Car is a '67 Dart, 440, eddy heads, Accel ignition, mighty demon carb, mechanical fuel pump.

The car runs great most of the time. Cruises at about 190 degrees.

But every time I leave it sitting for maybe 20 or more minutes, the engine is not running right when I start it. The temp isn't excessive when this is happening.

When it's running rough after starting, it sounds weaker, and when I put it in gear or reverse, the RPMs drop way down. When I step on the gas it takes an extra push to get it to go, then as it decels again the RPMS drop and sometimes it just poops out. I restart it, rev it up, it sounds like it's running weak etc.

Once I get out on the road (out of the parking lot) and I've gone a few hundred feet, it runs fine again.

I think it's a carb issue, but can't figure out what would be going wrong with it.

Any ideas? I've considered vapor lock, would that give these symptoms that resolve after driving a few hundred feet?

Gasoline isn't formulated for carburators any more... The situation your describing is called heat soak... The engine is up to temp, you shut the engine down & heat rises & everything cooks (soaks up heat) the fuel isn't designed for sitting unpressurized, it doesn't have stabilizers to resist boiling... On a fuel injected vehicle all the fuel remains under pressure so it can't boil... After you car starts the fuel pump will begin to refill the fuel bowl(s) & the new fuel will be cooler helping mask the effects of heat soak....

Install an electric fuel pump back by the gas tank and bypass the mechanical fuel pump. I guarantee you will never have this problem again. Been there, done that, got the t-shirt.

Wow, unanimous answer, thanks guys!
I'll be switching to an electric pump.

I run an A/F sensor on my car and what is happening during the heat soak period is that the A/F guage shows super lean (16-17) during this time. When I startup after it has been setting, it will show 16-17 and I just keep feathering the carb until the A/F starts to come back down.

thanks

I have the same problem,I just upgraded to an aluminum duel plane intake manifold and used the race gasket set that blanks off the heat cross over and am putting a cool carb spacer setup under the carb,need to replace the gas tank before I can test the new setup.hope to have it done this week,I will let you know the results.


http://www.coolcarb.com/

1/2 inch phenolic spacer between the carb and manifold worked for me. Still using only the mechanical fuel pump.

More good stuff, thanks. After I wrote my last message I started reading about heat soak here and people seem to have luck with either heat dissipating spacers, or carb heat shields like made by Holley, Mr. Gasket and Cool Carb.

So before I try the electric pump I'm going to try something thin to block or isolate the heat.

I also read that another symptom of this is the smell of fuel when the car is sitting in the garage, without having any noticeable leak. I have definitely noticed this too so it's nice to confirm it's heat soak causing these problems.

I have a DP4B and an Edlebrock carb that had the same problem. I picked up a plastic 1" spacer from Advance auto and at 1" it made my stuff a little too tall. I cut it down on my table saw to 5/8"... be prepared to fiddle with the throttle cable, brackets, and maybe a new stronger return spring. The whole project including re-adjusting the carb took me about an hour and a half. The results where like night and day, the other day it was about 87, I went for a ride and stopped for lunch when I came out it started right up and runs like a clock. I think I can even feel a little extra torque too.

I think I have at most 1/2" clearance now, but I think I can go to a thinner air filter element and gain enough to put one a bit taller or just have more wiggle room with the 1/2" one.

'1/2 inch phenolic spacer between the carb and manifold '

Bullseye...

I run a fairly common induction system... Performer RPM (Eddy heads, so no crossover heat) with a Holley.

I'd been running an electric pump for years, with the fuel line routed away from the exhaust... up the firewall and approacing the carb from the rear of the engine. When E10 became the only fuel we could get in this part of the country, I began getting the same symptoms. I added a Moroso wooden carb spacer and a sheetmetal heat shield. That helped, but it still wasn't as good as it was with non-ethanol gasoline.

The thing that fixed it for me was a vapor return line. Yes, it's a PITA because (in my case) there is not a fuel sending unit available with the return fitting already in it, so I had to fabricate one. I bought a 5/16" fuel line kit from Fine Lines that fit my car, and made slight modifications to it and used it for the return line to the tank. 1/4" is the "normal" size, but I wanted a pre-bent set that fit nicely, being too large won't hurt anything, and 5/16 is the smallest size they sell for my car. Of course, you could bend your own out of a roll of 1/4" tubing. If you go this route, be sure to add a 1/16" (approximately) restrictor in the return line.

My advice, for what it's worth ( ), is to try all the stuff that's already been suggested, but be prepared add a vapor return line if the problem persists.

J

Thanks Jim, that will definitely be helpful if the spacer doesn't work.

I have a 1/2" phenolic spacer on the way so next weekend I'll be able to test it out.

Got my 4 hole spacer today, 13.99 off ebay, and find out that the butterflies on the Mighty Demon do not clear it. I had a feeling it might be an issue.

So I'm going with an open spacer rather than fiddle with this thing. Will give the 4 hole to my dad.

It's a Performer RPM dual plane intake.

I thought the smaller the spacer height, the less the overall difference (in terms of performance) between 4 hole and open might be which is why I thought to just get the open spacer.

Right now it's hanging up on the secondary throttle blades when they start to open, once it gets past a certain sticky point it seems to be clear. It wouldn't seem to require much filing/sanding to clear.

So with a performer RPM, should I just file/sand this one to clear?

You might be fine with a 4 hole open spacer , carters like the 4 hole spacer, I don't know how a demon/holley reacts to the difference.

It's either buy one or put a little time and effort into making what you have work?

I'd be inclined to use the 4 hole or make it a 2 hole and mimic the opening of the intake.

Yeah it couldn't hurt to tinker. These are cheap enough that if I mess one up too bad it wont kill me.

I'll start by clearancing the 4 hole spacer, maybe adding a little taper to all the bore tops, then I'll run it. If it doesn't run right I can try the open spacer that's on its way. I do like the thought of increased low end and carb signal with the 4 hole spacer.