454 back fires at 1/2 throttle or more

[nemesis_pyros]

ok so I took my truck out after about 2 years of not being driven on the street. (it was started and moved from time to time) Well it runs pretty good but when I get on it at 1/2 throttle or more it starts to back fire through the carb like crazy. Like i said when I parked it, it worked fine and we did start it a few times to move it and all was well. The only thing that appears wrong with it is I need to adjust some of the rockers as they are making a bunch of noise, but I don't think that pertains to this problem.

454 in a 49 chevy p/u with a th400 and a 10bolt with 3.08s


thanks
Brent


oh I have tried adjusting the timing from retarded to advanced and it makes no difference.

 

[thatK30guy]

Vacuum leak?

 

[k5guyva]

Could be distributor vac advance probl. There are some springs and weights in there.

 

[nemesis_pyros]

Good point didn't think to check the distributor advance wieghts, I'll do that this evening and see how things go. No vacum leaks as far as I can tell. But I'm gonna go over it thoughly this evening. thanks guys i'll keep you informed

 

[thor]

If it's an HEI check the wires from the inductive pick up, they like to crack from where the vacuum advance moves the assembly and will cause this. have to change out the pick up if they are cracked.

It could also be an intake valve hanging open. a compression test will catch this. SeaFoam makes a terrific product for this, pour one through the carb while running, add a second to your oil....

 

[diesel4me]

might be..

ok so I took my truck out after about 2 years of not being driven on the street. (it was started and moved from time to time) Well it runs pretty good but when I get on it at 1/2 throttle or more it starts to back fire through the carb like crazy. Like i said when I parked it, it worked fine and we did start it a few times to move it and all was well. The only thing that appears wrong with it is I need to adjust some of the rockers as they are making a bunch of noise, but I don't think that pertains to this problem.

454 in a 49 chevy p/u with a th400 and a 10bolt with 3.08s


The noisy "rockers" might very well be the trouble!...sounds like a few valves might be sticking in the giudes,that will cause backfiring and noisy valves...I'd add some kind of "Valve Medic" or "Marvel Mystery Oil"to the crankcase and run it awhile and see if the noise goes away (and your backfiring with it) before pulling off the valve covers..your accelerator pump might have croaked during its long nap too-and a lean mixture will backfire easy..

 

[sweetk30]

same problem with my 454 was bad cam. flat lobes. ran great till more than 1/2 gas . will almost be sure worn out cam. the bbc eat them more than sbc.

 

[nemesis_pyros]

New Problem, Old problem fixed thanks for the Help.

Fixed the rockers, runs nice and quiet now . Also found a vacuum leak at the vacuum advance port on the Distributor. Runs much much better now! just needs some tuning.

I got a new porblem now the bushing in the tail shaft on my TH400 is chewed and it ends up walking back on the yoke and pushing the output seal out of the tranny allowing me to lose all my tranny fluid. Question is can I just clean up the yoke a little if its scored a bit, then get the bushing and put it in myself with the tranny in the truck. I talked to a shop and they said i had to take the tailshaft off and the bushing had to be perfectly seated. The out put seal is brand new and so are the u joints. The yoke wobbles so bad that it feels like really bad u joints.

 

[dave_90_blazer]

Fixed the rockers, runs nice and quiet now . Also found a vacuum leak at the vacuum advance port on the Distributor. Runs much much better now! just needs some tuning.

I got a new porblem now the bushing in the tail shaft on my TH400 is chewed and it ends up walking back on the yoke and pushing the output seal out of the tranny allowing me to lose all my tranny fluid. Question is can I just clean up the yoke a little if its scored a bit, then get the bushing and put it in myself with the tranny in the truck. I talked to a shop and they said i had to take the tailshaft off and the bushing had to be perfectly seated. The out put seal is brand new and so are the u joints. The yoke wobbles so bad that it feels like really bad u joints.

one other thing to check, take your distributor cap loose and see if you can wiggle the rotor, also check the pickup reluctor wheel, ive seen both worn out and loose, causing reallly wierd drivablity issues