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  1. #1

    new member with 91 650sx rebuild questions

    hey all, if this isnt the right forum mods please move it......Just bought a 91 650sx for 250 bucks. Has a pipe, looks like it has a head on it also, IT says "pro" dont see any parts numbers, ride plate etc etc. Bought it with the dude saying it has low compression.

    Story is that he rebuilt it, let a friend ride it etc then sat since 2012. Took a compression reading and it was 120/130. Poured gas down the carb and it fired right up. But once it got hot compression dropped to 90. So off came the head. Looks like its had some piston slap. i can move each one about 1mm side to side. Has STD pistons in it. Who sells the best rebuild kit and what size should i go with? Thank you. Already has the oil injection taken off from what i can tell but still has the crank case drain so i know i need to block that off.


  2. #2
    DavidR's Avatar
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    Moving the top of the piston isn't a good indicator of wear, pistons are smaller at the top. Measure cylinder wall to piston clearance about an inch above the base of the skirt. You may be able to just hone and replace the rings or new STD pistons.

    If the cylinders need to be bored, the machine shop could tell you what oversize to get. WSM seems to have a descent piston for a good price, I just got mine for $61 each on eBay, and they have a coated skirt for about $10 more.

  3. #3
    SplishSplash's Avatar
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    Post an engine pic. So we can identify the aftermarket parts. Pro is probably ocean pro

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  7. #7


    Stock cylinder?

  8. #8


    The choke plate and screws are gone, I wonder if they went through the motor

  9. #9


    Said ride plate was aftermarket....is it?

  10. #10
    DavidR's Avatar
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    No way choke plate and screws went through the motor without MAJOR damage. Previous owner removed them and probably installed a primer. Pistons don't look too bad. Measure skirt to wall clearance and if it's in spec, hone cylinders, replace the one scored piston and re-ring the other one. If out of spec, new over-sized pistons and bore cylinders.

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