1969-383 stroker-10.5-1. It's 93 degrees outside, car runs at 190 degrees. When I stop for say 10 minutes, up to an hour and restart the car, it idles with no problems. When I start pulling out,it will either stumble and stall or run really rough for 10 seconds and clear up. Headers are ceramic coated and wrapped with header tape. Fuel line is clear and not touching anything. Someone suggested that the float level in the bowl may be set too low??? What this got to do with anything?? I could see if it was running hotter, but 190 degrees, come on.
Runs flawlessly any other time. (Cooler out) I thought about installing an electric fuel pump but don't want to if that's not the issue.
HELP!

 

do you have a vented gas cap? I had an issue like that a long time ago.

 

my 69 used to do the same thing in the summer. Stop at the grocery store for 20 min, then when you pull out on the highway, it stalls, out of fuel for 5-10 sec, then runs fine. Are you using the stock fuel filter? I think you are getting vapor lock in the filter, might want to check and make sure the the return line is not kinked. Honestly I think they only used that fuel filter 68-69 because of this problem. My 71 does not have the problem.

 

Yes to the vented cap.

Fuel system has been updated. Large filter unit mounted by the tank with 3/8 braided stainless line running to the carb. No return line. Holley 750 Double pumper.

Still think it's vapor locking. Do you think changing the thermostat to a 160 degree unit would help?

 

If You Find A Solution For Your Problem E Mail Me And Let Me Know What The Fix Is. I Have A Stock 71 Lt1 And It Does The Exact Same Thing As Your Car.

 

I can see two problems here. Using an aftermarket mechanical fuel pump with no provisions for a fuel return line and second...... you shouldn't have a fuel filter before the mechanical pump (suction side). It must be located after the pump (pressure side). Eliminate the fuel filter before the tank and switch to a Corvette style fuel pump with a return line output. There are certain factory GM Corvette pumps that are available that should have your flow requirements. Hopefully other guys can chime in about them.

 

I agree that a pump with a return line may remedy the situation. The filter before the pump is not an issue. My other car has the same setup. The prefilter (Aeromotive 100 micron) just needs enough flow so as not to restrict the pump. A different filter with a tighter screen (Aeromotive 10 micron) is used after the pump. I had a mechanic make 2 other suggestions. He said I could try wrapping the exposed fuel line with a type of reflective heat tape, and also install an aluminum sheet(1/16 thick) between the carb and the intake to reflect radiant heat from the fuel bowls, which may be cooking the fuel on a hot day. It would have to be large enough to stick out past the bowls a bit, maybe 1". I'm not sure I would like the look of it. Maybe if I polished it up a bit...

 

I solved my bad "vapour lock" issue with the installation of a cheap low pressure electric fuel pump at the fuel tank. Left stock mechanical fuel pump on the motor & the car now works perfect
An old street rodder friend told me that was the cheapest most reliable fix for the "vapour lock" problem & he was right IMO.

 

I agree with Ken on this one. The fuel filter location may not be the immediate problem since your problem is not under accleration. The fuel is boiling after letting it set resulting in a problem on start-up. The return line keeps cool fuel in the line and being pumped to the engine. You can buy an insulator sleeve for the fuel line above the manifold that attaches with velcro. You can also buy a carb insulator/spacer if you have the room under hood. A carb insulator can be made from wood if you are handy and so inclined. My 69 has the stock fuel filter set up, but with a Holley mechanical FP and with a fuel return line and no vapor lock issue. GM engineers had it all figured out and working without a problem.

 

There are 2 issues that are both often referred to as "vapor lock." Only one of them is really "vapor lock."

Vapor lock occurs when a mechanical pump on the engine is "sucking" fuel from the tank and the fuel in the suction line starts to vaporize. The fuel in the suction line can vaporize due to low suction pressure (remember your high school chemistry class when you made water "boil" by decreasing pressure in the container..?) or from high ambient temperatures. Usually, a combination of both factors will create vapor in the suction line (aggrevated by any fuel filter installed on the suction line). When a mechanical fuel pump receives a "slug" of vapor, the pump's output drops to 'zero.' Mechanical pumps will not pump vapor. Thus, "vapor lock" is when the suction side fuel line produces enough vapor from heat or low pressure to cause the pump to stop pumping. This will usually result in the car "falling on its face" or complete engine stoppage from lack of fuel. Cure for this problem is to remove any suction side retrictions (such as any filter installed on the suction side) and to eliminate any heat sources applying heat to the suction side fuel line (such as header collectors running close to the steel line). A fuel return line running from the pump to the tank will also cure this problem, since the return line lowers fuel temperatures in the suction line and keeps fuel flow velocity in the suction line high enough to eliminate vapor in the suction line.

The second problem, often referred to as "vapor lock," is fuel boiling in the line between the pump and the carb, or boiling in the carb itself. This condition is not a vapor lock condition - it is a fuel percolation problem induced only by heat. After engine shutdown, engine compartment heat can raise the temperature of the fuel in the fuel line, and in the carb, to a point where the fuel starts to vaporize. If the fuel in the carb starts to boil, the fuel will shoot out the accel pump discharge nozzles and will flood the engine upon re-start. If the fuel in the line is boiling, the engine will run erratically upon re-start until the vaporized fuel in the line has "cleared out:" The alternating liquid and vaporized fuel going through the carb's needle and seat causes erratic float level control in the carb until the fuel flow has stabilized. Solutions to the fuel percolation problem are a little more difficult, but include running a carb heat shield, a correct carb base gasket, and limiting heat input into the fuel line (by not using black rubber hose...). A custom fuel system can effectively utilize a return fuel system that runs the return fuel from the carb inlet to the tank rather than running the return fuel from the pump to the tank - this works very well. The key is in keeping the fuel as cool as possible and allowing vaporized fuel to be removed from the system through a return line system.

Modern fuels with high ethanol contents have made the problem worse for our carbureted cars - modern injected cars do not have this problem since they run high pressure fuel from the tank to the injectors, and no fuel is stored in a "float bowl." Eliminating the problem on an old carbureted car requires some mods to the system to compensate for the high vapor pressure of modern fuels. Often, restoring the operation of the stock fuel return line will solve the problem - it is very popular to eliminate the return systems when installing "high performance" parts, and this will make "vapor lock" issues worse.
Lars

 

I stumbled on this from a few years ago and thought I would update it. I had been running dual electric fans on the car (radiator pointing down at 45 degrees) and had the vapor lock issue. I installed the mechanical fan back on the car and the problem went away. I have to assume pushing cooler air across the fuel pump and motor cured my issue. Maybe this will help someone down the road.