idles well, but still runs poorly in the off-idle to 3000 RPM range where the around town driving is done and where the idle jets provide 90% of fuel delivery.

Last year I swapped out my too-lean .55 idles for .60 idles and it made a world of difference in how the car runs, but it didn't fix my backfire at shutdown. After tweaking the mixtures for a while, I found a happy medium where the car doesn't seem to backfire. It's tricky....too lean and it pops and hiccups, too rich and it backfires when you shut it off.

Anyway, if your car backfires when you shut it off, close the idle mixture screws a bit. If that makes it run badly just off idle (hiccuping and hesitation), your idle jets are too small. You can increase the size of the idle jets to get the car to drive well around town, but still use the mixture screw to cut down the mixture for idling so it doesn't backfire.

Of course, having the carbs in synch to begin with is very important. If they are in synch, the car will idle smoother, and it is possible to set the idle a tad lower. Lowering the idle speed ALWAYS helps the backfire issue even if it's way too rich, because the engine is turning slower when you shut it off, so less fuel gets dumped into the exhaust. This is the reason why using the clutch with the car in gear to slow the engine just as you shut the key off works. As soon as the ignition is off, with the load of being in gear with the brakes on, the engine stops immediately and doesn't turn long enough to dump fuel into the exhaust.

Hope this helps.

Birdman
Click to expand...

post of the month!!!! the holy grail of backfires has been found, and we can stop it! my 77 doesnt do it, my 78 was a mess.

pma1010 · May 5, 2005

Great insight, Jonathan. Try this one:

Motor has been sitting for 5 months out of the car. Lots of work done to the oiling system (sep thread). All seems good. No leaks.

Car won't idle. Has to be proded constantly with the throttle. 6 inch flames coming out the trumpets.

Ignition setting: ring gear position on the harmonic balancer is OK (it's crank triggered) and power on cycle: fine.
Plug wires are fine.
Plugs are fine (3rd set gapped to 0.032).
Carb settings are fine. Fuel delivery appears OK (fuel in the bowls).
Cam timing is correct.

Tomorrow we are going to check for:
- dirty fuel
- power up of the relay which fires the coils (I have an Electromotive HPX).
- ignition timing when running

It seems like something is wayy off. Like the ignition timing being off by 30 degrees or more. Doesn't seem to be. Timing light on it tomorrow eve.

Anyone add an informed view to the list of tomorrow's "to do's"?
Sorry to hi-jack.
Philip

Birdman · May 6, 2005

ham308 said:

I was wondering if it would help to set up the idle with the air by-pass screws open say one turn so that the butterfly valve position is no longer as critical to the whole process.
Click to expand...

Another Fchatter suggested this in the past. This is not the recommended way to acheive a good idle, but given the twitchy nature of the throttle stop screws in setting the idle (1/8 of a turn can make a big difference) it may be a way to get more consistent results. The problem I see with it (I would have to go look at a diagram of the carb because offhand I can't remember how the air bypass valves are designed) is that you might be bypassing the venturis so you don't get the same effect as opening the butterfly valves a little smidge.

I have heard of people drilling a hole in the butterfly valve in order to get a small flow with the butterfly completely closed, thereby assuring that only the idle progression hole is supplying fuel. My car seems set up fairly well to idle with all bypass valves closed except one (on the brake booster carb) that needs a smidge to synch.

Philip,
Six inch flames? That would be outside my realm of experience! Definitely check the timing. If the timing is OK, check for a consistent spark. (That gap is way over the factory spec, but you have a powerful electronic ignition) Also, lean conditions will cause popping out through the carbs. What idle jets do you have, and are they clean?

Birdman

ham308 · May 6, 2005

Birdman, sorry to highjack this thread slightly, but it seems as good a place as any to continue with the never-ending thoughts on setting the idle...

I think I've got it cracked, (famous last words). The car now idles regularly even when hot. The by-pass screws are all still shut so I don't know if that would have helped.

