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07-20-2006, 05:47 PM #1
GM 572 crate motor problems - need help/suggestions/opinions
I'll try to keep this brief, but need to list all the things I've tried.....
I have a brand new GM 572 (620hp model) installed in my '70 Nova. I'm running a Centerforce DF flywheel/clutch/pressure plate, a Tremec TKO 5-speed and 3.42 gear. Running 110 leaded race fuel.
The engine idles great and when accelerating ANY amount it runs out smooth and STRONG all the way to redline. The problem is when trying to hold a constant mph. When doing this, the engine surges and stumbles, NO popping/backfiring, more of a back-n-forth surge. The mph, rpm or gear doesn't matter, if I'm holding the throttle still it's the same surge/stumble. When stumbling/surging you can hear the change in engine tone through the exhaust which makes it sound like a miss or carb loading up, but it's not.
I initially thought it was a carb problem when I posted my previous thread (http://www.pro-touring.com/forum/showthread.php?t=20887), but have since ruled that out as a cause. Here are the things I've changed/tried to try and resolve the surge/stumble:
- Changed the primary jets in the 850 Demon both up and down from baseline - no change
- Plugged the power valve in the 850 Demon - no change
- Swapped out the 850 Demon with a BRAND NEW 950 Holley - no change
- Changed the initial timing from 20 degrees (44 advanced) to 12 degrees (36 advanced) - no change
- Swapped out the MSD HEI distributor and GM wires with a new electronic distributor and wires - no change
- Changed the ACDelco plugs for Bosch Platinum plugs - no change
07-20-2006, 07:14 PM #2Registered User
- Join Date
- Nov 2004
It sure sounds like it's lean on the primaries. When you jetted up the Demon, did you only go from 85 to 87? Or did you try larger than 87 as well? My understanding is that sometimes you won't see any change with a small jet change because of the tolerance of the jet orifices (i.e. the 85 could be on the large side and the 87 on the small side).
07-20-2006, 07:27 PM #3
Are you getting constant/correct fuel pressure to the carb?
Check for vaccum leaks?
hope ya get it going.-Ed Nelson
1970 Corvette. -Sold
07-20-2006, 07:29 PM #4
07-20-2006, 07:32 PM #5Originally Posted by Hidro
I haven't checked for vacuum leaks, but will tomorrow. I'll spray some carb cleaner around the carb base and intake gasket areas.
07-20-2006, 07:34 PM #6Registered User
- Join Date
- Aug 2004