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  1. #1

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    3/4 Lean on a 951 - true or not ?

    I keep reading about a 3/4 lean spot on 951 engines caused by aftermarket filters.

    Standard flame arrestor causes air flow restriction so lower pressure in the throat aiding fuel flow. Free flowing filters mean less restriction so no (reduced) low pressure so less fuel flows.

    The theory i understand but is it a real issue? The fuel flows into the throat by a pressure difference but surely the main reason for that pressure is the fuel pump which is driven by crankcase pressure.

    Has anyone used aftermarket filters and had a problem with 3/4 lean ?

    I do big distances on my ski to waves so i really need 3/4 lean reliabilty,

    Thanks,

    Phil


  2. #2

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  3. #3
    PEST!'s Avatar
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    I was trying to get rid of the "lean spot" on my wifes 02XPL . it has a coffman pipe, vtech reeds a ADA Rec head. bunch other stuf but you get the drift. I ended up looking at MicroTouch's race ignition and discovered 98xpl had less advance in the range wear the lean spot is. That and the 2 deg retart over 7k helped make an engine that doesnt melt pistons any more and can run through a wide rpm range without excessively lean spots. Im very happy with it. Those mods and a scat pump have made an excellent wave jumper and cruiser. Took lots of time and testing. My notebook was the biggest help. so I could remember how it ran last week to effect the next change. You can get an mpem with 98 curve or get yours done. Dont bump up the timing as it will make it worse. IF you want to see the curves go to http://www.revlimiters.com/index.html they have curve charts on the page for their ignition

  4. #4
    PEST!'s Avatar
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    it also has to do with the throttle switch and its timing curve

  5. #5
    If i knew,...I wouldn't ask sea-one's Avatar
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    I read on PPG's site awhile back stating that you can't get rid of it...i dunno

  6. #6

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    So although it's lean at 3/4 - does it cause issues? is it so bad to destroy pistons ? I read the groupk stuff and also read "bens secret mods".

    Groupk reckon there is a lean spot - ben - seems to think it's not an issue.

    I suppose if it were a destructive reliability issue there would have been heaps of posts saying how their ski's melted.

    So anyone had a 3/4 lean issue bad enough to damage engine?


    oh yeah - mine is a 98 xp so from what you say it has the better suited curve already

  7. #7
    PEST!'s Avatar
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    "mine is a 98 xp so from what you say it has the better suited curve already" and its cheep to put on a rev limiter only one coil. There is a lean spot and it will be a problem the closer you get the ski to maximum output. Stock there is a built in buffer. As you hop up the output you get closer to the point were the engine will meltdown from lean condition or detonation. Detonation can be caused by lean conditions , compression, and heat. they can be caused by any number of things. OLD gas, to much water in the exhaust, not enough water in the exhaust, dirty filters, bad or wrong plug, leak in the exhaust manifold, plugged or dirty jets ect.ect.ect. . I have had a ski stick lots of times from riding at 3/4 throttle even with just a level 1 mod. By modding your engine you are changing the way the engineer had it set up. They set them up so your average dope can run it WFO all day and 99% of the time something can go wrong without a major meltdown. Yamaha is so popular because the buffer is so big its near impossible to blow them up. for the first 100hour. Then the exhaust valve falls into the piston. But there old engines are almost indestructible. 701-760-1100- It takes a 1100 to do what an 787 can do on a Doo though

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