I have a 351m in my 78 f150. It runs great most of the time, except for mid to heavy throttle. Once the motor is nice and warm it is not as bad, but still has extreme hesitation and sometimes backfires under mid to heavy throttle. Light throttle, and WOT are fine, but the mid to heavy range is what's killing me. I found this truck parked in a garage for over 20 years, pulled intake and heads off, completely cleaned top end and replaced lifters, pushrods, and seated the valves. Put in a new fuel tank, fuel pump, rebuilt the carb, and all new fuel lines, before running. Started up and ran great. The only problem is the hesitation and backfiring. I plugged the vacuum line to the egr to see if this might be the problem, and it ran horrible. Pinged continuously under any throttle. Runs much better with the egr hooked up properly, but still not what I would consider good enough. I've checked all of my vacuum lines and replaced most, but cannot find a leak anywhere. I'm pulling between 18 and 20" at idle, and have great compression. I do however hear a hissing noise coming from under the dash under extremely light throttle. Could these problems be coming from a vacuum leak? Or is it more likely to be timing? I've set my initial timing to factory specs, and checked the distributor vacuum advance which works well. Oh, I also replaced coil, plugs, wires, cap and rotor. Any insight would be much appreciated. Thanks.

 

Hesitation at part throttle is a sign of a lean condition. It will be worse when the motor is cold and wants a rich mixture. It sounds like a vacuum leak from the ported vacuum.

 

what coud cause a bog when opening the whole throttle at once? after the bog it dies, other than that its fine..

 

After it dies, is it hard to restart or does it start right up. If it's hard to restart, it could be over rich (flooding). If it restarts easily, it could be lean. Check your float and accelerator pump.

It could be a weak spark also. Check those new wires and cap, including the coil wire.

 

It starts easily, ill try and enrich the mixture and see, then going do the wires.... thanx

 

To the OP - if you hear hissing under the dash, the heater controls could have a vacuum leak since they run off of manifold vacuum. I wouldn't think it'd be enough to cause that terrible of a lean condition though, since they are tiny hoses. If you are sure the timing is dead-on, I'd look at your accelerator pump. Take a look down the throat of the carb with the truck off and pull back the throttle - you should see two nice streams of gas shoot against the venturis.

hollenjoe - it could be a couple of things; first it could be the accelerator pump like I mentioned above. You could also have a low float level. Basically everything danlee said is very reasonable. I would say though that to an extent, it is normal for a carbureted engine to bog just slightly if you slam on the gas as fast as you can for a brief second - don't expect the engine to instantly rev up really fast and settle back down. Only an EFI engine can react that fast. This is the purpose of the accelerator pump in a carburetor - when there is a sudden change in throttle position, the air moves a lot faster than the fuel can, so the accelerator pump adds a shot of fuel to compensate. But even that design isn't bulletproof. But it is definitely good enough that you shouldn't have any driveability issues.