454 erratic timing

I have a 1990 454ss truck that i recently bought, the truck was sitting for 6yrs. I Replaced water pump, all water hoses, intake gaskets, egr valve, distributor, cap and rotor, sparkplugs, sparkplug wires, two injectors and cleaned tbi. The truck has lost of power and backfires from the tbi when driving. When checked for timing, The mark on the balancer moves up and down. I checked for compression and it's good 145psi on 6 cylinders and 150psi on two cylinders. The guy that i bought the truck from told me that somebody had replaced the cam. Iam starting to think maybe there's a flat cam lobe, or the cam is to radical causing the knock sensor to not work properly. I need help.... :CRY: Thanks
SAL

Maybe a bad timing chain, or the cam not installed correctly. Bring #1 cylinder up on TDC and see where the timing mark is....

What timing light are you using? Be warry of newer advance style lights. I just helped a friend diagnose a problem with his from autozone. Timing mark was wandering around. Make sure 5 and 7 are not crossed.

Kitz

Timing light is a digital matco forgot part number. Engine is coming out this weekend.:(
Thanks. I'll keep posted on what I find.
Sal

Any digital or dial back timming light will show timming jumping.. I never seen one that don't yet.

I had a slipping damper pully and tried several timing lights turned out he dumper pully was rotating around the rubber ring. Ed

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Originally Posted by Racecar100

Any digital or dial back timming light will show timming jumping.. I never seen one that don't yet.

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My Snap-on did when it was new.... I figured for $400.00 it better not!!!!! Had my Snap-on man send it back to be checked.. Whatever they did to it, it works great ever since, even on the MSD boxes...