Rough Idle when Hot- any theories?

[MercuryMarc]

I have tried 2 different carbs (Holley 1940's) on my car and experience the same problem- the car runs great cold (with manual choke) and great when warmed up, but when hot I nearly stall pulling into my driveway. Runs fine at higher RPMs when hot. The glass fuel filer is full of fuel so I think the fuel delivery is fine. I wonder if it could be a timing issue or vacuum problem? I am running 12 degrees iniital advance with a 1982 D2 dizzy/module.

Thanks!

 

[wallaka]

The gas could be boiling off when the fuel pump isn't supplying the volume that high RPM's give it. Other than that, I have no idea.

 

[Anonymous]

Could possibly be a rich condition. My Vette is equipped with dual fours which runs better in the cold and at high rpm's. When it is hot or when I take it up into high elevations, it wants to die due to the lighter air / rich mixture. In the winter, I don't have a problem with it wanting to stall. Someday I'll get one of those proverbial Round-to-its and get the parts to tune those obsolete carbs.
Doug

 

[ludwig]

Needle valve worn, pitted, weak spring? Maybe the hot gas is pushing past the needle valve ahead of the need.

 

[65coupei6]

I would try the simplest thing first. Wrap the fuel line from the pump to the carb with some insulation and see if it still happens or change to a rubber line if it is metal.

 

[Redfalken]

Hi Marc,

I'm assuming since it doesn't drop below freezing in SF too often, if ever, that you're not running water through the carb spacer? That would really heat things up.

I have a DSII and run it about 14 degrees initial so you might be able to bump that up a bit.

 

[MercuryMarc]

Thanks for the suggestions Keny, 65C, Doug, Ludwig, and Wallaka- I changed the metal fuel line to rubber and still have the problem. I have not been using the heated carb spacer. I have had the same problem with 2 different carbs so I d not think it is a carb problem, but I'll try leaning it out a bit and see. Once I had a piece of foam debris in the fuel line between the fuel filter and the carb- but this lead to problems at high RPMs. I think I'll change the fuel filter - what the heck. Any other suggestions appreciated!

Thanks

 

[65coupei6]

Mark, did you figure out the problem?

 

[MercuryMarc]

Hi Marco-
Still working on it...
This AM I reset the initial timing (to 14 degrees)
Changed the fuel line to rubber, routed very far from the block.
Moved the fuel filer to just off the fuel pump (farther from the block).
Replaced the fuel filter
Reset the idle mixture screw to max RPM
Reset the idle speed to 1000 (yes - 1000) to prevent from stalling.

I wonder if for some reason I am runnning way to rich- that would explain the worsening when really hot? So If the above tricks do not work then my next plan is to gradually lean out the idle mixure screw 1/4 turn at a time and see if that helps. The ony problem with that theory is that I had the same issue with two different carbs.

Comments welcome!!!!

PS- Hope to make it to Fun Ford Sunday this year! Sept 10th, Vallejo.

Thanks-

 

[jahearne]

Marc,

I'll try to make out to Ford Sunday with you in Vallejo. I can't think of anything else that the other fellows hadn't already mentioned. All I can think of is vapor lock.

Good luck,