'97 LT1 breaking up at high revs

[brianko]

Working on a '97 LT1 Camaro, 58mm TB, 1.7 rockers, Holley intake, no cam. Pulls like a homesick angel through 4K. After 4K, it begins to (for lack of a better term) "break up," like it's starving for fuel or hitting the rev limiter. WB looks good, about 12.8, so plenty of fuel. Not even close to the rev limiter. I added 2 degress of timing between 4K and 6K, above 60kPa, and that seemed to alleviate some of the problem, but the stutter is still there. Without KR data (for some reason the HPT scanner won't log it), I don't really want to throw more timing at it. Owner thinks that maybe the valve prelash is set too tight.

Any thoughts?

[EagleMark]

You have no idea how many times I have fixed this with a fresh set of plugs! Even if they look good and only have a few thousand miles on them. Years ago we only got 3-4 1/4 mile runs from a set of plugs but they would run fine on street low RPM forever. OE plugs too! No fancy snake oil plugs.

Second place I would look is at EST/ICM. Again GM only... if the cap and rotor are within service limits.

HTH!

[96lt4c4]

I have seen on some highly modded LT1 engines running the stock ODB2 computer, breaking up at high RPM. They swap over to the earlier 94-95 PCM and it goes away. Not saying this is your problem as yours sounds more mechanical.

What I am describing is a more subtle break up at high RPM that may not even be felt.

[brianko]

Yes, that's it, very subtle. I think you're probably right about it being mechanical or electrical. We've cooled tge icm, replaced plugs, readjusted lash, no luck yet.

[1project2many]

Spark can be checked with oscilloscope. Same with injector pulses.

1.7 rockers... spring condition??? Floating valves can seem a lot like ignition issues. Lifter pump up can be more pronounced with larger rockers. Where's the preload now? Try running zero lash if not already there. Or switching to smaller rockers.

[brianko]

Well, problem has been resolved: Reinstalled stock pushrods, readjusted valve lash, found a plug wire arcing on headers. Went with hotter plugs (Autolite XP104), gapped to .045 (.005 under stock). Runs like a champ now!