Back firing thru carb
 

Hello again. I appreciate the help I got with my last question. I was able to get a rebuild kit for my Holley carb on my 401 C.I. engine on my 1966 F-850. Now I need some more help. The engine would only run if you left it mostly choked, if you un choke it even at operating temp it will back fire thru the carb when trying to accelerate. I was told it was probably a blown power valve which is why I needed the rebuild kit. I rebuilt the carb, replaced the power valve yet still have the same problem. What would cause this to happen? Any suggestions? Thanks for any help.

I'd blame ignition first, then vac leak or float
 

Ignition - spark and timing ok? If it's a part throttle problem is the vac advance ok?

On the carb, if you have to close the choke at IDLE to keep it running that's usually a vac leak somewhere.

If it idles good, it's probably not a vac leak, but a low float level or dirty main feed passage.

I dont think a blown powervalve would cause this problem.

I've got the same exact issue with a '69 F850 SD. My carb is leaking badly so, I'll follow in your steps with the kit. I thought it was timing since it seems to be firing on open valves. I'll hook up a vacuum guage tomorrow just for kicks. I'll know more later.

Backfiring and choke plate
 

If it responds to closing the choke at hot idle, its probably a vac leak or dirt in the idle carb circuits.

If it responds to this when accelerating, it could be dirt in the idle or mains of the carb but it could also be spark timing or CAM timing [stretched chain or slipped a tooth] - check these too.

If you have a spare carb [always a good idea] see if it does the same thing

Forgive me if I'm wrong, this thing doesn't have a timing chain does it ??? It's gear drive. No way it slipped a tooth. Spare carb? For the Fire Department, not. Vac leak is very probable, yes the choke does help. The carb is such a mess I've gotta go there first. I'll check static timing just to see where we are at. I didn't get to work on it today because of a big political pow-wow ribbon cutting thing going on. I'll get my carb kit Friday, at least I can stop the fuel leaks. Thanks Werby for the send.