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Thread: Engine feels like it's "breaking-up" after initial hard acceleration.

  1. #1
    Join Date
    May 2018
    Location
    Fair Hill, MD
    Posts
    76

    Default Engine feels like it's "breaking-up" after initial hard acceleration.

    Hey guys, late last summer my engine developed this issue where it'd feel like it was breaking up or coughing a little after a strong initial acceleration. Does it in 1st and 2nd gear, 3rd seems fine. I just started diagnosing yesterday as I've been busy with other projects on the car. Goes down the road fine and accelerates everywhere, but at full throttle just fine.

    What happens is I punch it from a slow roll around 3000 RPM and it'll initially pull hard for a second or two, then break-up for a second or two, then pull hard to 6000 RPM where I shift. It doesn't pop or backfire, nor are there any signs of black smoke. Other than the cough, it pulls pretty hard everywhere else.

    I tried a few different pump cams, and none of them have really done much as far as correcting the issue, just changed the severity of it. On the stock orange cam it felt the worst. White cam improved it as did the green, but they're on different ends of the spectrum right? I also tried changing the secondary springs and that did nothing to affect the issue.

    Carb is a Holley Street HP 750 with vacuum secondaries. It was rejetted to match my old Street HP (went to the aluminum model for the current carb) that didn't have this issue. I jetted that carb based on testing, so I assumed it would be the correct setup here as well. The only other difference between the setup with the old carb and the current one, is a switch to an electric fuel pump. Long story short the fuel system is what comes with the Holley Sniper EFI system with a bypass regulator to control the fuel pressure. It pulls harder with this fuel system than it ever has before, so clearly something was lacking in the fuel delivery department before. It may be a totally unrelated issue, but I felt mentioning the fuel system change was a good idea. It didn't do this breaking up when I first put this carb on, it developed after about a year with no other changes to the tune or setup.

    The rest of my setup is 10.3:1 (static) 350 SBC, Comp XR294HR cam, Edelbrock Performer RPM heads and Air-Gap intake with a 1/4 plastic spacer between the carb, Summit CDI box (MSD 6AL), MSD distributor that I've had for a long time, Blaster 2 coil. Timing is 14° initial, 34° all in by 3000 RPM. This hasn't changed in years. Primary float level shows fuel about halfway up the sight glass. Secondary fuel level is about 1/3 of the way up. This carb has the 4 corner idle circuit.

  2. #2
    Join Date
    May 2018
    Location
    Fair Hill, MD
    Posts
    76

    Default

    I should add that I raised the front of the car 4", and it didn't do this before doing so. 2" initially with springs, then another 2" recently with stock height spindles. It did start after the initial spring install. Could the float level and additional nose height be starving the secondaries for fuel after initial acceleration?

  3. #3
    Join Date
    Jan 2010
    Location
    Brisbane, Australia
    Posts
    1,661

    Default

    I'd say you might want to try a wedge spacer: https://www.ebay.com/itm/Moroso-Carb...AAAOSw-45eS5OU. If you have a secondary metering block, I'd add main jet slosh tubes, and no secondary power valve, main jets split primary to secondary by about 8, so 72/80 as example. Gary
    Regards, Gary

  4. #4
    Join Date
    May 2018
    Location
    Fair Hill, MD
    Posts
    76

    Default

    The wedge spacer is a good idea. I picked up a pair of jet extensions a while back, but never installed them. It does have a secondary block, but it's vacuum, so no secondary power valve. I forget what my jets are, but I did follow the traditional guidelines for the number split.

  5. #5
    Join Date
    Jan 2010
    Location
    Brisbane, Australia
    Posts
    1,661

    Default

    Sounds like you have some idea what to try now. Fit the jet extensions first as a test. Gary
    Regards, Gary

  6. #6
    Join Date
    May 2018
    Location
    Fair Hill, MD
    Posts
    76

    Default

    Switched back to the white pump cam on hole #1 and raised the secondary float level a hair. Improved the flat spot, but it's still there. Interestingly enough, it now spins the tires after the flat spot. Didn't do that before. I'm guessing it's a compound issue.

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