'84 Carrera running Webers - Strange dieing issue?

[aircld]

I have an '84 Carrera 3.2 running Webers and have been experiencing a strange dieing issue that has me stumped. The car generally runs pretty well, but when warmed up to operating temp and driving nicely every once in a while it will die immediately following a throttle blip.

For example, when blipping the throttle for a downshift it will halfheartedly rev (almost like a cough) and then the tach will peg at 0 and the car will die. Often times when this happens it will almost bump start itself right back up, but a small handful of times I've noticed it happening and just pulled over and restarted the car normally.

The car is running Italian 40 IDA 3C Webers with a 36mm venturi, which are on the small side for a 3.2 but seem to be fine outside of this strange issue. It is running an Electromotive Xdi with crankfire and electromotive coils for ignition.

It has only done this a handful of times (<10 times in roughly 400 miles) and seems to only do it on a downshift throttle blip - if I rev it lightly when stopped it behaves beautifully.

What could be causing this?

[Chuck.H]

What kind of tach are you running? If you notice a tach 'peg at zero' when you think the engine is still spinning (even a little), then you can suspect a crank trigger has stopped providing a signal.

Just a thought,
Chuck.H
'89 TurboLookTarga, 384k miles

[scarceller]

May have a faulty ignition switch? With car at idle tap on the ignition switch with the back of a screwdriver from under the dash. I had faulty switch and cause engine stall and tach drop like a rock to 0RPMs

[CCM911]

Don't bad coils cut out when hot?

[G50911]

i'd say no gas. i had the same behavior and i solved it by changing my fuel pump.

[scarceller]

Not sure the tach would go to 0RPM if no gas?

Originally Posted by G50911

i'd say no gas. i had the same behavior and i solved it by changing my fuel pump.

[LJ851]

Are you monitoring AFRs?

If the AFR is very rich on decel, a little accelerator pump hit from the throttle blip + no load on the engine could be just enough to stall it.

[aircld]

Originally Posted by Chuck.H

What kind of tach are you running? If you notice a tach 'peg at zero' when you think the engine is still spinning (even a little), then you can suspect a crank trigger has stopped providing a signal.

Just a thought,
Chuck.H
'89 TurboLookTarga, 384k miles

Factory tach, never modifed/opened and seems to work perfectly. When it pegs at 0 the engine is dead, however due to how fast it happens it has restarted as I let out the clutch to downshift if I didn't catch it in time. When it dies it gives off all the signs that it would if it was stalled normally (like by an inexperienced driver). The oil pressure light comes on, can hear fuel pump whirring but engine isn't running.

Not really what you were asking, but at the moment I have no reason to suspect a crank trigger failure - tach seems to work fine.

Originally Posted by scarceller

May have a faulty ignition switch? With car at idle tap on the ignition switch with the back of a screwdriver from under the dash. I had faulty switch and cause engine stall and tach drop like a rock to 0RPMs

This is possible, but it only ever does this on a quick throttle blip. And when its dead the ignition is still on.

Originally Posted by G50911

i'd say no gas. i had the same behavior and i solved it by changing my fuel pump.

Possible, however I believe the PO went through the fuel system when he converted to
carbs, so it should be ok. The pressure gauge on my PMO fuel pressure regulator never budges, seems to have a steady supply of fuel.

Originally Posted by LJ851

Are you monitoring AFRs?

If the AFR is very rich on decel, a little accelerator pump hit from the throttle blip + no load on the engine could be just enough to stall it.

This seems like the most plausible reason... I have not been monitoring AFR, need to find a wideband to do so. The carbs aren't dialed in 100%, but are probably 90% there.

The first time this happened I actually had a friend driving behind me and he reported a "cough" of dark smoke out of the tail pipe as it restarted. I pulled over immediately and shut it off, inspected everything as well as I could and drove away fine afterward.

Based on a conversation with a local Porsche shop I have been worried that the carbs might be too small for the 3.2. Apparently the person I spoke to has blown up a 3.2 running 40mm carbs due to it running lean at the top end. However, from my own research it seems like I should be fine for street use and despite leaving some power on the table, I am safe. From my (limited) understanding of carbs, having too small venturis would cause it to be rich on deceleration, no?

[wayner]

What about stuck or clogged components indie the carbs?

I've been chasing jetting issues for quite some time but they are absolved now after a rebuild and cleaning.

Could this be the case with yours?
My were gummed up from the pref owner putting them away wet.

[aircld]

Originally Posted by wayner

What about stuck or clogged components indie the carbs?

I've been chasing jetting issues for quite some time but they are absolved now after a rebuild and cleaning.

Could this be the case with yours?
My were gummed up from the pref owner putting them away wet.

It's definitely a possibility, but the tank/fuel lines were cleaned just prior to my purchase in March. Since then I've put around 1800 miles on it without issue except for this which is a more recent problem.

I do know that the jets on these carbs are picky when it comes to gas and debris, but the car seems to run pretty good except for this one problem. Wouldn't clogged jets manifest themselves in several ways?

