I have a 79 Ford Bronco 400 T18, that is back firing out the carb and exhaust and bucking only under a load. Here is what I did. About two months ago I drove out to my buddy’s with no trouble, Took off the factory intake and carb. Swapped on a Edelbrock intake and a rebuilt Holly carb(600cfm),with Felpro valley pan. Did it with out removing the distributor. Idles great, revs out nice. Took it for a test drive starts bucking and backfiring. But not to bad. Put the air cleaner on for the drive home made it 100 times worse. Took air cleaner of has not been back on yet. Seamed to run butter when cool. Started to look for the problem. Adjusted the floats that helped some, but the more I drove it the worse it got. Found the advance was not working, changed the advance, retimed the engine for the 1st time with 4bbl. Switched the vacuum advance from the carb to the manifold and back to the carb. No help. Found that the vacuum port on the metering block was not working at all. Got another rebuilt Holly 600cfm carb, swapped it on NO CHANGE! Got the starting fluid out. Found a leak on the intake manifold, so I pulled it new Felpro valley pan Black RTV around every port both sides of the valley pan. Still no change! Just this week I did a compression test 5 cylinders with 110psi 2 with 115psi and one at 120psi. Plugs all light golden brown. Found the Accel coil was leaking oil replaced with house brand coil, found the plastic in back of the ICM was loose, test good at 1st. Tapped it while testing and it failed every time after that! Replaced with new. Still no change! So right now She will start with just a turn of the key Idle like a dream, Drives nice tell it warms up then Right back to backfiring and bucking. I have found if I drive in a lower gear at a higher RPM its better but still not right.

Sorry for the long story, I tried not to leave any detail out that might help.

Im out of Ideas any thought would be of great help! Please and Thank You!

 

you sure you dont have plug wires crossed? they might be crossfiring too, check it.

As simple as it is, check your distributor cap.. sounds like it could be it, really.
Even if you didn't touch it, you think you possibly leaned on it while working on the carb and cracked it? I just did this a few months ago and didn't realize i had busted it. My Bronco ran terrible when warm, so I remembered leaning on it, figured I had broke it, replaced it and it fixed the problem.

 

I priced out new cap, cap adapter, rotor and pick up At that point it was only $4 more to get a whole new distributor. Should be here in a few days.

 

good luck, i hope that was the cause of it.

and you're absolutely sure the intake is sealed now? if you went from leak to no leak and there was no change in the way the truck was running, that's a bit odd.

 

Thats a good point I check again

 

No intake leaks.

 

Hmm interesting story. Is it the edelbrock performer 400 intake? I have that and a carter fab 4bbl now on my 78 and save for the entire exhaust being one big leak, it runs fine.

one thing to think about, maybe you need a spacer. the heat from the block could be transferring into the carb and causing it to burp. I haven't had that issue but it could be a possibility. Next make sure the carb is tuned right, choke is working, all that good stuff. Hmm I know there are other things but I can't think of them at the moment.

 

Could be a blown holley power valve..

 

I Know it has to be blown, but not point fixing tell I find out why?

 

i'd offer more but I know nothing about aftermarket carbs.. stuck with my stock one until the end

 

I don't know much about Holley's but if the power valve blew on the old one AND the replacement then something is seriously wrong... I will keep my chips on the spacer issue.

 

A single backfire can cause a loose timing chain to jump, have you checked it?

Pat

 

New double roller 5000 miles ago, give or take a few hundred miles

 

yeah, it would be tough to get a brand new chain to jump.. but since it wasnt that long ago, you sure the chain is set right?

 

Ran great for a year with the 2bbl on it. Got the distributor at work, but had to spend to much on the Jeep this week so the bronco will have to wait a few more days.