In the past I have tried using a Colortune, but this seemed to show yellow when it was far too rich and then blue for a large range of idle screw adjustment, before being obviously too weak. So for me it was not accurate enough although probably with more experience it would be.

A cheap CO meter was good for the overall levels at the exhaust pipes, even showing the same values as a proper garage device when it was being emission tested. Not much good though when the individual sniffer ports are blocked up, (and the ones that are free get very hot indeed.) On several occasions the CO meter did however show two different readings from the tailpipes which, with my standard silencer, seemed to be consistent with the front bank going into the right side and the rear bank to the left.

In the end I got the best results with (and I'll be patenting this soon) putting three fingers over each cylinder choke. Yep, this is all you need guys. No fancy devices.

I found by accident that the restriction of the carb synch device caused the engine to speed up on some cylinders, slow down on others and have no effect on the rest. I got the same effect with just blocking the air supply with my fingers over the Weber adaptor of the synch device.

I guess it is like SU carburettors where you lift the damper piston slightly to let more air in and see what happens. This does the opposite. Fingers over the choke equals less air which causes speeding up when weak, slowing down when rich and no effect when you get it right. In the end it was very quick and sensitive. It is also more or less what the Haynes book recommends - making small changes to the mixture and watching the speed, only using the air supply and not the fuel.

I'd be interested to hear if anybody else gets the same results, or agrees/disagrees. (I might just have imagined the whole thing, having spent all afternoon in the garage with the engine running )

Richard

Sean F. · May 6, 2005

My '77 doesn't back fire, hickup or spit. Stock jets, points, OEM gap on NGK BP6ES.

You guys have jacked up your cars so much with bigger jets, different AC jets funky ignitions that may or may not be set up right....you need to slow down and get one thing right before you start screwing with something else.

pma1010 · May 6, 2005

1 maybe 2 problems.
MAP sensor ground wire gone. Fixed. Was causing idle at 10 Atdc. Disconnected the sensor, runs at 20 degrees Btdc.

Coil driving cyl, #1, #4 seems to be more intermittant than the others.
Philip

Bryan · May 7, 2005

Birdman said:

I have heard of people drilling a hole in the butterfly valve in order to get a small flow with the butterfly completely closed, thereby assuring that only the idle progression hole is supplying fuel. My car seems set up fairly well to idle with all bypass valves closed except one (on the brake booster carb) that needs a smidge to synch.

Birdman
Click to expand...

A separate 365 GTC4 group discussed and decided that drilling the hole in the plate is not a good thing to do. What works well for getting the throttle plates synched is to adjust the throttle linkages so that the same number of progression holes are seen in the progression well, when you shine a light down the throat.

Here's photos of a sidedraft DCOE. I don't know 308 carbs well enough to know if this is workable in downdrafts.

http://home.earthlink.net/~fml2811/DCOE%20Progression%20holes.htm

ashsimmonds · May 7, 2005

not the same engine (effectively half a 308) but more similar than different, is my 1608 (Lampredi twin-cam - 124BC). i used to get run-on after shutting off the ignition, and occasional backfires after it stopped.

after pulling the head, i saw that the tops of the pistons looked like a dirty bbq plate, with little jaggies and whatnot all over the joint. these little bits would heat up and act as a glow-plug, providing an ignition source for anywhere between 1 and 5 seconds after shutdown.

what i also noticed here was that it was only on one end of the motor, i had higher air velocity at one end of the intake chamber, providing a leaner mixture to cylinder #1 and #2, to the extent that #1 was clean enough to look brand new, but cylinder #4 was running really rich and dirty and was as pretty as an Englishman's teeth.

(attached pics of before and after)
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Birdman · May 7, 2005

Sean F. said:

My '77 doesn't back fire, hickup or spit. Stock jets, points, OEM gap on NGK BP6ES.

You guys have jacked up your cars so much with bigger jets, different AC jets funky ignitions that may or may not be set up right....you need to slow down and get one thing right before you start screwing with something else.
Click to expand...