[LJ851]

Originally Posted by aircld

Based on a conversation with a local Porsche shop I have been worried that the carbs might be too small for the 3.2. Apparently the person I spoke to has blown up a 3.2 running 40mm carbs due to it running lean at the top end. However, from my own research it seems like I should be fine for street use and despite leaving some power on the table, I am safe. From my (limited) understanding of carbs, having too small venturis would cause it to be rich on deceleration, no?

Carb size has very little to do with leanness, that is all about jetting. Sounds like the shop is not familiar with carburetors. Too small of a carb is generally easier to tune than too large since the vacuum signal is stronger.

[aircld]

Originally Posted by LJ851

Carb size has very little to do with leanness, that is all about jetting. Sounds like the shop is not familiar with carburetors. Too small of a carb is generally easier to tune than too large since the vacuum signal is stronger.

That makes sense now that you mention it, as the Venturi size would control airflow and the jetting controls fuel... following that through it seems as though a too-small carb would run rich when taxed to maximum, which somewhat supports what LJ851 said above...

I'm definitely a little out of my comfort zone with all of this, but not so much so that I'm completely lost. Thanks to everyone for the advice.

[LJ851]

Originally Posted by aircld

That makes sense now that you mention it, as the Venturi size would control airflow and the jetting controls fuel... following that through it seems as though a too-small carb would run rich when taxed to maximum, which somewhat supports what LJ851 said above...

I'm definitely a little out of my comfort zone with all of this, but not so much so that I'm completely lost. Thanks to everyone for the advice.

A wideband 02 setup will GREATLY improve your comfort level in regards to carb tuning or ruling out the carbs for another tuning issue.

[aircld]

Is there a recommended setup that feeds live data? What is the preferred route?

[Chuck.H]

I ask about the tach to learn about your system - is the tach connected to the electromotive ignition? I wasn't aware this was possible, so cool if it is. I thought the only way to connect the tach was to the DME output, but for all I know this is a standard square wave and any ignitions 'tach output' will work?

If you suspect the carbs aren't supplying enough fuel (or enough air), perhaps you can try raising the idle as a test?

Chuck.H
'89 TurboLookTarga, 384k miles

[aircld]

Originally Posted by Chuck.H

I ask about the tach to learn about your system - is the tach connected to the electromotive ignition? I wasn't aware this was possible, so cool if it is. I thought the only way to connect the tach was to the DME output, but for all I know this is a standard square wave and any ignitions 'tach output' will work?

If you suspect the carbs aren't supplying enough fuel (or enough air), perhaps you can try raising the idle as a test?

Chuck.H
'89 TurboLookTarga, 384k miles

I'm actually not sure what the tach is connected to, I'll have to figure that out and get back to you. I've had the car since March, and frankly have just been driving it and trying to get to know it better. Still haven't extensively gone through everything on it.

[jpnovak]

A few key questions.

1. What ignition system? Is it still Motronic ignition control or is there a converted distributor and some additional ignition box?

2. What is your idle speed? If the idle speed is too low this may happen. You might have to idle higher than a stock Motronic since the air-bypass valve and idle control are no longer there.

3. If the air plates (throttle butterfly) are set wrong they will cover the lower fuel delivery hole. This can happen if the idle screws are too far out and the butterfly valve drops low enough to cover the hole. There is an excellent post by Paul Abbott (1QuickS)of Performance Oriented about this problem.

4. Where are the position of the air correction screws? Are they all the way in or several turns out? If they are out far, this is a symptom of #3 above. Meaning, the air bypass is making up for the throttle plates being too far closed.

Do you have a shop that is familiar with multi-throat carbs like webers? There is a systematic approach to balancing and setup.

[aircld]

Originally Posted by jpnovak

A few key questions.

1. What ignition system? Is it still Motronic ignition control or is there a converted distributor and some additional ignition box?

2. What is your idle speed? If the idle speed is too low this may happen. You might have to idle higher than a stock Motronic since the air-bypass valve and idle control are no longer there.

3. If the air plates (throttle butterfly) are set wrong they will cover the lower fuel delivery hole. This can happen if the idle screws are too far out and the butterfly valve drops low enough to cover the hole. There is an excellent post by Paul Abbott (1QuickS)of Performance Oriented about this problem.

4. Where are the position of the air correction screws? Are they all the way in or several turns out? If they are out far, this is a symptom of #3 above. Meaning, the air bypass is making up for the throttle plates being too far closed.

Do you have a shop that is familiar with multi-throat carbs like webers? There is a systematic approach to balancing and setup.

It's running Electromotive ignition with Crankfire and Electromotive coils.

Idle speed is just about 1k when warm. Cold start idle is around 800 rpm, rising slightly as it warms. It never has a problem settling into idle, even if I rev it when stopped. Only in this weird transition from under load to throttle blip.

As for questions 3 and 4, I'll have to double check and report back.

I'm located in the Boston area, I know of a handful of shops that may be familiar with multi-throat carbs, but I'm not positive. Rensport has lots of experience with Porsches, specifically the air-cooled ones, and I have seen several carb'd older 911's at the shop there before. I would assume Rene is familiar with these carbs. The other shop that comes to mind is Boston Sports Car Co, which specializes in older Ferraris but also seems to have experience with Porsches. Iirc many of the older Ferraris use similar downdraft Webers so I'd imagine the process is similar.