The Stovebolt Site Forums The Shop Area Engine Shop Hard starting when warm.

Truck starts fine when cold, 2 pumps on gas pedal + choke out and it fires right up. Problem seems to be after I shut it off (after driving for a bit) and it sits for 10-15 min or more. Then, when I try to restart it, I have to crank it several times. Any ideas??

what drivetrain? original? sounds like fuel perculation. May need a heatshield between carb and manifold,
Scott

Well, it's not likely that the compression changes from start to start, so it's down to fire or fuel.

Be more specific about "cranking it several times". Does that mean cranking for 10 -15 seconds with no fire, resting for maybe 20 seconds, then cranking it again for 15 seconds? When does it finally start when warm?

Do you pump the gas on the warm start? What if you don't? If it cranks a time without firing, I'd pull a plug or two and see if they're wet or dry - too much fuel, or too little. Does choke make a difference?

For me, fire is a little harder to diagnose, and seems less likely. You hear about coils or condensors that have increased resistance when they get warm - it seems like this would present when it was still driving though.

Could be down to a loose or corroded connection in the ignition circuit, say, at the key switch, ammeter, etc.

But I'd guess fuel first. Percolation, as Scott said, definitely a possibility - so is fuel seeping past the float valve and flooding the engine, though you'd likely smell that . . .

Just some thoughts - hope they help.

-Michael

That's where I put my money. The heat from the exhaust manifold is radiating to the carb and boiling the gas out into the intake runners. It's like trying to start a hot engine with the choke on. It's actually a quite common issue on these old engines. One thing that has helped me is when I restart after sitting a few minutes I arch my foot over the gas pedal to press the starter pedal so that I'm not squirting even more fuel into the fuel rich intake.

Percolation is most likely.It will not only occur in the carburetor bowl but in the fuel trapped between the fuel pump discharge valve and the carburetor float valve.
When the engine is cut off and air flow from the fan ceases,this trapped fuel soaks up heat, expands possibly to boiling and the increase in pressure opens the float valve to raise the bowl level and spill down into the carb throat.
Some times you can get a indication of this by vapor/smoke issueing from the carb throat after this "heat soak" has commenced.or some times liquid fuel leaking from the throttle shaft.
some things that can help:
metal line instead of rubber
line routed to avoid hot areas
if an in line filter is here keep it installed low and as cool as possible
make sure the heat riser valve is free to open --I might even block it open
insulating spacer-as thick as possible between carb base and manifold
insulate fuel line from pump to carb

I'm not sure I've seen it on chevy but Pontiac and Olds in the 60's-70's had a small bleed off line either on fuel pump or filter that bled down the fuel trapped under pressure in this part of the fuel supply system.

And when all else fails-- the old cure for a "flooded" engine.Crank it with accelerator on the floor.

Aint this modern gasoline wonderful?

Thanks for the replies. Truck is all original, 235 with 3 on the tree. As far as my restating technique I think I've tried em all. Pump the gas 1x, 2x, don't pump, both with and without choke. I'll do some more research into how the fuel system is laid out.

Percolation makes sense, it's already in the upper 80's here so ambient temp isn't cool, which prob doesn't help. Bob, great explanation of the problem. Time to get under the hood.

Is your automatic choke working properly?

Tim, Still running the original sgl barrel with manual choke. I'll check and make sure PO didn't adjust the cable incorrectly. Although I don't think it's a choke issue as it runs fine once started.

Is this a 6V truck? What size battery cables are you using??

Keith,

Are you referring to your 1954 hydra-matic? The original carb had a manifold-heated automatic choke and a complicated throttle/transmission linkage. Maybe these complications are adding to the quandary?

Regarding Sweet's pertinent question, in your first post it looks like you indicate that the engine is cranking OK when it is warmed-up?

Sweet, It's got an 8v battery, not sure of cable size.

Tim, It's my 54 with 3 on the tree.

It cranks fine, no lagging or slowing down. Just takes awhile to fire up after the first start up. First start she fires right up, most of the time.