06GT85Z28

I have an 85 Z28 with a 350 swap. The engine is new, somewhere around 2-3,000 miles on it. I've been having a backfire problem where it will backfire though the carb when accelerating from a dead stop and will sometimes backfire when climbing up a hill. I really have to feather the throttle to get it moving and even then it will sometimes backfire and stall out. My idle RPMs are sporadic it moves between 500-1000. I have 6" of vacuum at idle. I have bought a new distributor and If I remember correctly my timing is set at 36 degrees and has been checked by two shops that said it was correct.
Since then I have replaced my power shot to a 2.5, replaced the orange cam to a blue cam in the 1 position. Replaced the "squirter" from a .31 to a .35 and I'm still having the same problem. I'm at my wits end and need help! My motor specs are posted below.


350 ci engine

Comp Cam XR282HR roller cam
RPM Range 2200-5800 Lobe Seperation 110 Intake Centerline 106 Duration Intake 282 Exhaust 288 Duration @.50" lift Intake 230 Exhaust 236
Valve Lift Intake .51 Exhaust .52

Valve Timing @ 0.006 Lift:
Open Close
Intake: 35 BTDC 67 ABDC
Exhaust : 78 BBDC 30 ATDC

Comp Cam hydraulic roller lifters
Comp Cam Magnum HP Chromoly Pushrods
Comp Cams Magnum double row timing set
Melling High Volume Oil Pum
Speed_Pro Hyper-eutectic race pistons
Eagle SIR I_Beam race connecting rods
Eagle Crank
Mahle Performance Piston Rings
Edelbrock Performer RPM Aluminum Heads
Edelbrock Performer Intake Manifold
Holly 670 cfm Street Avenger Carb
K&N Air Filter
Champion 3 Row Aluminum Radiator CCC951 with mechanical fan and shroud
World Class T5 manual transmission
2'73 Rear End.

pancherj

Lets hear about the timing that the shops said is correct. 36 degrees doesn't tell us much. What is the timing advance at idle with the vacuum advance unhooked? With it holed back up (you are using a vacuum advance distributor...right?)?

Once we get that all nailed down, we can move on to touching the carb. For now, I would not do ANYTHING to the carb except set the idle. You don't need a 2.5 power valve. I would put in an 8.5 it was me and start from there. Timing first...carb second.

JaBoT

Like pancherj is saying the timing probably isn't right. You should have way more than 6" of vacuum at idle. As an example comp got 9" @ 800rpm from their test motor. So something isn't right. http://www.compcams.com/Technical/Dy...2HR-10_001.asp
I would probably have a competent mechanic use a piston stop and degree wheel and make sure 0 on the balancer and timing tab are actually 0 to start. Then reset the advance in the distributor, but that depends on which one you have.
Then after all the timing is done you can adjust the carb.
O and a 2.5 power valve (not power shot) is crazy low. There is no reason yo should ever need to run a power valve that low. At least for now put the 6.5 back in it untill the timing is fixed.

bestracing

Like the two post above we need more info.

I'm also interested to know what distributor you have. New/Old, Factory/Aftermarket, HEI/Points/Other, Vacuum advance/Mechanical advance?
Ignition Coil - New/Old, Factory/Aftermarket, HEI/Canister/Racing?
Timing at idle?
If vacuum advance, ported vacuum or manifold vacuum?
Checked for any vacuum leaks?

If you would of had a flat tappet cam I would have wondered if you had a cam lobe or two gone bad. I had that happen to me and my vacuum levels were in the dirt, choppy idle, carb would back fire similar to your situation. But the longer I messed with it the worse it got after the initial break in. I had 4 lobes worn down with 0 miles on the car and ~4 to 5 hours of run time on the cam. This was before most people knew about ZDDP and not much was posted on Crane or Comp Cams web site about it.