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  1. #1
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    Default 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
    I've called Barry Grant, my GM Performance Parts dealer, and anyone else that would listen to me whine. I've tried all their recomendations - nothing has helped. PLEASE HELP, I'm open to ANY opinions/suggestions. I'm at a total loss.

    Thanks!


  2. #2
    Join Date
    Nov 2004
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    Default

    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).

  3. #3
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    Default

    Are you getting constant/correct fuel pressure to the carb?
    Check for vaccum leaks?

    hope ya get it going.
    -Ed Nelson

    1967 Firebird.......
    1970 Corvette. -Sold

  4. #4
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    Yes, when I jetted up the Demon I only went from 85s to 87s on the primaries. Interesting thing is when I changed to the 930 Holley (#78 primary jets & 6.5 power valve) the 'surging' decreased by 40%-50%.

    78

  5. #5
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    Default

    Quote Originally Posted by Hidro
    Are you getting constant/correct fuel pressure to the carb?
    Check for vaccum leaks?hope ya get it going.
    Thanks Ed. Fuel pressure is a constant 6-6.5. If it were a fuel pressure issue I'd expect it to fall on its face when I 'stomp it'. Instead it runs like a scalded, pissed-off cat! :3gears:

    I haven't checked for vacuum leaks, but will tomorrow. I'll spray some carb cleaner around the carb base and intake gasket areas.

  6. #6
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    It sure sounds like a lean condition. Any chance of a vacuum leak away from the carb. Maybe the back of the intake, vac. line going to brake booster, the booster itself? Sure hope you get this figured out. I want to here some good reports on this motor...and maybe a good burn-out video too.
    Brett

  7. #7
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    Quote Originally Posted by Norwoodx55
    It sure sounds like a lean condition. Any chance of a vacuum leak away from the carb. Maybe the back of the intake, vac. line going to brake booster, the booster itself? Sure hope you get this figured out. I want to here some good reports on this motor...and maybe a good burn-out video too.
    Brett
    OH it'll do a burnout, I just can't drive it down the freakin' road and hold a constant speed!

    I don't have power brakes so the booster is out of the equation. BOTH carbs I've tried have only 3 ports, one for the PCV (which is plumbed to the valve cover), and two on the carb base plate which are both plugged. The only vacuum leak it could be would be around the intake.

    If I had that huge of a vacuum leak wouldn't it have trouble idling?

    Thanks for the assistance Brett!

  8. #8
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    Nov 2004
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    Quote Originally Posted by dhardison
    Yes, when I jetted up the Demon I only went from 85s to 87s on the primaries. Interesting thing is when I changed to the 930 Holley (#78 primary jets & 6.5 power valve) the 'surging' decreased by 40%-50%.
    Demon and Holley jets do not follow the same numbering scheme. If I'm not mistaken, a 78 Holley jet is larger than and 87 Demon jet.

  9. #9
    Join Date
    Aug 2004
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    Default stumble

    I may be way off but here is a thought. Your fuel pressure may be good but the fuel volume may not be enough. There is going to be an amount of fuel in the bowls to allow short blasts without starvation but a constant steady speed may drain the bowls and eventually starve and lean fuel to the the enigine.
    What size line do you have and what is the volume capacity of your pump?
    I have a friend who had a 500+ c.i. motor in a mopar and it never ran right until he changed his fuel line. Good luck!

  10. #10
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    Quote Originally Posted by Josh
    I may be way off but here is a thought. Your fuel pressure may be good but the fuel volume may not be enough. There is going to be an amount of fuel in the bowls to allow short blasts without starvation but a constant steady speed may drain the bowls and eventually starve and lean fuel to the the enigine.
    What size line do you have and what is the volume capacity of your pump?
    I have a friend who had a 500+ c.i. motor in a mopar and it never ran right until he changed his fuel line. Good luck!
    Thanks Josh. I'm running a 1/2 aluminum line from the tank to the engine compartment. From there I have #8AN line everywhere else. My pump is a Carter street pump but will flow up to 8psi.

  11. #11
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    If possible,re-install the Holley 950 and jet up the primarys.

  12. #12
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    Nov 2004
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    Quote Originally Posted by USAZR1
    If possible,re-install the Holley 950 and jet up the primarys.
    I agree.

  13. #13
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    Quote Originally Posted by USAZR1
    If possible,re-install the Holley 950 and jet up the primarys.
    Thanks guys. I'll give that a try as soon as I replace the broken axle the 572 (and M/T drag radials) caused last night at the track.

  14. #14
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    Quote Originally Posted by dhardison
    Thanks guys. I'll give that a try as soon as I replace the broken axle the 572 (and M/T drag radials) caused last night at the track.
    Dan, did you get any times before the carnage? Sorry to hear she's down again.....
    71' Nova....LY6/6 speed Swap In Progress

  15. #15
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    Quote Originally Posted by NvrDun71
    Dan, did you get any times before the carnage? Sorry to hear she's down again.....
    My first run I cut a .500 light and ran a 7.68 @ 95mph (1/8th mile track). I ran three times before the axle let go and the best was a 7.60@95. Considering my suspension is in NO way set up for drag racing I was pleased. I figure it's a 11.60-11.70 at around 120-125mph car in the 1/4 mile.

    Oh and my worst light of the four runs was a .508 and that was when the axle snapped.

  16. #16
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    nice!! remember, if you're not breaking any parts, you're not making enough power!!!
    What kind of 60' were you pulling with those drag radials?

  17. #17
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    Yeah, I really wasn't all that bummed (or surprised) when something broke. When it happened I thought my TKO had let go, luckily for my wallet that wasn't the case.

    My best 60' was a 1.7810. Not bad considering I'm running shortened Hotchkis front coils, slapper bars and I still had the front sway hooked up. When the car launched it immediately lifted the front end then immediately slammed back down due to the front coils and sway. It was a WILD RIDE!! Here are the details of the runs.

    Run #1
    Reaction: .5000
    60': 1.8408
    330': 5.0719
    1/8 ET: 7.6819
    1/8 MPH: 95.36

    Run #2
    Reaction: .5026
    60': 1.8721
    330': 5.1529
    1/8 ET: 7.7841
    1/8 MPH: 95.36

    Run #3
    Reaction: .5078
    60': 1.7810
    330': 4.9910
    1/8 ET: 7.6020
    1/8 MPH: 95.20

    Run #4
    Reaction: .5014
    60': 1.7970
    330': 5.0228
    1/8 ET: 7.6432
    1/8 MPH: 95.10

  18. #18
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    Aug 2005
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    My 396 always ran like what you have described, and I shrugged it off as having too much overlap on my cam and therefore confusing the air in the chambers as to which way to go while just cruising. Any other 572 owners who can post their timing and jet specs? How do the other 572's cruise?
    68 Chevelle SS396. Pro-touring in Progress
    Pro-jected 396, TKO 600, 3.90'd 12 bolt, LS1 brakes.

  19. #19
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    Scrap the Demon carb there junk,nothing from Holley will interchange with a demon.I had a Demon on a 427 and it ran like crap tried every tuning trick nothing changed it.Switched to a 750 holley and cured all problems on install.

  20. #20
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    I've spoken with 6-7 other 572/620 owners. None of them have the surging problem or valvetrain noise mine has. At this point I'm in the process of trying to get GM to take it back and get me another motor. It would suffice if they'd just fix the issues at hand. I've done my homework and the GMPP service guy has no additional suggestions for a fix. He's working with GMPP corporate to see what they'll do.

    I've already swapped the Demon for a 950 Holley. There was no improvement to the surging with the Holley. I'm convinced it's an internal valvetrain issue.

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