BLACK Z !

Hi, Okay hopefully you guy's can help, I have had no luck at any of the local garages and dealers (yes I have also tried to fix the problem myself many of times). I have a 1986 Z28 camaro, it had a 305 lg4 and a auto from the factory, It now has a gm 350 replacement motor and a wc t-5 (the t-5 was behind the 305 for awhile). When ever I am decellrating and the rpms are above 2,000 I get a constant after fire thought the exhaust, if I give it a little bit of gas it will clear right up. I can sit and rev it up and it will not backfire once, it will not do it while I shift either. The motor runs great, the only mod I did to it that the 305 did not have is a edelbrock performer intake and the msd 6al. I have tried disconnecting the 6al and it still does it. I also had the carb rebuilt ( it is on the car now). I have tried a jet perf stage 2 carb and still no change. I currently have there stage 2 chip for a 305 in the car. I did change the knock sensor and the module over to the 350 pieces. The car is in perfect shape tune up wise. People tell me that is this is normal for carbed motors, but the 305 never did this, and I also had a summit 350 in the car before this and that motor never did this It had a 650 double pumper and a vac advance dist. If you could please give all the help you can it makes me want to sell the car.

dj haf

uhhh, my car backfires too when i let my foot off the gas to cruise... it's normal. it happens more often now because i took off my cats i think it gives the car a dope image when im passing by ppl and all of a sudden my car starts to go "pop pop pop!"... then i usually hammer the gas so that ppl who heard me backfire forget about it cuz they're too busy noticing how loud my car is lmao

motorhed

Did you change the exhaust at all , I belive that has to do with less back pressure. It would probably do it more with performance exhaust and no cat. Also I am assuming it does not have the air pump on it anymore, a bad air bypass valve off the air pump will cause this as well .

90rocz

Yeah, what happens is, fresh air gets sucked back into the hot flaming exhaust and mixes with left over fuel and ignites. Usually from a short exhaust, like open headers, or an exhaust leak.
Normal for these conditions....

If the exhaust is OK, maybe the valves aren't sealing good or a vacuum leak is causing a lean backfire.

BLACK Z !

The exaust is the same has been on the car with the stock motor, the summit motor and the New motor, The whole air pump system was new, and it was doing it so I took it off to see if that was the problem and it is still doing it, so I just left it of. The cat was gutted on all 3 motors and I have since taken it off and put a straight pipe in it's place and it is still doing it. The thing is it never did it with the 2 other motors never, and the summit motor had a lot more pwer then the current one.

BLACK Z !

The exaust is the same has been on the car with the stock motor, the summit motor and the New motor, The whole air pump system was new, and it was doing it so I took it off to see if that was the problem and it is still doing it, so I just left it of. The cat was gutted on all 3 motors and I have since taken it off and put a straight pipe in it's place and it is still doing it. The thing is it never did it with the 2 other motors never, and the summit motor had a lot more pwer then the current one.

P.S I have replaced all of the gaskets on the exuast and went over it a bunch of times trying to find a leak.

c1989rs

check ur timing and make sure itz not to high or low the other problem might be itz not tuned right i had this problem andi fiddled wit the timing and it doesnt do it ne more and i have the borla exhaust which flows well just my opinion hope it helps

c1989rs

check ur timing and make sure itz not to high or low the other problem might be itz not tuned right i had this problem andi fiddled wit the timing and it doesnt do it ne more and i have the borla exhaust which flows well just my opinion hope it helps

DanD 96 SS

my money is on - the valves need to be adjusted. Just my thought does anyone else agree?
Dan

IROC-T

I agree with you if what he is talking about is a down shift compression kind of back fire,you get this kind of back fire alot with solid lifters. The rocker could need adjustment,and,or the timing could be off,check the cam degree also. Is your vacuum advance hooked to the right port on the carb(constant/metered),and have you checked to make sure the advance diaphram is not leaking? Just a few things to check.

BLACK Z !

I have the timing set at either 6 or 8 btdc, I forgot and that is with teh electronic connector unpluged, I don't have a vacumm advance on this car, the lifters are not soilid, and teh motor is a new GM 350 replacment motor so the vavles should be set right.

IROC-T

My bad,I thought your post said you had vac advance,just because it's new does not mean it's adjusted right. They are adjusted dry many times and this will get the motor started,and running but after a run in period you should check and readjust everything. If your cam degee is off by one tooth you could get this problem also,and in a assembly line situation this is intirely possible,could even be a sticky lifter(try some Marvel Mystery oil). Is this popping excessivly loud it could be a bad exhaust leak at a donut seal caused by back pressure durring decelleration,most engines do this it's just not as pronounced with stock exhaust. I may be way off base but I really think it's in your timing,wether it's distributor,or cam related,or valve train.

BLACK Z !

Is there anyother way to check if it si the vavles with out pulling the vavle covers?

IROC-T

Not really,I know they are a pain to pull,so start on the drivers side that's the easiest.