This topic comes up from time to time. I have been told by many people WAY smarter than me the culprit is today's gasoline which lacks the 'boil-off' additives of yesteryear but I was wondering if anybody has had any success with additives or other things. I have had many people find success with carb spacers/insulators but non-stock parts are not an option for me. The situation ONLY occurs when I try to restart my car after it sits hot for 20 minutes or so. If I start it before then, it fires right up. If I wait an hour/day/week or more it fires right up. Even after a 20-minute hot soak it starts but I have to hold the pedal to the floor and it fires up but sputters for a second until it 'cleans' out (Black smoke from the exhaust for a second) then it's fine again. It's definately something I can 'live' with because it always starts and runs perfectly otherwise. I was just curious if anybody had any ideas. Also I have found a Gas station that sells Ethanol-free gasoline and filled the tank yesterday. Maybe that will help.

 

Do you pump the gas pedal prior to starting the warm engine? It could also be timing, carb issue, vacuum leak, out of tune, old plugs, fuel filter, etc...

I used to press my gas pedal just enough to open the throttle plates on my old 330.

 

I had a similar problem with my original numbers 68 400 with A/C. I installed the stacked laminate style spacer under the 251 carb. That helped but didnt cure it all. You have to really look to see it. I'd drive to the local burger joint. Park it. Then try to restart after sitting 20-30 min. So the carbs good and heat soaked. It would actually fart so hard back out of the carb sometimes that it appeared as I had an engine fire! Quite embarrasing when you got a good looking car and it spits sputters to a start.
I finally broke down and installed cross-over block-off plugs into the intake verses the heads. I welded them in. I removed the theromstatic flap and spring set-up out of the drivers side exhaust manifold and welded the hole (not original to car, WZ 455 manifolds).
I had to tweak the divorced choke. It takes a while to warm up. I dont drive in the colder months. I do start it in the winter. Yes its now a cold blooded car but the heat soak problem is gone!

 

You have issues way beyond the sorry Ethanol laced fuel. My first inclinations are you have a failing fuel pump, possibly a weak spark coil, and a carburetor that probably needs a rebuild. The problem is not fuel percolation or evaporation related, though with Ethanol the latter can sometimes be a problem when a carburetor needs rebuilding.

 

I've actually got the same problem. It starts right up cold. When hot I have to floor it. I have an HEI with the hotter coil, new plugs, wires, cap, fuel filter. Set the timing, air/fuel mix. The carb probably does need rebuilt and I suspected a vacuum leak. I'll fix the vacuum leak and see how long I can keep using the old carb. My old car had the heat x over welded shut and never had the problem. Of course it was a new carb too.

Texas...How would a failing fuel pump cause this problem?

 

You mentioned not going with non stock parts, but may be your best option, if you want to drive the car on hot summer days. They're only bolt on's anyway, and can be quickly removed. As mentioned, could be to much initial timing. Happened to a car of mine, with to much cam, and timing. Ended up frying a starter, so careful. Believe Mr Gasket did or does make a carb heat deflection plate kit. I always run fresh intake air hoses before the radiator on my cars. My Olds has a rear mounted electric fuel pump, I shut off, to run the carb bowls dry when parking.

 

A failing fuel pump may be still pumping fuel to the carburetor but not enough to completely fill the bowl and preventing the float from doing its job. Fuel pumps in a failing mode will also often be pumping fuel into the crankcase. Check the dipstick for a gasoline smell or what has become an over fill indication on the dipstick, oil will also likely look black as pitch on the dipstick from the gasoline in the oil breaking deposits loose from everything in the block. If either condition is present the pump is basically shot and should be immediately replaced before dilution of the oil ruins main and rod bearings, or worse ignites and blows your oil pan into the next century.

Just went through this type of fuel pump performance problem with no outward signs of the failure going on, but with similar restart issues, with my '78 Lincoln and a fresh Holley fuel pump solved the problem. Just sayin'! Your issue might actually be different.

Much of what you describe also sounds like there could be an issue with the choke not releasing when the engine has reached normal operating temperature. As for a vacuum leak idea, vacuum leaks will typically provide a noticeable increase idle speed and can often be found around carburetor mounting gaskets by simply squirting 30W oil where the throttle plate mates with the intake manifold in small quantity, but sufficient to momentarily plug the leak. Of course you might also be able to hear the "hiss" of vacuum leak using a piece of flexible fuel line or plastic tubing as a stethoscope while moving the hose around the carburetor base.

 

I always used the 1/2'' base gasket under a Q-Jet, just to avoid the situation.
Seems that and a 160 or 180 thermostat solved it, as I never had a problem.
I like that gasket as it has plastic sleeves around the bolt holes to prevent over tightening.
One of the GM divisions had a bulletin about using that to solve your exact problem, back in the day.

 

Just to clarify the fuel percolation after hot soak causes pressure in the bowl (and sometimes the pump as Texascarnut suggested) allowing fuel to 'flood' onto and by the throttle plates and into the intake manifold resulting in the flooding condition not an 'out of fuel' or vapor lock condition. This is why it is remedied by holding the throttle wide open and it will start (hence the Black smoke). Carb well plugs are not leaking, tune-up, ignition, coil, vacuum, etc. are fine, choke is fine. This has been a very common (and well documented) problem since the mid-60s. I just wanted to see if anybody had anything that worked for them. I think I'll try the thicker base gasket like Rickman suggested - worth a shot I guess. Also I need to check my vent return line on my fuel pump (which is not that old either). Supposedly (according to some early GM literature) this was supposed to prevent this kind of problem.

 

i had the exact same problem. i used the insulating gasket and a fuel pressure regulator. major difference! i use an electric choke now so i can block the crossovers too.

 

BTW, ethanol will SIGNIFICANTLY lower the boiling temperature of the fuel. around here, winter gas has around 15% ethanol. summer gas has about 10%. we have rec.gas or marine gas with no ethanol. the non-ethanol does not do what you described.

 

It may not just be Ethanol content in gasoline that is lowering its boiling point. Injector cleaners may also be adding to that issue. No one aside from refiners exactly the nature of injector cleaners being placed in gasoline nor is the formulation of injector cleaners uniform from refiner to refiner.

Shell just put up a bunch of banners around some of their stations proudly stating their gasoline now has 20% more cleaners. Twenty percent more what and in what actual volume? One thing is just about for certain; anything good for cleaning stainless steel injectors is likely incompatible with most components in a fuel system for normally aspirated engines, and most particularly carburetors typically cast of Zinc alloys and/or aluminum, and their various gaskets and diaphragms.

 

Update: There's a place here that sells 93 octane Ethanol-free gas and I've filled up twice and have not had the aforementioned hot soak/hard start problem since so draw your own conclusions from that.

 

I put a 1/2 wood spacer under my carb and could tell the difference in the carb and fuel line temps. Also it starts right up now when it's hot.

 

This is the main reason I blocked the heat to the crossover on mine when I rebuilt the engine. Ethanol free gas does help this problem.

 

Are you guys using the 4 hole or open spacers?

 

I was thinking of getting this one sometime soon... I have similar problems as the OP.

http://www.amazon.com/Mr-Gasket-3406...hu-rd_add_1_dp

I hear the 4 hole spacers can give you a slight boost in torque.

Is 1 inch too much though?

 

Depends on your set up. If you need more top end then use the open. I'm using the open. I don't think 1 inch is too much. I wouldn't go over 2inch though unless you have a high revving motor.