Okay. its my old 98 ls1 block with new balanced ls6 crank/rods/pistons/rods with arp rod bolts with all new bearings a highflow oil pump with trex cam. The intake is a vic jr with 90mm tb. Speed density tune.

After getting the tune done (mail order) it idles great and after it warmed up I gave the throttle a few cracks to about 5k. Well it backfired on one of the revs and now its snafu.

It still idles normal but has trouble reving to anywhere near 5k.

It has full oil pressure still. All the springs are good and it has 0 compression problems. I dont hear any knocking, but then again the motor is kind of loud. All the plugs looked a little rich except for #6 and #1 which looked like they were running a little bit lean.

Would a rod/main bearing spun cause my symptoms?

 

No engine codes or misfire codes either.

 

Are you monitoring fuel trims, maf and spark advance as you do the throttle snap? Have you used a vacuum gauge during the snap. You should have 16 and you should see a dramatic drop during the snap and a jump to 24 after the snap ending up back at 16. A pressure transducer and labscope may be needed to narrow this down. Could be a sticking valve or poor sealing, but you need to confirm that before you do a tear down.

 

Thanks for the input. I found that the flexplate bolts came loose and that the flexplate (doesnt look damaged) was clanking around a bit on the crank. Everything is tightened up now, but the motor still stumbles at higher(4k up) rpms. Could the flexplate being loose have caused internal damage?

 

I just found out the starter wasnt disengaging. Could that cause backfires at higer rpms too?

 

If you have found those 2 major issues, i would make sure everthing is all tightened down, injectors seated right, etc.