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Thread: Step up to 600's or Not?

  1. #1
    No Drama
    So here it is in short, 21' 4300 # cat, 468 chevy/ tunnel ram TRX2, 450 holleys, jetted 58Â’s and 81Â’s, still show running fat on plug check. Tried every conceivable power valve and jet combo. Absolutely no difference. Idle is smooth, go to take off, it falls right on itÂ’s face, just stumble. Throttle up fast into secondaries runs like a stud. I bought a pair of 600 Holleys, as per recommendation. Spoke with my Dyno man, he sais, IÂ’m running to much fuel as it is. What to do? IÂ’m ready to sink the damn thing. Any suggestions?

  2. #2
    pgf127rt
    No Drama, there is something terribly wrong with your setup, how much HP did your engine make? What kind of heads are you running? Are those carbs vacuum secondaries, if not have you changed the squirters, the jetting sounds real lean on the primary side, I have a built 468 with good square port heads on a 20' Glastron Jet which is close to the weight of yours I am running a holly 850 annular discharge carb which is jetted at about 80 square, with 50 squirters, and it runs very well, put up more info on your engine and maybe some of the experts can help you.

  3. #3
    RumRunner
    No Drama:
    First you do not want to have more than a 10 number split from the primary side to the secondary side on your carburetor. Secondly since carburetors work based on air velocity, putting larger carburetors on this engine will make it run leaner !

  4. #4
    Thunderbutt
    N D I run 2-750s on a tunnel ram with 50cc pumps on both end of the carbs with out power valves and have no problem. It seem your to lean on the primary side. I don't see why your builder is saying you have too much fuel. I think it stumbles at take off because it needs more fuel and when the sacondarys kick in it gets it. I assume you have the right heads and cam to go with the tunnel ram on the 468. Did the 450s come with the manifold? I've never seen 450s on a tunnel ram and a bbc. but who am I! Originally posted by No Drama:
    So here it is in short, 21' 4300 # cat, 468 chevy/ tunnel ram TRX2, 450 holleys, jetted 58Â’s and 81Â’s, still show running fat on plug check. Tried every conceivable power valve and jet combo. Absolutely no difference. Idle is smooth, go to take off, it falls right on itÂ’s face, just stumble. Throttle up fast into secondaries runs like a stud. I bought a pair of 600 Holleys, as per recommendation. Spoke with my Dyno man, he sais, IÂ’m running to much fuel as it is. What to do? IÂ’m ready to sink the damn thing. Any suggestions?

  5. #5
    No Drama
    First I want to thank all of you for your replies, very informative and gave me some other avenues to look at. Secondly, I have stock square port heads, vacuum secondarys, and the smallest squirters available. CanÂ’t give you the size because the boat is in the shop having the outdrive serviced. I do have another question pertaining to the same problem: Have any of you used or find it reasonable to run progressive linkage on my set up? My thought is if both carbs are linked together itÂ’s squirting twice the fuel for this motor. Hence if they opened individually, one before the other, allowing vacuum to generate and giving a more proportional air fuel mixture. Am I talking out of my ass?
    Thanks again,
    No Drama

  6. #6
    bruleracer
    take the (list number) off the carb, go to the Holley catalog,put stock jets and pv's back in it. I'd bet your close.
    jet sizing is based on venturi diam. in general,, the fact that you have a 58? in the primary is the problem!

  7. #7
    bruleracer
    ok. i just went back and read your question again.. what the heck is a 450,, never seen one never want to. Carb size does not equel
    (to much fuel) carb size = air flow,(cfm, cubic feet per minute) to much fuel is the wrong jet for the venturi diam. the engine is an air pump, if it pumps less air the signal to the booster is redused, and does not pull as much fuel through the same jet.
    An example wuold be the fact that when a carb has the correct jet, it will run on a 454 as well as a 350 with almost no changes.
    again, go to the Holley catalog and start with the basics...

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