69 400 FB - After 15 to 20 minutes the idle drops, engine surges, then stalls
 

This is a new problem and one I can't figure out. I can run the car for 15 to 20 minutes and then if I stop and idle for more than five seconds or so the idle drops, then the engine surges, and if I don't give it some gas it will stall. I can keep it running if I keep the idle up with the pedal but if I don't it will stall. Besides that the car runs great.

FYI, I have swapped carbs and I just swapped out my coil with a new pertronics flamethrower. I have a pertronics ignitor e-ignition. Any ideas?

Do you have a voltmeter?

If so when the engine starts to act up check what you have for voltage at the coil.

You may also have a fuel system issue such as the pump sucking air and not fuel due to a rubber part of the system not sealing good enough, and yes the system can pass in air yet not leak fuel really.
Air is 400 times lighter then fuel for a pump to suck.

Just as a quick test, back out each of the 2 idle mixture screws 1 full turn to see if that improves the situation. If no real change, right when it's acting up, watch the timing marks with a timing light. Petronix modules are famous for cold solder joints among other weird glitches with their Chinese electronics. Finally, check for strong/clean, white/blue spark with a spark checker at the time it's behaving badly. Those are all quick and easy checks to eliminate some common issues. I am assuming you have a Q-jet for a carburetor. If running a Holley, just 1/2 turn should make a big difference. Full turn on an AFB or Edelbrock AFB clone.

Thanks, Steve. Yes I do have a voltmeter and I will check that. I'm also going to check the fuel pressure after i get my new gauge tomorrow.

Interesting. Thanks for that info. FYI, Right now I have a 750 Eddy performer on it but I've also had 2 different Q-jets.

OK, you have had 3 different carburetors on this engine with the exact same running results and symptoms? If that's correct, the carburetor is unlikely the problem. I would then concentrate on an ignition issue.

My Pertronix, that was installed by the previous owner of my LeMans, gave me pulling my hair out type problems similar to yours. Hard to say if that’s the problem in your case, but just don’t overlook it. Some people have run them for years with no issues, but I went back to points. Now I have no problems. :)

Gas tank vent plugged?

yeah, put some points back on your engine.

I had problems with msd- it was old- and my points. Removed msd and ok. SD revs 5400 easily. Msd seems to be a choice for many. I try to keep things simple

When it goes to drop off on you also try closing off the PCV hose with pliers to see if that helps.

Also where is your vacuum advance hooked up to?

If your getting vacuum advance at idle then that’s no good.

Also intakes manifolds that can take spread bores or square bores are prone to vacuum leaks in your not very careful.
As stated in post 6 your issue most likely at this point due to the things you have tried with 3 carbs, fuel supply related or ignition.

That's my initial thought. Takes awhile to develop a slight vacuum in the tank that fights your pump. Easy enough to check by loosening your gas cap.

Eric

Curious how manifold vacuum at idle is “no good”?

It's not that it's No Good, Just not needed and can really change idle.

Case in point, my 455HO. While it fires, runs great with manifold vacuum. I had much lower throttle setting for idle. The transfer slots were partially uncovered with ported advance. But, when hot, at idle, 660RPM in gear at a stop, if my electric cooling fan would come on, the sudden load on alternator change, would drop RPM slightly, this RPM drop also drops the vac signal slightly, leading to MORE RPM drop, more vac drop....... See the pattern?
This would be noticable even with turn signals, esp if headlights were on. RPM would fluctuate with each flash. Any different electrical load on or off, would change my RPM and timing.
I increased the Idle Air bleed sizes, Now I'm ported advance, Engine has the same solid choppy idle rpm (660 in gear 780 in park) hot, cold, no matter what electrical load comes on. And doesnt die at intersections.


But I would check the tank vent. Leave the cap loose and try

My 350 was the same as your description when it had the choppy cam. I’ve since swapped it for a smooth 066 cam. Now this engine idles better and cooler with live vacuum all the time.

Check for about 14 volts at coil + terminal engine running.

Points will run at about 11 volts.
That´s too weak for the "pertronics ignitor e-ignition" to function properly.

As alluded to in post 14 a motor that’s on the edge with too big of a Cam will have vacuum fluctuations such that if vacuum advance is being used at idle it will only serve to make idle speed even more unstable.

What you really need with your 350 HO is to rework your mechanic advance such that your initial timing is such that at cranking speeds the advance is low enough to not fight the starter and then once the motor fires you get a big kick in of mechanical advance.

Then with using ported vacuum advance you can add in 8 to 12 more degrees of timing.

This allows fine tuning of the idle and off idle circuits in any carb that’s not on a flat out race only motor.

Yes, that's an excellent thing to check. GM cars, since they don't use a physical ballast resistor, people forget to bypass the resistance wiring when converting away from a points system. All those electronic conversions need 12+ volts to run properly.

Yeah, after reading some of these suggestions I will be checking out fuel and ignition. Fuel will be easy because I now have a fuel pressure gauge that I wiil use tomorrow to see if my pressure drops and I will use my volt meter to check voltage at the coil. Is there any other way to check for ignition issues? Thanks.

Doc, how can I check if that's a problem?