76 Chev truck stumbles when hot outside

Have a 76 Chev 3/4 ton, 350, Q-Jet -- new fuel pump, fuel filter.
Runs great all winter and when its cool out--when it gets in the 80's it will stumble when pulling away from lights as well as after going down the interstate for about 15 miles.

Assume its some form of fuel issue as it will catch itself if you get off the gas or will idle fine if you shift to neutral at the light.

Have rebuilt the carb, bought new rebuilt and have had the same issue for years. I know I have a small intake gasket leak as it will vary the idle when shooting some carb cleaner over that area. Have also heard about fuel percolating in this year range as well as the carb secondaries bogging down and causing this. If it dies it will restart with a quick shot of starter fluid but without will just crank. Too much fuel, not enough? Truck isn't worth spending a fortune on tracking down tons of theory but if anybody has ideas or have had this happen I'd appreciate any of them to narrow it down. Don't mind spending hundreds but can see this escalating out of hand in a hurry with somebody not familiar with this problem--especially since it will be very hard to duplicate the problem.

Any thoughts toward coil getting hot? Have you looked into the possibility that you may be getting a fuel vapor lock??

Ever try putting some insulation on the lines? You should repair the vacuum leak first. A vacuum leak at the manifold will make the engine run lean and cause the symptoms you are stating. Did you change the fuel filter in the carb? discretesignals wrote:You should repair the vacuum leak first. A vacuum leak at the manifold will make the engine run lean and cause the symptoms you are stating. Did you change the fuel filter in the carb?


Filter was changed

Had thought about that -- why would it have a problem only when hot outside though? One reason is that when the engine is at operating temp cold dense air has more oxygen per CFM(cubic feet per minute) and requires less fuel for a correct fuel to air charge ratio for proper combustion. This enables you to back off the throttle cause the engine is making more power and you save gas. When the air is hotter the air charge is less dense, so the oxygen amount is less per CFM. With the vacuum leak the ratio will be leaner cause the amount of fuel to be added isn't coming out of the carb cause some of the intake air is being bypassed by the leak after the carb depending on how big the vacuum leak is.
Drag racers are constantly tuning their cars according to the temp and the barometric pressure of the area that they are racing in because of that principle. Fastest speeds are achieved at sea level with cool temps.

I am not positive that the vacuum leak might be causing your stall but it might be causing your stumbling. Marvin might be onto something with the vapor lock. If you had an ignition problem, the engine probably would not start if you had sprayed the carb after it stalled. Another item to ponder is the float level. If the float level isn't adjusted correctly that will affect the fuel to air ratio. You may have multiple issues. I love Q-Jets, rebuilt many.

discrete pretty much covered it. Fix your vacuum leak first. Double check your float settings. Make sure the pump rod arm is in the right hole for your engine. It has a winter and summer holes. Make sure the choke is functioning and not hanging up. Last, make sure the mixture screws are around 1 1/2 turns from seat, then fine tune with a vacuum gauge if you have one.

Once the carb is right and leak fixed, recheck the timing and idle.