Hey all, my 79 f250 with a 351m is driving me a little nuts right now. First of all i am suffering from the oh so dreadful stuck distributor which is very common for these engines. I have managed to free it up where i can tap on it with a punch and turn it a little but that its not my problem. My problem is that i set the ignition timing to specification - 14* BTDC - and now i have trouble starting the engine when hot. It acts like the timing is way out but it runs like a charm, seemingly better than ever. But now with it idling it has an occasional pop in the exhaust and i did the "paper trick" and it sure enough acts like a leaking exhaust valve. But i have true dual exhaust and it does the same on both tailpipes. I know it is an almost 40 year old motor and it has at least 300,000 miles on it so it wouldnt be impossible to have leaking valves but i never noticed it until now. I dont think screwing with the distributor would have anything to do with that? But im pretty aggravated with it and wondering if anybody has any insight.

 

The paper trick doesn't work well on the 335 series engines because of the unique firing order.


As for the hard starting, does it not want to crank or not want to fire?
Not cranking is timing too advanced
Not wanting to fire is probably a vapor lock issue

 

I would definitely try to retarding the timing a little. When the engine is hot the spark ignites the mix while the piston is still coming up but the engine is not turning fast enough for inertia to force it through TDC. Thus the starter stalls until the pressure bleeds off enough to let the crank go over TDC.
Then the next cylinder does the same thing.

 

Not cranking like its advanced. Sorry i should have been a little more clear.

 

14 degrees is getting up there but it shouldn't cause that. Heck I run 18 and I don't have any trouble starting.

Is your starter and battery up to snuff?

 

Thats the thing, i retarded it A LOT but it didnt help my starting issue much. I made the mistake of not seeing where the timing was at before i turned the distributor in the first place.

 

I have dual batteries they are both brand new. I just replaced the starter with a used one and its done fine so far. If i let it cool down for a while it will fire right up without hesitation.

 

Bad cables will do it too, and neglected grounds. Hot temps will put the whammy on starts as well. Very common.

What you really want to do is maybe put a vacuum gauge on it and see what you're pulling for vacuum adjusted, for Sea level. This will give an indication of optimum timing as well as the overall mechanical health of pistons/rings, valves, vacuum leaks, etc.

For all that if you were to disable the ignition as a test, the starter should not kickback regardless of what the intitial timing is set and would eliminate timing as a source of the trouble.

 

You're setting initial with vacuum disconnected/mechanical advance only right?

 

Thats a good idea i didnt even think about that. I will run it tomorrow then pull the coil wire off and spin it over. I am confident it is a timing issue because it didnt do any of this until i messed with it but stranger things have happened. I planned on doing a vacuum test anyways.

 

Yessir, just for reference how much should the timing advance when i connect the vac. back up?

 

I don't know for your specific application, but probably in the 10 - 15 degree range.


I guess it's possible your starter is failing and it's not related to timing. Maybe someone smarter than me will have another idea.

 

Timing should not change with vac hooked up. Not at idle speed. You want to use ported vacuum, not manifold.

 

I had a vacuum advance stick on one of my 400'S took a while to figure that out.

 

Okay, was able to pull the truck inside today after driving it. I yanked the coil wire off and the starter spins free and fast. I also did a vacuum test and it pulls a steady 19 in/hg idling and lowers to about 5 then raises to almost 30 when opening and closing the throttle. Proper readings if im not mistaken. I turned the crank by hand until the ignition timing marks lined up and checked to see where the rotor was pointing and it pointed dead at plug wire number 1. Any ideas?