Good starting range of jet sizes on holley carb?

Discussion in 'Engine Topic' started by AllGoNoShow, Oct 18, 2019.

  1. AllGoNoShow

    AllGoNoShow Veteran Member

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    Trying to get my old motor started as it has been sitting for a few years. It is a 350, roughly 9.8 compression, AFR 195cc heads, 230/236 solid cam at about .500 lift, Dart intake.

    Carb was a Holley 650 double pumper but was modified sometime ago with a 750 Proform main body, Quick Fuel Billet Metering Blocks and Billet Throttle Plate (I guess it's like a knock off Holley HP now). When I took it apart I noticed I had 75s in the primary and 89s in the secondaries. These seem way too rich and I'm not sure what I was thinking. What is a good starting point for my combination?

    Nick
     
  2. 1980RS

    1980RS Veteran Member

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    Most Holley 650's are like 68-70 pri jet and 76 sec jet, but this is with the pressed in air bleeds. With a Proform main body the main air bleeds might be larger which then would change the emulsification that's why it may need larger jets in the rear. 89's in the sec are pretty large but what matters is the fuel ring on the spark plugs at their base what the A/F ratio is at WOT. I run a QF 750 and it has 80 front and 82 rear on a 461 BBC right now that has gone 11.67
     
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  3. G72Zed

    G72Zed Veteran Member

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    Looking at your engine & carb parts parts combo, and to keep it simple & balanced I would suggest you start with 72 in both the Primary and Secondaries with a power valve in place in both locations, the QFT metering blocks should be machined for this, and the starting place for their PVCR's are in the .059 range. Considering the Proform/QFT 750 main bodies usually come with a 73 IAB and a 36 HSAB you will be close. Also take note of the metering blocks E package, I think they are a 4 channel setup in most cases set @ 28/28/plug/28.

    If it's a dual plane, with fully divided plenum, you could possibly go down to 70/71's as it may in fact have a very good booster signal. either intake, you would have to test the mains on a steady state cruise RPM to see for lean surge, or if you have an AFR gauge, it would tell you this right away, and WOT as 1980RS states.

    A 12-14 step jet spread would indicate no rear PV and somewhat large primary PVCR's to have a carb properly calibrated front to rear on WOT if the carb's venturii is the same.

    Hope this helps out.
     
  4. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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  5. AllGoNoShow

    AllGoNoShow Veteran Member

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    Thanks for the tips and references. It does have the 4 corner idle, the air bleeds that are mentioned, but no secondary power valve. I ended up putting a 72 in primary and 80 in secondaries and will go from there (seems like Holleys have a spread of 6-10 jet sizes up on most of their double pumper carbs with no secondary PV).

    Started her up today and did well...I have an innovative O2 sensor that surprisingly still worked and I got the idle Air/fuel at high 13s/low 14s. However, even just reving it in neutral there is an immediate lean spot when you hit gas (goes to 16-17)...guess I need to adjust the accelerator pump cam (has a 31 squirted on it now). Was also surprised it only pulled 13 inches of vacuum (230/236 solid cam at 110 separation) at 800 Rpms with 24 degrees timing (10 +14 vacuum)...would have thought it would have been 15 or so.
     
  6. Z28zz383

    Z28zz383 Veteran Member

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    Make sure you have a 50 cc squirter set up or it will go lean on tip in. I run 73 pri and 80 sec with no rear pv
     
  7. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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    A 50cc squirter on a 9.8:1 350 cid? Does anyone else think that's a bit overkill? I never ran that on my 470hp 383cid.

    I had a 750 DP with a Proform HP style main body. Plugs were a lot cleaner with 70/79 jets. With that Proform main body, you can also tune with idle and main air bleeds. Honestly, I got tired of constantly tuning for weather and whatever and bought an Eddy ProFlo 4 setup. Best thing I've ever bought for this car.
     
  8. G72Zed

    G72Zed Veteran Member

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    The Holley P-S jet spread is based mainly on the primary PVCR size, everything else being equal if not using the rear PV.

    First off, do not "tune" the idle to a specific "predetermined" AFR ratio, use a vacuum gauge and/or tach to set you idle circuit to highest vac/rpm setting. If you set it on an engine that has not been fully brought up to running temp, with full intake heat soak, it will run lean once it is.

    I asked before, is this an open plenum intake? if so, the "750" Proform/QFT main bodies sometimes comes in a larger venturii size, like in the 1.400+ range, and I have flowed some of them, they are more than "750", that being said, what you are seeing is common with your setup.

    Dial in/adjust the pump cams, add in 6-10* initial timing, take away 4-6* of the vacuum timing with a limiter, and it should come around.

    What is the CR of the engine now, cranking pressure test done?
     
  9. G72Zed

    G72Zed Veteran Member

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    Perhaps he meant 50cc REO pump.

    Most "650-750" are in the 28-35 "shooter" range, and do fine with the 30cc pump.

    I hardly tune for weather, the last 2 races are late in the year, no jet or bleed changes. My carb WON both races, 10k cash, no tuning.
     
  10. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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    I agree that on a drag strip, an EFI system will not out perform a carburetor. On the street, and with NASCAR going EFI, that's a different story.

    For shooters, I ran 32's with a 30cc pump.
     

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