Ok so i have a 79 f150 4x4 with the 351m in it. i recently rebuilt the entire truck and i had it running, timed it in and it idled fine reved fine and everything.

so one day it was idling and just died. after testing and found no spark i troubleshooted and found the distributor was bad(i used an ohm meter and it was not fluctuating when cranking the motor)

also i found that the coil was bad (it would only spark when the key was first turned on or first turned off, just that split second)

i replaced both.

when taking out the old distributor(which isnt that old) it was a little hard to wiggle out, also the new one was a little stiff wiggling in

so i reset to TDC and reset the new distributor, i had a mark where the old one was and just wanted to make sure it was all set correctly again

so it still wouldnt start, i would get spark a few times then i wouldnt and i found that the rotor inside the distributor(the entire shaft) was not turning with the motor

first thought was the cam wasnt spinning so i pulled the dist. and shined a flashlight in and jumped the motor at the solinoid and the cam gear is spinning

i double checked the new dist. with an old one i had laying around and everything matches up as far as gear placement and everything

teeth on both the dist. and the cam are in like new condition

had the rod for the oil pump seated properly and the dist down all the way and the shaft of the dist. does not spin

when i was seating the dist. on the oil pump rod before i wiggled the dist. down (i used the sparkin solenoid technique to crank the motor) the shaft of the dist. turned a few times, once it was seated on the oil pump rod it didnt spin anymore

this leads me to beleive the dist. was not sizing up somewhere, but it matched the old one that was in the truck when it was running a week ago...

i am totally lost, i hope i was detailed enough in my discription

 

Is the gear on the distributor shaft spinning free on the shaft? Check for a pin or a key that might be sheared.

 

I have been there and when it happend to me the rotor was not seated all the way down. Another possibility is that it is making contact with the cap and not allowing it to rotate all the way.

 

The pin is still in the gear, and the gear is tight

 

i tested it with the cap off to make sure there was no conflict there and the shaft itself does not move, but i did notice once in a while when im cranking the motor over the dist. shaft seems to surge, almost as if it was trying to turn?

 

You might have a few problems, the distributor roll pin might be sheared (i know you said it looks like its still in there) If the roll pin is sheared it could be from the cam walking, or the oil pump being siezed up, or stuck.

Have you tried holding the gear in one hand, and turning the rotor with the other hand?

Might also want to make sure the oil pump shaft turns and turns the pump.

My 400 sheared a few roll pins on the old distributors i had, did it 3 times just after rebuilding the engine, and has not done it since ( the last 3 years)

 

My '77 would only show spark or run w/ key in start position, found it to be the ignition switch. If the pin is sheared, there may be enough of the sheared pin left extending out from the shaft to give you 'resistance' when you check it but it may not be enough to give you/the engine the necessary rpm's to start. I would physically check the pin to make sure it is intact and not compromised, then set it in , make sure it is seated all the way down, this being after you have set it in the proper position for timing, and turn the engine over w/ the cap off to see if the dist. shaft is turning, make sure the rotor is seated and try it after it is all put together. I hope this helps, my explanation in the typing form has a lot to be desired!!

 

i have held the gear in one hand and the rotor in the other and it is tight

i had the oil pump rod in the motor and turned it with a 5/16" socket and it felt like it was spinning freely

i have tried it with the cap off of the dist. and that is how i know that the dist. shaft is no spinning

i was thinkin of taking the oil pump rod out and just setting the dist. in to see if it turns without the rod in, this would eliminate the possibility of the rod interfearing...

 

I would limit my turning it over w/ the oil pump shaft out, remeber, no oil circulating. If the shaft is in, and the dist. is seated all the way , and there is no spinning of the rotor w/ the cranking of the engine..., does the dist. shaft turn from the top?? somewhere there is a link missing??

 

if it's surging and trying to turn you may have not gotten the cam gear bolt tightened properly and sheared the the pin off the cam and thus it's actually the cam that isn't spinning. take a valve cover off and see if the valves are moving at all. If not your going to have to tear the motor down most likely especially if it was running at the time it sheared. If you have gone through two different dist with the same symptoms its probably NOT the fault of the dist.

Also your coil probably wasn't bad, what you described as the symptoms for it is typical of when the coil isn't getting a signal from the dist, it will spark when it first comes on with the ignition (power goes through creating a spark) and spark again when the power is cut (which is what the dist actually does to create the spark)

 

ok guys...i figure it out...here it goes...

so i took the dist. back today because it looked like the gear was in the wrong place...actually it was sheared off but so tight it didnt move when i tried turning it...the new one sheared off as well....

after scratchin my head for a little while it dawned on me...the oil pump

i pulled the oil pan off and the pump and low and behold the pump was seized...and brand new as well..luckily i had another one just layin around...i checked it out, looked like new inside so i cleaned it up and put that on

one more new dist.(3rd times a charm i guess) set for base timing....puppy fired right up

timed it in and it idles revs fine again, tomorrow ill have to check out all the fluids and retune in the carb :-)

 

Glad you got it going.