Sean, I'm delighted that your car runs properly. But this is a known issue with a lot of 308s, many of which are dead stock. I don't think my car is "jacked up" since it is completely stock except for the exhaust system and the idle jets.

Unless you happen to have an original, 100 pound thermoreactor muffler on your 77 (which I doubt), it is in fact NOT stock, and your idle jets are likely too lean. I'm glad it runs OK, but watch your exhaust valves.

These cars are pushing 30 years old and have been screwed with by many people in their lives, most of which didn't seem to have a clue. Many of us have our work cut out for us to get them tuned properly. Do you think stock jetting from the late 1970's was chosen for optimal engine performance, rather than to comply with emissions? Do you think the Euro 308's used the same jetting?

Your car runs OK, and you choose not to mess with it, great. Don't take pot shots at others.

Birdman

ham308 · May 8, 2005

There is quite a lot going on in this thread but I don't want to start yet another.

I suggest that using a restriction (fingers or whatever) in the air supply to the carburettor is a quick and efficient way to fine tune the idle mixture on a 308 and I'd be grateful for any comments.

Richard

ham308 said:

In the past I have tried using a Colortune, but this seemed to show yellow when it was far too rich and then blue for a large range of idle screw adjustment, before being obviously too weak. So for me it was not accurate enough although probably with more experience it would be.

A cheap CO meter was good for the overall levels at the exhaust pipes, even showing the same values as a proper garage device when it was being emission tested. Not much good though when the individual sniffer ports are blocked up, (and the ones that are free get very hot indeed.) On several occasions the CO meter did however show two different readings from the tailpipes which, with my standard silencer, seemed to be consistent with the front bank going into the right side and the rear bank to the left.

In the end I got the best results with (and I'll be patenting this soon) putting three fingers over each cylinder choke. Yep, this is all you need guys. No fancy devices.

I found by accident that the restriction of the carb synch device caused the engine to speed up on some cylinders, slow down on others and have no effect on the rest. I got the same effect with just blocking the air supply with my fingers over the Weber adaptor of the synch device.

I guess it is like SU carburettors where you lift the damper piston slightly to let more air in and see what happens. This does the opposite. Fingers over the choke equals less air which causes speeding up when weak, slowing down when rich and no effect when you get it right. In the end it was very quick and sensitive. It is also more or less what the Haynes book recommends - making small changes to the mixture and watching the speed, only using the air supply and not the fuel.

Richard
Click to expand...

Birdman · May 8, 2005

Richard,
It sounds like a clever idea to me, but I haven't got enough experience with carbs to know if this is an accepted method. Maybe someone else can chim in here!

Birdman

pma1010 · May 8, 2005

Sean F. said:

You guys have jacked up your cars so much with ... funky ignitions that may or may not be set up right....you need to slow down and get one thing right before you start screwing with something else.
Click to expand...

Sean, I agree with the second part of your sentence. Get one part right before you move on. For me, the issue here was trying to diagnose what had gone awry: inadvertent change to cam timing (unlikely but too critical to ignore), carb settings (requiring re-setting after having the motor inverted on a stand), ignition, bad fuel etc. It is a process of testing and elimination. FWIW, my gut was the ignition timing was 30 degrees out and it was -- just not from the trigger wheel being replaced incorrectly but from a poor ground on the MAP sensor.

On the wisdom of changing ignition systems. Au contraire. When the Electromotive HPX is set right. You're done. Period. My error after 3 years of (trouble free) use was not checking the connections to the module when re-installing it after having pulled the motor out. Given I use the car almost exclusively on the track it's been modified. While most observers will opine it looks great, it is much quicker than stock. Experienced drivers will indicate it handles much better than stock. Clearly it is not original. That said, for my use pattern, I'll take the reliability and dependability of a well engineered, crank triggered, solid state ignition system over points and rotors anytime.

