I have a 1981 F-150 farm truck with a carburated 351m. I just rebuilt the carb and now the truck won't start after it is warmed up. When it's cold it runs great, when it's warm it runs great. If I shut it off when it's warmed up it won't stsrt until it cools down. It's even hard to start with starting fluid! This is the first carburated vehucle I've owned and I don't know where to start looking for the problem. Any suggestions?

 

Did this problem begin directly after having rebuilt the carb, it was working just fine
before then?

I can think of one possibility but I kinda doubt it's the cause if you're having problems
starting it with starter fluid...

The M-blocks have an exhaust cross-over in the intake manifold, not a coolant cross-
over like many other V8 engines. IIRC in the stock configuration, there is a valve in or
around the factory exhaust manifold(s) that is supposed to regulate the flow of gasses -
more when the engine is cold, less when warm. Exhaust cross-over was to help warm
the carburetor faster in cold weather.

But I understand those valves frequently got chucked when people eventually replaced
their exhaust systems, the result being that gasses are always flowing in the intake
manifold and creating heat even when not needed. This heat can be enough to quickly
evaporate the fuel in the float bowl (I could hear mine boiling and you could see "smoke"
coming from the "smoke stack" (carb bowl vent tube)) the result being the starter motor
needs to crank a while to get fuel into the carb again.

But I doubt that's your problem; instead, I more suspect your DS2 ignition-control box,
the gray box about the size of a sandwich on the driver's side wheel cover, the problems
you're describing are classic symptoms of a dying DS2 box. Remove it and take it to a
parts store to be checked, most such stores can do that for you. Go from there. The
fact that starter fluid doesn't help points me to the ignition but that'd be kinda rare as
most failures like this are the direct result of something having recently changed. That
being the case, I have to wonder if the carb is somehow flooding the engine? Have you
looked at your spark plugs, are they wet with gas?

 

Thanks for the reply! This problem started immediatly after I put the carb back on. I've had the truck for 12 years and have never had a problem until now.

I'll check the condition of the exhaust valve and make sure I hooked everything up right. The truck has the stock manifolds, a flowmaster muffler and no cat converters. I'll also check to make sure the fuel lines are properly routed and not up against something hot.

When this problem first happened I thought it was the ignition module too. I replaced it, warmed the truck up, turned it off, and it still wouldn't start. I have a feeling something isn't adjusted right or I installed something wrong. I haven't checked the condition of the plugs when this happens but that will be the next step. Any other advice?

 

After shutting the truck off hot look down the venturi and watch the boosters in all barrels. Do any drip fuel? Are the throttle plates wet? Any puddling?

If so check float level. Also since you say you've never had a carbureted vehicle, when it's hot try holding the throttle to the floor and starting. If it's dripping fuel or over-rich for any reason this will lean it out and help clear any puddled fuel out of the intake.

Other than that go over the basics, float level, ensure all air bleeds are clear,ensure idle mixture screws are set properly, ensure choke is not coming on at all/too much on a hot start, timing, plugs, etc.

 

Thanks for the advice, I'll be checking that stuff out this weekend!