Hey guys, Thanks in advance for the advice.

I recently bought a 1979 Ford F150 Ranger, with the 351M engine.

It was running smooth, until recently I parked it at the store, and when I came back out to drive home it was shaking on acceleration.

It seems like it may be idling a little weaker than it was before also.

It only shakes at lower RPM and speed. When I accelerate harder, it smooths out, and when i let off of the gas it smooths out as well. It does it harder when I am starting from a stopped position.

Any Ideas what this could be?

I was told by someone at work to unplug each spark plug wire while the truck was running to see if one of the wires or plugs are not working properly, but each unplug changed the sound of the engine.

Could it be that one of the spark plugs may be firing, but very weak, or could it be a more serious issue?

 

Well it could be unrelated but today right after I washed my truck and degreased my engine it was running horribly, then stalled in the parking lot at sears and didnt start for about an hour. Then I got her to run again and it was still rough. When I left the sears car show, it was still rough but on my way to the next car show it started to run better and by the time I got there it was running normal again. When I left there it started up fine and ran great the rest of the night.

I think I just got water in the carb or something.

Any chance that you could have got water in something it shouldnt be in?

EDIT: it was running just as you described yours

 

Thanks for the reply but no, there is no way. It has been in my garage for about 2 weeks and it is still doing it. I am going to try replacing the plugs and wires, and if that doesnt fix it, I may have to replace the motor mounts. I need to pull it anyway to clean it up and make it presentable.

 

• Physically and visually check each plug wire for burn-through.
• Then check to make sure each plug wire is properly seated in the distributor cap.
• Check the contacts of the rotor and distributor cap. Look for any cracks on the cap.
• Check the coil wire contact for corrosion. Check the coil wire too.
• Check your spark quality. It should be bluish-white. A weak spark is yellow.
• When was the last time ya checked/changed the fuel filter?
• There could be crud in your carb. Pull it off and blast the passages with cleaner and shop air.

A possible issue after washing the engine is steam can form from running he engine and having condensation form inside the distributor cap which causes an erratic spark.