rapid backfire in the intake and through the carb. seems to be rpm and load dependent. Revving in neutral I can get it starting with rapid small backfires at around 2500 rpm in the intake and it gets worse as the rpms go up. it will not rev past 4500. Does the same under a load, but only worse. I am at a loss.

Car is a 1986 trams am, 305, used to be a tpi, but converted to carb i'm guessing that it was done in an effort to track the problem down, but who knows. the last owner was more interested in trying to rip me off than giving me information. All if the computer stuff is gone out of the car, and it runs a vacuum hei ignition and a non computer quadrajet

Idles and starts smooth
18.5 inches of vacuum, holding steady at idle
165 psi in each cylinder
6psi fuel pressure through range
timing set at 10btdc ( works best at this setting )
12 volts to the HEI feed wire

Things I have done:

3 different quadrajets , all the same no change
2 hei distributers (complete)
2 different coils
new cap, rotor, plugs, wires
new valve springs
swapped carb base gaskets a couple times
Adjusted valves a couple times, all were in adjustment at one turn past zero ( when I readjusted them I went 1/2 turn) -- no difference
Eyeballed the rockers a few times to see if cam was moving them all, and they looked like they were all working

PArts I have swapped came from good running vehicles if not new, the old parts were put on different vehicles and they run fine --

I just cannot understand whats going on with the backfires, Ive been through carbs, ignition, compression, springs and cannot figure out for the life of me why it has this rapid intake backfire. Please help.

I considered the cam, but wouldn't that show up it idle as a miss and on the vacuum guage, and the rockers should of been out of adjustment?

Help, I cant afford to throw parts at this car and get no results and I am tired of trying to find the problem with no progress.

 

You have not mentioned the exhaust , any possibility of a clog ?

Also , and I ask in all seriousness , are you really 110% really REALLY sure your wiring order is correct from your distributor to the plugs ? I ask because with 165 psi on all cylinders and 18+ inches of vacuum it sure sounds like the major mechanical aspects of the engine are OK . With the trying of different carbs and all the other parts you changed it made me wonder if it could be something as simple as an overlooked mistake in the firing order ???

 

Ignition related 99% of the time.

 

Wires are in correct order, even used the manual for firing order instead of by memory. I would of thought ignition but so far anything ignition related has made no difference.

One possible clue maybe? The timing cover was once off as evidenced by the silicone around the gasket. Possible to much camshaft advance? Maybe the previous owners advanced the gear to much?

Its been frustrating, trying to figure this out from before christmas. The issue seems to cause car to be gutless, and hog gas badly. Its like a smoke cloud of unburned fuel follows the car down the highway. It doesn't really use any oil either.

 

Funny that , But one of my first thoughts was cam timing , but I dismissed that notion when I saw 165 psi and 18 " of vacuum . My thought was that if the cam timing was off enough to cause backfiring , it should have shown up also as low compression and or low manifold vacuum , correct ? That's what got me thinking to things external to the engine like maybe clogged cat in the exhaust or a simple error in the firing order , both of which could cause backfire (although granted , a clogged exhaust usually will also produce a symptom of low vacuum , something you havn't got) ... Still , just for the heck of it , I'd try disconnecting the Y pipe as an experiment and see if it still backfires . If that don't pan out , you may just have to pop that freshly installed timing chain cover off and have a peek in there .

One other quick thought , if the timing chain cover shows recent work , it is a certainty that the Harmonic Dampener has been removed and reinstalled . There exists two possibilities with this , one , that the original one got broken in the removal and was replaced with another that fit but has it's timing marks in the wrong place ? Or that they messed it up while pulling it , damaged the rubber separating the two pieces of it , and now the outer piece has slipped relationship with the inner piece (the hub) and is now showing a timing mark in the wrong location ?

Have you tried the "time it by ear and give the engine what it wants" method , to see if the timing marks you are using with your light are actually correct for the engine ? For instance , if setting it by the marks at 10 degrees is causing the popping , is there ANY position that it doesn't pop at ? Cause if there is , that could be a great indicator of improper timing marks .

 

Timing by ear it ends up about where it is now

Would over advanced cam timing, build more cylinder pressure at a static test? And low rpm( under 2000)rpm. But as rpm raises be out of sync? Possibly causing something like this? I don't know

 

Sounds like it jumped time. Pull the timing cover off and have a look. May have had a improper cam install or worn gear and chain jumped time.

 

I was just going to ask something similar, how would it run if , they put the chain and sprockets one tooth off, advanced?

How would a car run like if it jumped time?

Anyway to pull cover and reinstall without dropping oil pan? Its a 1986 motor

 

For what its worth 165 is the highest that the gusts has ever read. It was cold engine and dry (w/out oil put in cylinder) with this guage 90 is a good reading on everything, a fresh stock l98 only reads 120.

 

How many miles are on this engine? Does the engine go back to smooth idle when you let off the gas after it has been backfiring?

 

181,000 on odometer.
Yes returns to smooth idle

 

Fixed,
The last guy had put a new timing chain in it but installed it one tooth off, works great now after fixed