1976 F250 Trailer special with 460, C6. Backfires flames throgh carb on cold start up. No other times. Runs fine after starting and warming up, no missing, no acceleration problems, no exhaust pops, no backfiring. Idles for up to 45 minutes without overheating (havn't tried it longer, no point, if it was going overheat

Had a few lifters ticking & running a bit rough, so pulled lifters and yep, 3 were seized. Seized as in sat overnight in a can of carb cleaner and won't free up seized. So:

-Cam looked fine everywhere, no wear visible at all. some of the lifters were dished.

- changed all the hydralic lifters & pushrods. Torqued rocker arms back all up to spec going around in firing order. (although it doesn't matter, with non adjustable pedestal rocker arms, you can just bolt em down, torque em to spec, then check again after warming it up) Slopped moly engine assembly grease everwhere on the cams, lifters, pushrods. when putting back together for good measure.
-put a new edelbrock 1405 carb on it. Manual choke- but don't have choke hooked up, its Phoenix area. plugged vacums not used, hooked up PCV valve to front, as edelbrock recommends.
-put a weiand stealth intake on it, torqued in recommended sequence.
-changed out my distributor for a rebuilt motorcraft in good shape (old dist was original, lot of slop in the shaft, and it had the wrong dist cap on it-real sloppy. New one fits nice and tight.
-changed ignition module
-found TDC compression stroke on #1 with my handy whistle tool -(spark plug extender with a dollar store party whistle super glued in)- works great
-dropped in the dist with rotor at #1 position, fiddled it to set down in the right place
-put all my wires back, checked em twice- firing order is correct (have numbers written on them with white out - checked em each just in case.

started right up, sweet, so- set the timing to 15 Before TDC with vacuum hose off & plugged at carb. fiddled a bit with idle and leaned out a rich mixuture -(leaned it a bit, was smelling a bit rich out of exhaust) - got it running smooth. Put hose back on, took idle up to 2500 (a guess, don't have a tach), total advance was around 32/34

but, now, whenever its cold and I try to start it up, its hard cranking, and it always backfires some flame out of carb on start up- 1 or 2 times. Then it will start up and idle fine. Runs fine- not as smooth as I would like, but its got alot of miles on it. Sprayed ether round the manifold, lower carb body- no vacuum leaks found. does sound like there is a nut or bolt rattling around in the carb body- but I've put my ear up there and figured its just the sound of air/fuel slurping from what i can figure- or maybe the wrong carb spacer (aluminum mr. gasket 1 inch).

any ideas on this? I can't figure it out. My experience is usually this is a timing issue.

- carb too rich/lean? I didn't do much to it after bolting it on
- timing advance too much? 10 is factory, I read alot of guys here on FTE set it to 15.
-wrong size or shaped carb spacer? (its a sqare bore carb, manifold and spacer) or put in upside down?
-not having a choke operating letting too much air in?
-other thoughts?

BTW - I don't buy in that dist can be "off a tooth" - dist can't be "out a tooth"- it either sits down in there in the right position at #1, or its off quite a few teeth- as its a 4 point hole that you have to sit down in these. Anyway, she idles well, doesn't backfire or sputter on exhaust when accelerating and driving, so timing should be close (but perhaps off enough to cause this problem - 15 is more advance than factory spec)

 

jbancroft,

Have you tried it with the choke hooked-up?

 

My first guess would be too much initial timing. I assume it backfires and doesn't start, then starts when it doesn't backfire? When in all that did the backfiring start?

 

Morning start backfires can be from being lean, the manifold absorbs all the initial fuel, hook up the choke and see if it gets better.