Short story... 86 Ford F-250 with 460 engine and Holley 4180C carb. I sucked in a valve head and thus blew out a cylinder wall and ate a piston. Engine dead. I replaced it with an engine from a junk yard that looks in good shape (interior surfaces/oil is real clean). Yes I know I probably can't trust a junkyard engine but the truck is old, abused and probably should have been thrown out. But now I'm in for a penny... and I've become obsessed, need to win this battle since I'm losing so many others in my life.

The problem now is that the engine is in, the truck starts. In fact when it is cold it has never started better and it runs quite well. Once it warms up it becomes nearly undriveable. The engine backfires (both tail pipe and intake). It idles very well, but when you touch the pedal to accelerate the engine dies (almost stalls). If you "blip" the pedal a bit to get past this "dead" spot the engine picks up ok. But under acceleration is starts backfiring a bit. The backfire becomes a little less noticeable at higher speeds with light acceleration. But as it is I can't pass smog because there is still a misfire so bad the truck won't stay on the dynometer (lurches).

So far we've replaced the engine ignition modules, the ignition coil. The Distributor pickup, spark plugs, plug wires, fuel filters. The timing of the engine has been done and is within specs. We've rebuilt two different Holley 4180C carbs and adjusted them. (They produce slightly different but pretty similar behaviors such as idle-acceleration transition stall and intake backfires. The second carb produces more tail-pipe pops than the first but the first stalled out at speed more. Both operate better when the engine is cold but suck when it is hot. ).

And ideas on what's wrong? What have we done wrong with the carb? How possible is it that this is not an ignition/carb misfire problem and is related to a mechanical engine problem inside the engine? Exhaust is clean, neither blue, black or white so at first I don't think I've got a gasket/ring problem. But at this point I'm flummoxed and frustrated. It could be anything. Wish I had just thrown the damn thing away when I blew-up the original engine.

 

I would take the vacuum advance vacuum line off the dist and plug it. Then see what happens. If it's better, it sounds like the timing is too far advanced. I would experiment by turning the dist back in the direction that makes the engine slow down, and then test drive it.

 

Flummoxed?

I agree with Franklin, do some checks with your dizzy.

I really doubt there're any sort of internal engine issues causing this....

Hey, BTW, please fill out the rest of your profile so we can see where you're at, OK? Click
on User CP in the top-left corner to get there.

 

Check the timing chain and the distributor shaft for slop.
Is the EGR connected and working properly?

It does sound like too much advance.

If the timing chain or distributor shaft is worn and sloppy it may run fine under no load but throw the spark off when trying to actually drive it. (BTDT)
I would hook up the timing light and rev the engine quickly and let it drop back.
The timing marks should advance and retard steadly, with no bouncing around.

These carbs are jetted very lean and if the EGR is not functioning will cause lean misfire once the choke comes off.

There are also thermally controlled vacuum valves, one of which affects distributor advance.
Do you know how much vacuum (in "Hg) you have at idle?
How much total advance do you have with the advance connected and the aircleaner in place?
Is that filter clean?

 

I don't know how much vacuum I have. I wasn't the one who set the mixture screws or adjusted this. I'll check this as soon as I can (probably weekend) and report back on that.

I'll also look at the vacuum advance and the dizzy slop. The EGR is hooked up but I have no idea how to test if it is working properly or not. (This is what you get when you have a graduate degree in computer science... you can work out Pi to the millionth digit but you can't do anything actually practical or necessary in life; like simply get something heavy from point A to B.)

I can't work on the truck in a timely fashion as I'm mentoring high school children in the impractical stuff on weekends for the next four weeks but I will report back as I find information (or if I decide to just set the truck on fire and BBQ.)

 

Los Angeles.... Is this a California truck with California emissions?