I don't know the term "jacked up". It implies, at least to me, "changed from stock in a uncertain way by someone that doesn't know what they are doing". We all hate to see anything like this. Without taking "pot shots" at anyone, it feels like the guy asking how to fill gear box oil contemplating doing an engine rebuild.

It also implies condecension. All I can say is that all of us learn, sometimes through painful and expensive mistakes. We learn how to read a picture and hopefully ask the right questions before something comes apart so we can put it together right and recognize when something is not quite right, I am fortunate in that I have a group of people, race techs, top class mechanics, Ferrari techs at dealerships that are willing to help with advice and sharing their knowledge. They'll also come to my house when needed.

This board can also be an excellent resource for sharing and asking questions. It is most valuable when people contribute data and detailed experience, just as Jonathan, Richard and others have. It is least valuable when people take pot shots, as, at least in this case, you did. I suspect I've wasted enough of all our time on this.
Philip

chrismorse · May 26, 2005

Philip,

Your are a diplomat.
best,
chris

vincenzo · May 26, 2005

Hmmmm.....
- fingers over the chokes to tune a carb
- 6" flames shooting out of the trumpets
All in the same thread.
What's wrong with this picture?

Take Care Out There,
Vince

Sean F. · May 27, 2005

My Appologies Birdman. I was not attempting to take pot shots. Merely trying to point out that a lot of changes/tuning attempts on these carbs seems to be posted every week.

Reset timing from stock.
Different Main Jets
Different Idle Jets
Different A/C Jets
Removing Insulation from the air box.
Putting fingers over chokes to tune carbs.
Drilling holes in butterfly's to tune carbs (I know the Weber book does talk about this).

When we would make suspension changes to my FF2000 car, we would only do ONE thing at a time so we would know what that one thing would do to the car.

I understand you're trying to extract a little more power from the cars. I'm all for it. But I've seen a dozen threads about solving this and that on the weber carbs when I think what it boils down to is that a lot of poeple have made many changes and they're now at a point where they need to stop and think about what they've done.

A dyno and exhaust gas analyzer are needed to really see what these changes are doing.

Good Luck.

PS Yes, I still have the OEM Thermo muffler. It needs to be replaced but it's still hanging on.

Birdman · May 30, 2005

I'm actually not that interested in getting more power from the car. It's perfectly fine the way it is. I'm working towards getting it to run well (i.e. as reliably as possible). I am guilty of doing the airbox mod though! Not because I need more power but I can't resist the sound!

I didn't mean to jump down your throat. My car WON'T run on the stock idle jets with an Ansa muffler and modern fuel. It simply runs far too lean.

;-)

Birdman

Air_Cooled_Nut · Jun 12, 2005

ham308 said:

...
In the end I got the best results with (and I'll be patenting this soon) putting three fingers over each cylinder choke. Yep, this is all you need guys. No fancy devices.

I found by accident that the restriction of the carb synch device caused the engine to speed up on some cylinders, slow down on others and have no effect on the rest. I got the same effect with just blocking the air supply with my fingers over the Weber adaptor of the synch device...
Click to expand...

I don't consider this too fancy (carb sych on the left) and it's far more accurate and allows finer tuning than the multi-finger method: http://www.aircooled.net/images/pphotos/tze0021.jpg
I use it on my dual Dellortos.

In a pinch, however, I think the multi-finger method is pretty darn clever!

Air_Cooled_Nut · Jun 12, 2005

Birdman said:

...This is the reason why using the clutch with the car in gear to slow the engine just as you shut the key off works. As soon as the ignition is off, with the load of being in gear with the brakes on, the engine stops immediately and doesn't turn long enough to dump fuel into the exhaust.
Click to expand...

I about fell outta my chair when I read this! Are you nuts?! Talk about a shock to the entire engine and tranny That's like telling a kid that in order to stop himself from running that he should aim for a wall instead of slowing down to a jog and then to a walk before stopping.

Holy cow, swallow the pride and let it backfire. Shocking the engine/tranny like that simply isn't good for it.

