3.6 Engine Shuttering/Jerking On Track (50 PMO's)

[daltvater]

Help me this riddle....

1979 with a 3.6 and 50mm PMO's. On the track this weekend. Hot day, 80F.

On full throttle the car started to cut out. Engine stayed on but acceleration started to stop and then go again. (jerky)

I was running low on fuel and it was only happening on the crest of a hill so I thought maybe fuel level??? Filled it up, back to the track and same thing started happening again only worse.

It feels like the fuel is cutting out on WOT. Under normal driving (1 hour home) there was no issue.

I don't know jack about Carburetors, but I'm guessing they could be the issue?

Thanks for the help, Dan

[Hi_Fi_Guy]

Was it only cutting out at a specific RPM range? Or at any RPM under throttle?

[stownsen914]

Unfortunately what you describe could have a number of different causes, including ignition. Have you experienced this before with the car in this configuration?

[john walker's workshop]

Fuel pressure trying to keep the bowls full @ WOT?

[dannobee]

Full throttle is usually a fuel delivery problem. Check fuel pressure and volume, (and fuel filter) and tell us what you find. Fuel system should be able to deliver over a quart/liter of fuel in 15 seconds.

[juanbenae]

Originally Posted by dannobee

Full throttle is usually a fuel delivery problem.

that was my 1st though. especially if you ran it lower on fuel than normal and the sloshing of the track duty you may have picked up some junk from the bottom of the tank that clogged filters.

are any of you filters clear so you can see if there is crud in em?

[daltvater]

Originally Posted by Hi_Fi_Guy

Was it only cutting out at a specific RPM range? Or at any RPM under throttle?

Good question, not sure. I want to say it was at the same RPM each time which would have been 6,000 or so??

[daltvater]

Originally Posted by stownsen914

Unfortunately what you describe could have a number of different causes, including ignition. Have you experienced this before with the car in this configuration?

Nah first time. Last carb adjustment/tune was done in 2014. Car has 6 years and probably 10k miles on it since then.

[daltvater]

Thanks everyone, I think you've given me enough to start working on a diagnosis. I'll let you know what the heck the issue is.

Thanks, Dan

[macssc911]

Are you running MSD with rev limiter

[daltvater]

Originally Posted by macssc911

Are you running MSD with rev limiter

The car has a Atom ECU. If it has a rev limiter in it I haven't hit it. I've taken the revs to 6,800-7,000 but typically shift around 6.

Do you think a Rev Limiter would have something to do with this?

[Hi_Fi_Guy]

Originally Posted by daltvater

Good question, not sure. I want to say it was at the same RPM each time which would have been 6,000 or so??

Without knowing more it is difficult to guess but in my experience in a variety of horizontally opposed engines from Porsche, BMW Twins, Corvair, etc. if the stutter/bucking is always at the same RPM range it can often be the carbs falling out of sync side to side at that point in the RPM range. Typically linkage related.

That said, the rule of thumb is to check compression including valve adjustment first, then check ignition timing second, before adjusting carbs. Lots of problems are blamed on carbs that are not carb related so it is always best to check & verify the other two areas first before messing with the carbs and possibly introducing a second problem.

[1979-930]

Every fuel starvation issue I have had was accompanied by a lean backfire through the carburetor. Was the cutting out accompanied by pop sound in the engine bay and not out the exhaust?

I've never had a flat six with individual carbs, so the lean pop may not be a standard result from fuel pressure running low.

[stownsen914]

I'd agree that checking fuel delivery volume is a good idea. I had a problem similar to yours on my 911 racecar that has PMOs. A higher output fuel pump cured my problem. A good and easy way to check fuel volume is to disconnect the fuel return line. This way your fuel volume check accounts for any reduction in delivery volume caused by the fuel pressure regulator.