Ok here we go. I have a '65 F250 with a 390, 2bbl, auto, with a stumble at takeoff and will backfire while driving, which seems to get worse as the engine heats up. I have replaced the dizzy with a stock rebuilt and reinstalled the pertronix III. My total advance is 38 deg. with an intial of 12 deg. I have also rebuilt the carb twice (autolite 2100) taking special care to completely clean the body and blow out all the passages. I have replaced the rubber fuel lines and installed a new mech. fuel pump. What stumps me is that the engine revs nicely in park, but starts to miss when put in gear. The transmission shifts fine. What am I missing?

Kurt

 

Vacum leak.

 

Does a vacuum leak only show up when the engine is under load?

 

Have you checked the compression in the engine? My sons 66 with a 390 would do the samething, it had 2 bruned valves

 

I haven't checked compression-don't have a tester. May have to invest in one.

 

Start the engine and hang a paper towel next to the exaust pipe and see if it sucks and blows. That is a old way to see if you have a bad valve.

 

Thanks Allen. No sucking, just blowing. There seems to be good flow coming out of the pipes. I'm going to replace the hose to the trans and see if that helps.

Kurt

 

I would still check that compression.

 

It can and it can also get worse as the engine warms up. You need to do some testing.

 

Thanks. The intake leaks a little oil at the back, so I suppose it could be the culprit. I have been dreading that particular job, since the intake I have is cast iron.

 

The oil leak in the back is fairly normal on a FE engine and I hear you about the cast iron, I have two FE's that I have to pull the intakes over the winter because of the leaks in the back. But a leak in the back does no mean it is leaking vacum.

 

I have done a number of things to the engine, since the initial post. Replaced timing chain with double roller. New glasspacks (blew out old muffler because of bad timing chain). Replaced old 2bbl intake with Performer RPM and Performer 600 cfm carb. Blocked off exhaust crossovers at head. Used right stuff to set intake manifold. No oil or vacuum leaks that I can find. Set timing @ 12 deg with total advance around 38 deg. Truck ran great initially, then started missing and stumbling while driving. Finally got around to a compression test. #1 - 135, #2 - 130, #3 - 125, #4 - 138, #5 - 128, #6 - 140, #7 - 130, #8 - 130. Any suggestions before I start tearing things apart again? Thanks.

Kurt

 

Did you change out the dist.? Are you running points? Check the vacuum advance to see it it is working.

 

You are right on the edge of your compression being out of spec. 10% max low to high. Since you had a bad timing chain it's possible having bent push rods or a bad valve, valves.

 

The dizzy is a reman with Pertronix Ignitor III and flamethrower coil. Vacuum advance is working. The push rods looked okay, when I last had it apart, but that was before the timing chain change. A leakdown test next?