I have a 73 F100 with a 360. I just put a four barrel iron manifold (65) on it and a 600 CFM holley with a 1 inch spacer for clearance issues. I have rejeted both lean and rich as well as adjusted the accelerator pump and the dam thing still backfires through the carb WHEN IT IS COLD. When the truck heats up it rarely does it. This happens when going from a stop and when I punch it at cruise. Hope someone has had this issue before too. If it helps the truck and myself live at 7200 feet and it has 9-11 inches of vaccum at idle with a 6.5" power valve.

 

Vacuum seems low for a 360. Check for leaks. A vacuum leak would/could cause a lean backfire. Spray some WD40 around the carb, if there are leaks around the the base, the engine RPM will go up when you spray. You did use gaskets on both sides of the spacer, right?
gw

 

I did put gaskets on both sides but I will try the WD-40 trick.

 

Sounds like a choke issue if it clears up when warmed. The mixture needs to be richer on a cold engine which is what the choke is for. Make sure it is operating properly. Even with ambient air temps getting warmer now the engine is still far from operating temp when you start up.

 

i have the exact same problem with my 390 with a holley 4160 600CFM. i have blown out so many power valves trying to drive it to get it warmed up to tune it it aint even funny. and the one time i did the idle mixture screws dont do crap to the vaccuum. im pulling about 15.5" at idle. some times if i hit the gas going to take off it pops 2 or 3 times and then gets up and goes, also when its cold and im cruseing aroud and i hit the gas to go up a hill it will pop once or twice and go up it.

 

If your mixture screws don't change anything when trying to adjust them you either have a vacuum leak, or your main idle stop is already set too high. To properly adjust the idle you have to get below 1,000 rpm. After that the intermediate circuit begins to come in. Also recheck for any vacuum leaks.

 

WD-40 trick did nothing. I spray tons of that around the flange and manifold-head areas. No noticeable increase in RPM. The choke is working so that is not it. Interesting that two of us as the exact same issue on the same motor with the same carb.

 

Well while you're at it, recheck the timing, it'll affect vacuum readings too. My 390 idles down to 300-400 RPM, if you can't idle down that far, chances are good that there is a vacuum leak somewhere. Also, retorque your manifold bolts. There are a couple of holes in a intake that must either be used or plugged. Make sure that neither of them are sucking air.

 

I'm having the same problem, but i have a motorcraft 2150 carb.

 

WYO100, do you have power brakes and an automatic tranny?

 

I know you didn't ask me, but I have both those things and I"m having the same problem...

 

yep the vaccuum modulator is fine and i plugged all the vaccuum ports accept the brake booster one when i took the vaccuum reading.

 

Hey how were you able to tell that your vacuum modulator was fine? How did you check your modulator?

Also did you check your brake booster? If so, how did you check it?

Another thing I don't understand is how PCV works... it seems like PCV would cause a major vacuum leak...

 

If you didnt check your PB booster just pinch the line to it completly closed with vice grips. If your idle speed changes its leaking. Also if it seems to be low on fluid and you have no brake leaks underneath the truck its leaking. You can check the auto trans fluid and if its low and you have no tranny leaks the vacum modulator is bad.