Hi Im new to this forum and hoping that I can finally get the help I need that isn't $80-$100 per hour.
I have a 91 Firebird Formula that has a 350 TPI with automatic trans. The Engine runs alright but just doesn't quite seem right. When it is in nuetral it has a slight shake to it (you can feel in in your seat and the steering wheel). Then when I give it some throttle and rev it to about 1500 or higher rpm it shakes more violently. The rpms don't jump or miss when it is idling though; however, I also noticed that the car takes longer to crank over than it used to, but only by a couple seconds, and one time I started it it backfired right when it fired up, but that only happened the once. It also surges slightly when I am driving through town just feathering the throttle. But it does not hesitate and it runs strong when I step on it.

If anyone has experienced this before or knows what the problem could be please help me out. Here is a list of things that have been done just to eliminate some suspicions.

-100 miles on rebuilt engine and heads
-New fuel Injectors
-New fuel Filter
-New plugs, wires, cap, and rotor(autolite platinum plugs gapped to .035)
-New air filter
-New EGR valve
-deleted smog pump and catalytic converters
-checked all vacuum lines for leaks(found none)
-Checked intake gasket for leaks(tight seal)
-Tested TPS .61v closed 4.35v Open
-Car is completely stock, I know everything ever done to it because my dad bought it brand new.

 

has everything you listed really been done within 100 miles?

If so, you either have

to check the timing
have a bad plug wire
have a cracked insulator on a plug
bad rotor
bad cap

in other words, it's ignition related.

 

I just changed the plugs and it didn't change the way it ran. The timing is right at 6 degrees BTDC(stock timing).

How to you check the cap and rotor? How high is to high for resistance on a plug wire?