Jetting size
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Jetting size

  1. #1
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    Default Jetting size

    Need help
    Stock 460 bbf 18 ft sthwnd
    780 holly vac sec.
    What size jets should I expect to find in prim and sec. Ran poorly popping backfiring surging. Ran great last week on the first of the season but had more weight in and skiers today. plugs are stark white on quick inspection. I've always suspected it runs lean, perhaps tuned for altitude at arrowhead. Oh and it ran real hot today. I'll pull the jets tomorrow. I run it at sea level by the way.

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  3. #2
    Senior Member ol guy's Avatar
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    you might want to check the fuel pressure at the carb. If you do a jet check anything less than a 78 primary is lean. I have a feeling the carb is starving for fuel I.E. clogged filter or weak pump.

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    Quote Originally Posted by ol guy View Post
    you might want to check the fuel pressure at the carb. If you do a jet check anything less than a 78 primary is lean. I have a feeling the carb is starving for fuel I.E. clogged filter or weak pump.
    Brand new filters i have a clear one that is full of fuel so I'm thinking I'm ok there ... It's an elect pump and I can hear it running to prime it...

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    You need a inline fuel pressure gauge to be sure ,at least 7psi and check if it drops below 2 psi on acceleration had the same problem on the kids boat put a new FPR and good to go.
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  7. #5
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    Hooked up a gauge haven't run yet will find out . How about plug gap ? Auto lite 24s

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    Fuel press appears to be okay

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    72 in the prim 84 sec. Anyone?

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    How about pwr valve size? G 6 5? I know y'all are out on the lake but help a brother out

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    test
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    Is 72 in the primary too lean? Also it seems to start acting up when hot so I checked and found a bunch of sand in the block I've cleaned as much out as possible. Someone suggested vaporlocking. So we'll see if it runs cooler and better? Any input on pwr valve? Thanks for testing.

  13. #11
    Some guy obnoxious001's Avatar
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    First step, what is the "list" number on the carb choke horn.

    If you have a used carb, good to look up original specs. If it's a 780 vacuum secondary carb,, the 3310 series had several versions,, like 3310-2,,, 3310-3,, etc., and jetting will vary.

    6.5 is probably the stock power valve, but it might require changing if your engine creates lower than 12 lbs vacuum at idle. You can get on Holley's website, but the power valve number is related to measured vacuum at idle, so that when you put a load on the engine and vacuum decreases, the power valve will open and supply additional fuel.

    If, in fact, your carb is a 3310 model, 72 would be the correct primary jet, but the secondaries would not be in the 80's. The later models do not have secondary metering plates with jets as they came from the factory.

    3310 780 (F)72,(R)76 10.5,8.5 4150
    3310-1 780 (F)72,(R)76 10.5,8.5 4150
    3310-2 750 (F)72,(R)plate 134-21 6.5 4160
    3310-3 750 (F)72,(R)plate 134-21 6.5 4160
    3310-4 750 (F)72,(R)plate 134-21 6.5 4160

  14. #12
    Some guy obnoxious001's Avatar
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    PS,, the power valve adds 6-8 jet sizes, depending on who you talk to, in effect making the primaries 78-80, instead of 72.

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    Air horn says 6r 3385 b

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    Still pops and backfires under even a light acceleration. Tried a double pumper off my car and runs the same as the vac sec. I suspect it's the msd. Any thoughts on diagnosing it. It's a ready to run unit

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