My 77 2wd f250 worktruck has the 400 cu in engine. I had been really lazy about maintenance, (too busy building my home) until it got to barely running. So I finally got to work and I just threw a lot of parts at it: factory rebuilt carb. with new egr, new pcv vakve, new distributor with new cap and rotor, plugs and wires, oil change wi filter, new fuel filter.

I've a little less than 20 inches of vacuum at an idle 650-700 rpm. I can't get it to adjust any slower, adjustment screw is backed out until it doesn't make contact (which is kind of interesting). I've timed it to the specced 12 degrees btc, and it doesn't float at all, sits right on the mark. The engine is a factory rebuild with about 20k on it, burns no noticeable oil, doesn't seem to be any lash in the crank like you might get with a worn timing chain. It is quite quiet at idle and at speed, good oil pressure, temp stays where it should.

It idles OK, not great, and if I put my foot down it seems to have pretty good power and the miss gets significantly better, but under mild acceleration and cruising it runs BAD. It doesn't die, but jerks and misses in a sort of random fashion. Coasting downhill I get some mild backfiring.

I'm stumped. I'm no mechanic, but a decent part bolter oner. I don't mind throwing money at the old girl, because can I do a pretty major rebuild for one months payment on a new model. I may have done something dumb or overlooked something simple, so I'm hoping some of you mechanics could give me some ideas about where and what to check out next? Thanks

 

Check for any vacuum leaks, that's about it

 

Vacuum leaks, timing, and then verify mixture after that. Is your vacuum advance working properly?

 

Concur. Vacuum/timing issue would be my first suspicion. A lil crack on a hose can wreak havoc.

 

It's odd that the idle RPM can't be throttled back to stall speed. 20" of vacuum is good, but should be checked around 550 or 600 RPM for an accurate figure.

 

Check your floats and set them right for your carb. Just because it's "Factory rebuilt" doesn't mean it's good from the gate. And since your maintenance schedule is virtually non existent (understandable) maybe a carb rebuild/clean out might be in order - a dirty old fuel filter could have let some snot in the carb, partially plugging up the jets and other orifices. You'd feel this under load as you've described.

After you've verified the float setting and have a clean carb, use manifold vacuum to tune the carb. Adjust the mixture screws such that you have the highest inches Hg then set the curb idle. Maybe bump the timing up a wee bit...

A real exhaust backfire or afterburn would sound like a shotgun....mild afterburn or gurgling/rumbling is acceptable...unburnt fuel in the exhaust when going from a higher RPM to coasting or deceleration is the culprit. Maybe check the exhaust gaskets?