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  1. #1

    Explain carb setting differences?

    OK...after my "second" rebuild" (finally did a new top end like I should have the first time....hello!), will someone please explain the technical reason why my three mikuni carb settings are different on my '93 650. I understand the reason, basically, when I have a single fuel pump traveling sequentially thru the 3 carbs, but when I have a triple pump, why do the carbs require more or less fuel mixtures? I plan on adjusting them to be a little rich because I DO NOT want another burnt piston!


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    The three cylinders do not flow identical quantities of air.

    Each cylinder feeds into the exhaust manifold differently as the manifold itself is shaped differently for each cylinder.

    The cooling water flow is also slightly different from cylinder to cylinder, in part caused by the exhaust manifold.

    On the early Fuji engines the cooling water also flows through the crankcase which may add to the water flow variability.

    If you have the restrictive 'snorkle' style flame arrestor then the air flow into each carb may also differ slightly.

    It all adds up to differences that the carburetors need to accommodate.

    Polaris was still learning about proper jetting and tuning in the early years (they only started selling watercraft in 1992). Each production year there were changes to the carburetors and the factory settings. Some service bulletins were issued describing revisions to the prior model year carb settings.

    And when ethanol gasoline came around a few years later that affected what jetting was appropriate.

  3. #3
    thank you! That makes sense! I have made all the necessary changes to everything, but I have not changed the air intake. In fact, I just read last night about some minor changes that can be made to the rubber snorkle thing! I'm trying my best to make this thing real stable and reliable, since all my grandchildren will be on it! I don't care if it has peak rpm's or speed; I just want it to give them some fun and bring them back to shore!! Thanks very much!

  4. #4
    Click avatar for tech links/info, donation request K447's Avatar
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    Well, no matter how reliable the machine may be, always have back up options in place.

    Ride with another watercraft or boat whenever possible. Carry communications gear to summon assistance (VHF radio, cell phone, flag, flares, etc).

    Teach everyone who will be operating or riding passenger the basics. If the operator gets hurt, the passengers should know how to work the machine and get everyone to shore, if not back to the dock.

    If the machine fails, know how you are going to handle it. If it actually sinks (yes, they can sink) how will everybody get to shore?

    By being prepared for the worst cases, you can enjoy the water knowing the risks have been mitigated.

    And of course, driver training in proper operation of the watercraft and the rules of the water!

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