Its a 79 f150 with 400. I 've got it running good at almost all rpm exept higher rpm.( around 3500 and up) The intake is sealed up good, the holley 2300 (500cfm) has #80 jets. It used to backfire through the carb under hard acceloration untile I changed to jets up to the #80's. Now it doent backfire, but it still is gutless when you floor it. I just would like to know where to look next on this. Thanks

 

Refresh our memories. Is this a stock engine or if anything has been done to it what? What size tires and rear end ratio?

 

did you check the timing?? also check the ignition module...any parts store should be able to check it...they have a high rpm side to them that can break down....when I worked at Advance, I checked alot that was bad on higher rpms....

 

As far as I know it is completly stock exept the carb and exhaust. I had new exhaust put on today. A single 3" with flowmaster 40. I also changed back to my #78 jets and put in a power valve that opens at 3.5 vacume. It is doing pretty good just now it is backfiring at high rpm again, but it seems like it wants to go more than with the #80's. I think it still has the 4* retarded cam gear and right now my timing is set at TDC. I tried driving it a little today without the vac. advance hooked up and it only made the backfiring worse. I have also driven it at 10* ATC but the backfiring is at about the same level. The tires on the truck are 31x10.50x15 and I think my gears are 3.54's. Hope this helps a little. I am just getting started with ford stuff so I really appriciate the help. Thanks

 

your ignition module should be mounted on the fender well...it will be a electronic box with 2 plugs coming out of it and it should say duraspark on it...take it off and have it checked....I think that's where your problem is going to be

 

Ok, I will try that next. I will let you know what happens

 

Also check for timing chain slop.

 

The timing chain is critical to running better, though it should not be causing a backfiring problem unless it is stretched as mentioned in the above post. As far a running like a dog a timing chain that can be run straight up is crucial.

 

Also check compression. Possible leaky intke valve.

 

I would hope it isnt a leaking intake valve. THe heads only have 150 miles on them.

 

what is the intake manifold vacuum at idle? new heads, did you check and shim for lifter preload?

 

There are only a few things that cause intake backfire. (grclark might have it right there.) An overly lean condition due to jetting or intake leaks. An intake valve too tight or leaking. (It could leak from being blocked partially open by a speck of carbon or debris.) Or cam timing and or ignition timing overadvanced. Intake backfire is aggrevated by overheating.

 

My idle vacume is about 18 when it is warm at about 800 rpm. I dont know on the lifter preload. They probaly are not right. Is this a lifter system where you run it warm, loosen them one at a time untile it rattles, tighten untile it stops rattling, and then give it another 1/2 turn? Since my engine was a smog pump motor at one time, could the way the distributor advances be off and adding to the problem? Also my vac. advance is hooked to the vacume port on the carb that doesn't pull any vacume at an idle. Only at higher RPM's. Is that the right one to hook up to? I am probaly going to get new timing gears and chain to get it TDC in a couple of weeks. So I know that will help alot. I would bet the one that is in it is stock and about to fall off. I havn't had time yet to test the ignition module, I will try to do this on thursday or friday. Thanks for all the tips, I hope to have this thing running nice and strong someday.

 

if you have a steady needle at 18" of manifold vac at idle that's really good. if you have a vacuum leak or too much lifter preload holding a valve or more open you'd not have a steady 18" of man vac. that's were my questions were going to find out if you have a vacuum leak causing a lean condition, or an intake valve staying open and letting combustion up through the intake.

have you double and triple checked the plug wires/firing order? fouled plugs will also cause a lot of popping out the carb.

what were the stock jets in the carb? is the accelerator pump adjusted right?

 

At an idle actually it kind of bounces between 17 and 19. THe gauge doent stay solid at 18. It is hitting on all 8. I have checked it more times than I can count. The jets that came in the carb were #73's. It ran like crap with those compared to the # 78's. The accelorator pump has almost no slack in the arm, I couldn't fit a feeler gauge in the slack. Should I try backing my intake valve adjustment off a little and see if that helps?