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Thread: 850 carb pop on hot start only.

  1. #1

    Default 850 carb pop on hot start only.

    Hello. I have a freshly built BPE 496 with a Holley 850 installed. Truck starts fine when cold (just turn key don't have to press gas pedal), but when it sits for a while hot, it becomes hard to start and more times than none it'll instantly back fire on first turn over before I get it started. I'm running a 1/2 phenolic block on a Warrior intake, engine never runs over 200°F while cruising. Truck runs great once it's started. I have made minor adjustments to the float bowls and idle mixture screws prior to first test drives, but haven't made any carb adjustments for this issue. Any help from this forum would be much appreciated.

  2. #2
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    Can you give the list number of your carb please? How far out from seated are the mixture screws? Gary
    Regards, Gary

  3. #3

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    It's actually a brand new Proform 850 part number 67214, but I'm told by them tune the exact same as a Holley 850. I adjusted all four idle screws one at a time to get the highest RPM at idle for each. Thanks!

  4. #4
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    Are all your mixtures the same turns from seated?
    You DO NOT turn one out until highest RPM, and then go to the next, etc.
    They should all be out the same distance.
    And of course Proform will tell you same as Holley, a good copy.
    Some more details of your engine combo, camshaft, etc.? Gary
    Last edited by Gaz64; 07-09-2020 at 06:48 PM.
    Regards, Gary

  5. #5

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    BluePrint engines
    HP & Torque: 575 HP / 555 FT LBS
    Compression Ratio: 9.6:1
    Aluminum Heads
    Hydraulic Roller Cam
    Forged Crank
    Hypereutectic Pistons

    Block:
    NEW BluePrint Cast Iron Block, 454 BBC
    Manufactured and Machined in the USA!
    4-Bolt Main
    4.250" Stroke, Bored .060" over
    1-Piece Rear Main Seal
    Mech fuel pump provision: Yes
    Standard Deck

    Rotating Assembly:
    New BluePrint Forged Crankshaft
    4.250" Stroke
    OEM type 6.385" connecting rods with 150,000 psi bolts.
    Hypereutectic pistons
    External

    Cylinder Heads:
    BluePrint Aluminum Cylinder Heads - PS8013
    119cc chamber
    2.30" intake/1.88" exhaust valves
    316cc intake/124cc exhaust runners
    Cylinder heads feature a .300 in. raised exhaust port vs an OEM casting.
    Must verify header/exhaust clearance

    Cam Specs:
    Cam Type: Roller
    .566 Intake/.566 Exhaust
    242° Intake/248° Exhaust duration
    @ .050 - 112° lobe separation

    Ignition Timing:
    10-16° and total timing 32-34° at 3500 RPM

    Warrior single-plane intake, BPE HEI, MSD wires, entire engine is brand new, less than 5 running hours.
    The electric choke was removed for initial startup (did want the idle to stay high or it interfere with the very first time I started it) and has not been re-installed, I've wired the choke horn plate or lever open, but can re-install if you feel that's the problem.

    So to be clear on the idle mixture screws, if I adjusted the first one for highest RPM and used that same distance out on the next one, but it brought idle back down, that's the correct procedure and move on? I could use some clarity on how to adjust those screws. Thanks a million!

  6. #6
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    That's good info you have there. A backfire when hot is generally an ignition source, hot combustion chamber, ignition crossfire, etc. I'd seat all of your mixture screws, counting the turns in, and note how many turns on each, take the average, then wind them back out to your average of all four as a starting point. Note how each one reacts at 1/16 steps, and open or close all four to try to keep them the same or near close to each other. Sometimes one corner of the engine may want a bit more or less. A vacuum gauge also helps. If your screws average out further than 1.25 turns, I would say the idle feed restrictors could go up one step. I'd like 180°F while cruising. Gary
    Regards, Gary

  7. #7

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    Quote Originally Posted by Gaz64
    A backfire when hot is generally an ignition source, hot combustion chamber, ignition crossfire, etc."
    How would idle mixture screws improperly set cause those conditions or how are they tied together? Thanks again.

  8. #8
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    Do you have them all equal turns out now? And what ignition components are you using? Leads, cap, rotor, retard module etc? Gary
    Regards, Gary

  9. #9

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    I was able to get idle mixture screws adjusted for highest vacuum and all about the same. No pop for now on start but still rich, burns your eyes with a 3.5 power valve and 5-7 inHg manifold vacuum, jetted down two sizes from 78/86 to a 76/84 might have to drop two more sizes, will let the plugs tell me after some more test runs.
    Last edited by 496C10; 07-17-2020 at 04:10 PM.

  10. #10
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    You're getting close for best lean idle adjustment, but your camshaft is the reason for burning eyes.
    You need a wideband A/F ratio gauge, and datalogging capability, and ultimately the use of a dyno.
    A power valve even partly open at idle does not contribute to richness.
    Idling at 5-7 inches, a PV of 2.5 to 4.5 would be OK. Even a 6.5 would be OK if you want the the PV to kick in a little earlier on part throttle load. Gary
    Regards, Gary

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