Hi all: My 1939 Plymouth Pickup PT-81 has a Flathead 6 .It is all stock. Truck is very hard to start cold. Once running it runs ok until near fully warmed up. Once hot, it begins stumbullng badly.back fires and losses power when revving the engine or if I try to drive it..(Seems Like a lean condition when hot) Can find no way to adjust carb mixture.Gets impossible to start when hot. In the past 2 years, I have done full tune up with new parts. I have added heat shield for carb,carb spacer, electric booster fuel pump, 160 degree stat, all to address possible vapor lock issues. Checked for vacuum leaks, new gas,coil etc. Anyone have any ideas? Nice truck but getting to wits end over this one. Thanks in advance for any help anyone can provide.. gordyc
If it's so hard to start cold, how do you get it started? If you add fuel cold (like carb cleaner) does it seem to start and run better momentarily? I always say use a vacuum gauge first. Once the thing is running OK, what's idle vacuum? Should be 17"hg or higher and steady. Slow hunting of the needle is OK flicking of the needle is NOT. As the thing starts to run like **** what's happening to idle vac? My first vehicles forty years ago were Plymouth Pickups. 1938 PT-57 and 1940 PT-108(?) These flatties have a bi-metallic spring operating heat riser. With cold engine can you grab heat riser weight and move it? By this time the inside flap is usually gone but maybe yours is stuck shut? This would account for poor hot running but not poor cold start. Put a timing light on it and check for the three advances: initial, mechanical and vacuum advance. Usually something like 30 degrees plus TOTAL. To see if vac advance works check timing above idle with it disc and then connected. Should be big difference 10 degrees+? Pull a spark plug or two, what does he business end look like. Sooty? White? Oily? Vacuum leaks are ever present devils. We use a smoke machine to root them out but propane from an unlit propane torch can help you find them too with engine running. Other follks use water, oil on intake fittings and gaskets and listen for RPM change. Spray carb cleaner on a hot engine looking for leaks is a good way to light the engine on fire. In my shop we'd use an oscilloscope to check ignition. I've gotten negative remarks on this website for that comment, but it's the gold standard to check for ignition problems on older vehicles. Wihout a scope you can crank engine while looking at coil high tension wire about a quarter of an inch from a good ground and check for a fat blue spark.
Is your float level set to spec? The hot start deal is usually a result of a FLOODED condition due to percolation of fuel. Todays gas absorbs heat quickly, expands and over fills the float bowl, putting raw gas into the intake manifold. Check the next time you run the truck and see if the carb is wet under the throttle plate. If it is you have float and needle and seat problems. I have reset my floats on a dual carb installation about a 32nd lower than the stock setting. This has cured my hard hot start problem. Do you have a hand choke, are you using it for cold start? Initial timing should be set up for TDC #1. Are you still running 6 V? is your coil wired correctly with pos to points and ign to neg?
A friend has a 39 plymouth coup. 6 cyl engine, mostly stock. His wouldn't start after it was warmed up.....He tryed most every thing and some things like, timming and elect. ignition all helped. He finally converted to 12 volts and now it starts all the time!!!
I had similar problem, starting fluid had no effect on cold starts. Needle was not seated and fuel tricked in when sitting. The hot running issue is making me think coil, points or wire. Please let us know what you find out.
wraymen: When you say coil, points or wire. Coil and points are new. So are plug and coil wires. Do you refer to other wiring such as harness?
No, I never had a flooding problem get worse when the engine got hot. An earlier post stated this and I wont doubt him, It just never happened to me. I did have a coil let me down after it got hot. I realize parts are new and I am as baffled as to what it could be. Wires/components that are grounding or weak will get worse when under load. I would also look for vac. leak again and check if points are gaped and aligned right.
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