OK you guys; I need some expert help on this. I have recently rebuilt my 67 F250 352 into a 410 with an Edelbrock 650cfm AVS carburetor.

I had about 500 miles on it and we had it properly jetted for the altitude in Denver and did an emissions check. Everything was perfect. You could stomp the pedal to the floor and burn rubber, no problem. Then I took the truck to the body shop to have them fix their screw-ups they had on the paint and bodywork. They spent 2 months doing what should have been 3 days of work. When I dropped it off I explained the starting procedure to the owner, the carb is equipped with a manual choke. He had that "deer in the headlight" look when I told him it had a manual choke; he is young and probably never heard of such a thing. The engine runs strong but is a cold blooded on start up until it runs for a minute or two so starting it takes a little finesse.

When I finally got to pick it up, the owner was conveniently gone for the day and the detail kid took care of me. He said more than once how it was all ready and all I needed to do was get a tune up and it would be a “real nice truck”. I could hardly get the truck started and then it backfired continuously through the carburetor. I pulled the air cleaner off and the carb was covered in black soot. I finally got it warmed up enough to barely run and as I took off it backfired terribly. I finally got it home and put a timing light on it and the timing was set to 35 degrees. After resetting it to 10 degrees it still idled at about 1500 rpm so I turned down the idle to about 700. I figured the idiots at the body shop didn't understand how to use a manual choke (which was confirmed by the detail guy who said he had much trouble starting it and when I told him it had a manual choke he got the "deer in the headlight" look) and that they started it and ran it in the shop and out the shop over and over during the two months and never warmed it up properly there by fouling the plugs and loading the carburetor with soot. And since it wouldn’t start right up and run like their fuel injected Jap cars they all drive they did me a favor by readjusting the timing and idle so it would run in and out their shop with no warm up.

Now the problem. It runs OK when warmed up but still backfires some during start-up. When warm, it runs fine during normal driving but there is a "flat-spot" if you stomp the pedal to the floor and often the flat spot is accompanied by a back-fire. It will also "diesel" for a few seconds when I shut it off.

I'd sure appreciate everyone’s expert thoughts as to what the problem is at this point. Everything’s new. I now have 600 miles on the truck after the complete restoration. I don't know much about carburetors but I'm wondering if all that backfiring caused a problem with the carburetor, maybe the accelerator pump is messed up now, I just don't know where to start. Everything was perfect before the body shop nightmare.

 

i think the first step is to go and beat the body shop owner!!! what an idiot, like you'd have minded going down and instructing them the right way to do if they called... pretty ballsy messing with a guys just completed truck.. anyways, when you mash on it and get a backfire, is it through the carb or back through the tailpipes? thru the carb is timing/valves and thru the exhaust is usually fuel... you could also hook up a vacuum guage to the manifold and see where it's running and that'll help point you in a general direction too... hope it's simple and doesn't make you pull out your hair...

 

the backfiring is coming back through the carb. I've set the timing at 10 degrees. Where do you think I should put it. I've heard 6 to 14 so I put it in the middle.

 

do you run timed or manifold soucre vacuum? It may be possible that the source was changed also. 35 degrees sounds a lot like a manifold setup. On the edelbrocks that i have seen, the passenger side is ported or timed and the drivers side is manifold. Another way to tell is, the lower port is probaly manifold as its below the throttle blades and the timed or ported is above the throotlle blabdes.

I highly suggest that you reset the idle mixture screws, use a vacuum gauge to do this. Rougly 1.5 to 1.75 turns out was ideal for my 390.

 

Yea, I was afraid they might have messed with the mixture screws and have not done anything with them yet. As far as vacuum advance, I'm running it on the passenger side, which is timed. They didn't change that. When I put the engine in and originally set it up, I used the manifold vacuum port since I do not have any emission equipment. Later when I had a mechanic help me fine tune the carburetor, he switched it to the timed side and used an exhaust gas analyzer to set the mixture screws. I'm not sure which way to go on the vacuum though. I'm using the original distributor that was re-curved by the guy that built the engine. Which do you suggest, timed or manifold vacuum? I'll probably eventually take it back to the mechanic that fined tuned the carb, but that always ends up costing me more than I'd like. I first wanted to find out if backfiring through the carburetor will damage any of the carburetors systems so I’d have an idea of what might be wrong.

 

the consenus is with timed source.

 

I think you are ok with your edelbrock, if you had a holley i would pay replace the powervalve, but edelbrock's are a lot better about that.

 

Are you aware that all of the newer Holley carbs have a Power Valve protection valve now to prevent a back fire from blowing them. And Gee Whiz guess what? You can upgrade the older carbs too and the problem is gone. Metering rod carbs are an old way to improve fuel economy and were never intended for preformance engines, even though alot of them find there way there, even from the factories.