No Doubt · Jun 12, 2005

Birdman said:

Lowering the idle speed ALWAYS helps the backfire issue even if it's way too rich, because the engine is turning slower when you shut it off, so less fuel gets dumped into the exhaust. This is the reason why using the clutch with the car in gear to slow the engine just as you shut the key off works. As soon as the ignition is off, with the load of being in gear with the brakes on, the engine stops immediately and doesn't turn long enough to dump fuel into the exhaust.

Hope this helps.

Birdman
Click to expand...

FYI: piston aircraft shut down their engines by leaning their mixture to pure air...whereas automobiles tend to shut down the engine by killing the electric ignition (turning the key to OFF).

And aircraft don't tend to backfire on shutdown.

Just a thought...

Technically, you *could* shut off the engine by shutting off the fuel or leaning out the mixture.

Birdman · Jun 13, 2005

Yeah!! I need to install a mixture control on the console next to the e-brake! If I drive up to high altitudes I can adjust on the fly! To shut it off, I'll just lean it out until it dies! (Sounds complicated, but sure to foil any attempts to steal it, right? Also would keep the wife from being able to drive it....)

Birdman

No Doubt · Jun 13, 2005

Birdman said:

Yeah!! I need to install a mixture control on the console next to the e-brake! If I drive up to high altitudes I can adjust on the fly! To shut it off, I'll just lean it out until it dies! (Sounds complicated, but sure to foil any attempts to steal it, right? Also would keep the wife from being able to drive it....)

Birdman
Click to expand...

"But Honey, why won't your car start for me?!"

Too funny.

Too bad the mixture control is so difficult for the carb versions. Of course, an electric valve *could* be fixed to allow or disallow all fuel flow to the carbs, thereby shutting off the car via fuel-cutoff instead of electric igntition cutoff...and *that* switch you could place on your dash or by your e-brake.

Ken · Jun 13, 2005

No Doubt said:

"But Honey, why won't your car start for me?!"

Too funny.

Too bad the mixture control is so difficult for the carb versions. Of course, an electric valve *could* be fixed to allow or disallow all fuel flow to the carbs, thereby shutting off the car via fuel-cutoff instead of electric igntition cutoff...and *that* switch you could place on your dash or by your e-brake.
Click to expand...

If you have a Lucas ignition you already have a cut off. Of course, it cuts off on the highway in the pouring rain at 2 AM....

Ken

bill308 · Jun 13, 2005

Hi guys. Let me add a little more color to this discussion.

I believe there are 3-causes that contribute to shut down "run-on" on carb 308's. They are, too high an idle, a rich mixture, and too much ignitiion advance.

A couple of years ago I was fortunate to acquire and install a set of new carbs for my 78 GTS. They were an earlier version of carbs, /45/46/47/48, instead of the /72/73/74/75, that were stock. I have an HPV crank triggered ignition fitted with a vacuum advance, to promote fuel economy at cruise.

Even with new carbs, synchronized with an 8-colume manometer system (by far the best method), I almost always had some "run-on" after turning off the ignition, usually 1 or 2 puts. The engine was ideling at about 1200 rpm and would not idle lower with all the carbs backed off as much as possible, but with all the carbs still in synchronization. It was possible to throttle down some of the carbs but at a cost of sacrificing synchronization.

I was not able to throttle down the carbs any more without ruining synchronization and for now, I like the driveablility of the richer idle jets. That left the only possiblility of affecting the system, to retarding the ignition at idle. This I accomplished in a rather unique way. I simply took advantage of a stock R1/R2 points switch to disable vacuum advance at idle.That is, the vacuum advance signal from the vacuum advance module, was interrupted by the R1/R2 points switch. The bottom line is that at idle, there is no vacuum advance. As soon as the throttle leaves the stops, vacuum advance is enabled.

So where does that leave us. To eliminate run-on, I believe you must reduce the idle speed, lean out the mixture, or retard the ignition advance, or a combination of these approaches.

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