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  1. #1

    Mikunni carb jets

    I have a new engine 750 SL that I want to use premix fuel , do I increase fuel metering, if so to what?? thanks Tony

  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
    Join Date
    Jul 2007
    near Toronto, Canada
    Welcome to the Hulk

    What year and model 750?

    If you want to pre-mix the oil into the fuel, then you do not need to change the carb metering jets. You should open up the high speed adjustment screws on the carbs by maybe 1/8 turn from factory spec.

    Do you have a reason for changing to pre-mix? You will burn roughly twice the oil compared to running the oil injection system (if it has the variable rate oil pump).

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  3. #3
    Click avatar for tech links/info, donation request K447's Avatar
    Join Date
    Jul 2007
    near Toronto, Canada
    Pre-mixing the oil into the fuel is a completely separate discussion from running the engine 'lean'. Lean burn (as in piston damaging) refers to the ratio of air to fuel, not the ratio of oil to fuel.

    Some folks do use the terms 'rich' and 'lean' when referring to how much oil they are mixing into the fuel, but I view that as being a confusing use of the term.

    By far the primary factor in preventing lean burn combustion (which means there is too little fuel for the large amount of air going through the engine at high throttle levels) is the carburetors and the fuel system feeding those carburetors. The best approach is to keep the fuel system well maintained, and keep the carburetors internally clean and operating to specification..

    This is true regardless of whether you continue to use the factory oil injection system, or pre-mix the oil into the gasoline.

    The carb's internal jets and the metering adjustment high-speed screws determine how much fuel gets drawn into the engine at high 'speeds'. If there is too much fuel for the air, then the mixture burns incompletely, with sooty exhaust. This is burning too rich. Overly rich combustion also reduces the power from the engine. Surprisingly, rich fuel mixtures tend to leave the piston tops looking clean with minimal carbon deposits. This is called excess piston 'wash', since the piston tops look washed clean.

    Too little fuel for the amount of air going through results in much higher temperatures during combustion. The extra heat can burn carbon from the fuel onto the piston tops, making them look very black all over.

    If the fuel to air mixture is far too lean the temperatures get so hot that the tops of the aluminum pistons literally begin to melt, usually right in the center. If the engine continues to be run like this, in short order there is a hole melted right through the piston top - which is not good.

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