I have an 80 model F150 that I have recently restored almost completely. It has had this horrible dieseling problem since long before the restoration or engine rebuild. It currently has a 351M that was converted using rods and crank into a 400. It has an Edelbrock aluminum intake and matching 600cfm carb with a K&N filter, but that is about it for modifications. The engine was built to completely stock specs using factory cam and timing set as well as all stock spec internals. All smog and EGR systems have been removed. My problem is that after the engine warms up and has been driven even a slight distance the engine will diesel very badly unless the engine is killed with the truck in gear. This problem does not occur if the engine is started from cold and just sits and idles in park. You can rev the engine as much as you want as long as you keep it in park or neutral it dies with no problem. I have tried replacing SEVERAL components including vacuum advance (dual diaphragm) basic tuning stuff (plugs, wires, cap, rotor, fuel filter) the control module (blue grommet on the inner fender) and even put a performance fuel pump on it (80gph holley) and nothing has seemed to help at all. I have timed the truck from 6 degrees all the way to 12 degrees with no success (8 to 10 seems to be the sweet spot). It has an original Ford distributor and coil also. Please help if anyone knows anything about this problem or can think of anything I may not have tried yet.

 

What is the idle RPM? Too high idle can cause this type of issue.

 

900 roughly

 

Too high. Should be lower, around 600-700 RPM.

 

If I set it that low when I put it in gear it barely runs. Rpm drops to below 200. Headlights dim power steering stops working etc

 

then you are running too lean as well. Running lean will make her run hotter, which in turn can cause the dieseling as well.

Set it for 600 rpm, then adjust idle mixture for best vacuum and then reset the idle to 600 again. should be good as gold.

 

I bet you have your vacuum advance on the dist hooked to manifold vacuum. On your carb, set the idle down, and the nipple up high on the pass side front of the carb should have NO vacuum on it. This is the port you should hook your vacuum advance to. Then reset the mixture, the idle speed, etc., and then you when you put it in gear it won't drop down anymore.

 

Thanks a lot guys. I'll try adjusting the idle mixture and resetting the idle tonight. I do have the vacuum to the dist on the passenger side port of the carb, but I do need to get ahold of a vacuum gauge and check it at idle. I'll check all that tonight and let you know. Thanks again

 

couldnt the floats be adjusted to high or jets are to big??