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Thread: 1390 big bore

  1. #1

    1390 big bore

    I purchased a 1999 1390 Group k XLL without any other specs available from the previous owner so i have a few questions.

    My compression is 125, 122, 127 psi across, what is the highest compression i can run on pump gas?
    Can the 1390 head be shaved or will further mill work be required?
    I have the 48mm Novi / Mikuni Carbs with 125 low speed and 150 High speed Jets and the craft is very rich, what Jet size should I start with when i reducing the size? It starts perfect and idles for a while without loading up.
    I dont have my Tach as yet but I GPS it at 60 MPH with no additional response after 2/3 - 3/4 Throttle i would like at least 68 - 70 mph.

  2. #2
    Happily Self-Employed WFO's Avatar
    Join Date
    Nov 2005
    Houston, Texas
    yes, you can mill the head for more compression. Depending on what fuel you have available, usually you can go up to 150 psi.

    Group K leaves the compression lower for reliability, so that's something to consider. If you want more power, you can look into aftermarket reed cages or maybe an advent ignition for better timing control. Also if you are riding in rougher water, the HO pump and dynafly impeller will yield better hook up and accel, and more top speed than the older style pump.

    You really need a tach to see where your rpms are.

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