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Thread: 4150 750 double pumper mechanical secondaries, hard start choke doesnt help.

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  1. #1

    Default 4150 750 double pumper mechanical secondaries, hard start choke doesnt help.

    Hi All Carb gurus out there I need a hand.
    Engine 351C 2V CHI heads
    Edelbrock Air-Gap
    Camshaft
    Intake Duration @ .050": 227°
    Exhaust Duration @ .050": 227°
    Advertised Intake Duration (@ .006"): 281°
    Advertised Exhaust Duration (@ .006"): 281°
    Intake Valve Lift with OE Rocker Ratio (1.73): .542"
    MSD 6AL Distributor without vacuum advance.
    Automatic transmission FMX Stall speed 2400 RPM.
    Electric fuel pump delivers 6-7 psi, with gauge.

    Okay so here goes. The carb is brand new from factory, stock jetting should be 70 main.
    Fuel bowl check.
    Ignition timing is 19°.
    Idle four mix screw set.

    I'm having a hard time starting it, I have to give it some gas otherwise it will die.
    If I put in some choke it dies directly. I have an Innovate Lambda meter inside the car.
    When the Engine is at operating temperature the idle is about 1200 rpm in park and AFR is 12.3
    In gear the idle drops to 800 and AFR is 13.4-13.6 just right I would say.
    If I shut down the Engine and restart it start directly without even giving some gas.
    Cruising is about 13.7 - 13.9, which is good I believe.
    WOT is about 10.6 - 10.9, which I Think is a little bit low.
    No bogs, no backfire no hesitation.
    Plugs look just fine.
    Is is that this combination cant idle in gear with an AFR that is around 13.6
    when Engine is idling at about 12.6 the plugs get fouled.
    Should I back off the timing a little bit? Best regards, Rob

  2. #2
    Join Date
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    Hi Rob. What curve are you using in the distributor if you have 19° at idle? You need to remove the carb, look at where the throttle butterflies are in relation to the transfer slots on both primary & secondary. The primary side is more important. The slot should resemble a square. Close the primary, and open the secondary to get to this preliminary adjustment, then refit the carb. Don't touch the primary side to get the idle speed you want, use the secondary. Gary
    Regards, Gary

  3. #3

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    Hi Gary. The distributor uses silver springs from MSD with an special weight. It adds only 10°. So 19°+10°=29° at 4000 RPM. The black original weight gives you 18°+18°=36°. That's too much for aluminum heads. And the 351C wants a rather high timing to run OK on idle. The slots are squared on the primary butterflies. The seconds are fully closed. I have the option to open them without removing the carb.
    Limiter bushings: https://www.google.se/url?sa=i&rct=j...54795534003510. Regards, Rob

  4. #4
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    Hi Rob. How far out are the mixture screws? Possibly wants a smaller idle feed restriction per metering block?
    I'm interested in this weight you have mentioned, I have seen a 10° bush for MSD distributors.
    You should check your timing at 1200, and see if it's the same in gear at 800.
    If you have an adjustable secondary screw, that does help.
    I have a 850 with XP main body, (idle bypass), Rapid jet metering blocks, externally referenced PV, etc. Gary
    Last edited by Gaz64; 04-08-2019 at 03:13 AM.
    Regards, Gary

  5. #5

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    Hi Gary. The mixture screws are about 1 turn out, I started with 1.5 turn. I'll check timing at 1200 RPM as you said. Here's the company that make them http://www.4secondsflat.com/Ignition.html. Regards, Rob

  6. #6
    Join Date
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    Hi Rob. What's the list number of your carb? Yes, I have seen the bushing you mentioned, I made my own. I though you meant an advance weight, not a bushing, back further in the thread. Gary
    Regards, Gary

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