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Thread: Two Quick Fuel Q-750 Problems

  1. #1

    Default Two Quick Fuel Q-750 Problems

    Setup: Quick Fuel Q-750 - nowhere near stock. I tuned this carb a few years ago and just started having a second problem. I’ve always had the first, but the second is new.
    Cam: 230°/236° at .050".
    Idles smooth at 14 inHg.
    Timing - Initial 16°, vacuum advance 10° connected to manifold, total 34°.
    I can get the IABs, MABs, IFRs, TSRs if needed. The carb was running great, then it wasn’t.
    All 4 corner idle screws at about 3/4 turns out. 1/16th of a turn in or out will drop the idle to 13 inches and it drops about 100 RPM. They are very sensitive.
    Plugging any of the IABs while idling does nothing to the vacuum or the RPM.
    Primary transfer slot is square. Secondary idle screw is used to set the idle. I don’t know exactly where this is on the transfer slot. The car is a kit-Cobra and essentially spends 75% of its time in the primary idle and transfer circuit. Maybe < 5% of the time are the secondaries opened at all.

    First problem - Always dies on hard deceleration. I have the vent whistles in both bowls, I have the jet extensions in the secondaries. I have adjusted the fuel level from mid-sight windows to where it is now at the lower edge of the sight window - no change. I have cut the vent tubes at an angle away from the venturis, no change.

    Second problem - recently developed. Popping on deceleration. Again, this is new. No changes to the carb, intake, or exhaust. I sprayed carb cleaner down all the air bleeds - no improvement.

    The plugs actually look rich. No oil fouling, not wet with fuel. Just a bit black. Smells rich - my eyes burn and I have to take a shower if I stand next to it for more than 5 seconds.

    I realize popping is usually lean. The carb seems to run rich. I don’t think I have an exhaust leak - my headers have safety-wired bolts. I will check the exhaust system closely for leaks. No intake leaks.
    Last edited by Texasdoc; 04-21-2020 at 04:45 PM.
    2012 LoneStar Classics 1965 427LS Cobra Replica, Navy Blue, Silver Stripes, 408W Stroker, 500HP, 600 Ft/lb, AFR 195 Renegade Heads, RPM Performer Intake, Quick Fuel Q-750, Tremec TKO600 5-speed, Ford 9-inch Rear End, 3.50:1 Gear Ratio.

  2. #2

    Default

    Engineis a 351W stroked to 408. Max RPM is 6250. Idles at 900. More data in the signature line.

    OK, I took the carburetor apart and measured everything. Just in case someone can look and see that something is way off. Maybe I'm masking a big problem with another cover-up.

    Confirmed that both the primary and secondary transfer slots were square. The primary is minimally over square, but really close.

    Primary Jets - 71
    Primary PV - 8.5 - (this works for my driving style)
    Primary PVRC - unknown, I didn't take out the PV to measure these

    Secondary Jets - 81
    Secondary PV - plugged

    Primary and secondary are equal for these:
    IFR - .029
    IAB - .060
    HSAB - .023
    TSR - .070

    These are much different than the stock Q-750. As it came from the factory, it was rich everywhere. 10:1 at cruise was not uncommon.

    With the current settings (back when I had my AFR gauge installed) the idle AFR was 13.0 Idle mix screws were about 2/3 to 3/4 out from seated and are very sensitive.
    Cruise AFR was about 16 under light load, progressing until the PV would open and then it would drop to about 13.
    WOT would start about 12 and stay around 12-13.
    Last edited by Texasdoc; 04-21-2020 at 08:00 PM.
    2012 LoneStar Classics 1965 427LS Cobra Replica, Navy Blue, Silver Stripes, 408W Stroker, 500HP, 600 Ft/lb, AFR 195 Renegade Heads, RPM Performer Intake, Quick Fuel Q-750, Tremec TKO600 5-speed, Ford 9-inch Rear End, 3.50:1 Gear Ratio.

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