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

  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
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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

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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

  7. #7

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    Hi Gary. Sorry mixing things up with the bushing. TGIS is the one I have HOL04779C. HOL04779C 750 CFM Four Barrel Street/Strip Carburetor. Mechanical Secondaries; Manual Choke; Model 4150 Double pumper. Regards, Rob

  8. #8
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    Hi Rob. Your 4779 is quite common. I have a 4779-3 a few years back. Not enough adjustability for me now. My carbs have replaceable everything.
    Do you have a vacuum gauge to tune your idle mixture by?
    A 400 RPM drop is way too much, and either the carb is to blame, or a looser convertor of about 2800 RPM could be on the cards.
    Your carb is also too rich at WOT, I'd rather see around 13:1 (12.7-13.1) & 14.5 - 15.0 for cruising.
    So we need to pull some fuel out of the idle circuit, and the PVCRs, some smaller mains to start with. Gary
    Last edited by Gaz64; 04-08-2019 at 05:59 PM.
    Regards, Gary

  9. #9

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    Hi Gary. I have the Innovate AFR wideband Lambda to tune it with. I have smaller jets 68 to put in. Should I change the secondaries too two steps down? I've now retarded the timing just a bit and the difference in idle to park is just about 200 RPM drop now. I have in idle with drive about 10-11 inHg vacuum. Should I use a power valve that's 4.5? Regards, Rob
    Last edited by Mach173; 04-09-2019 at 12:42 AM.

  10. #10
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    Hi Rob. I'd drop the primary to 68 or even 66 if you have that, and just concentrate on the primary side for now.
    Your vacuum at idle is OK to stay with a 6.5.
    So now if you have pulled the timing back to say 15° at idle, will you not have enough timing for power?
    You might need a 14° bushing, which you could make from the 10°, or make you own. Gary
    Regards, Gary

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