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  1. #1

    Rotary valve question on challenger 1800

    1997 twin 787 motors. I bought a short block and got a new rotary valve but I'm having trouble running it without getting backfire in the rear cylinder. I assume the timing is still off or I have the wrong size valve. My valve is 160 degrees. I have timed it to 147 on opening at tdc. I read on another site that you need 130 at tdc. Anyone really know what I need or if i even have the correct degree valve. thanks!


  2. #2

    Rotary Valve Timing on 787 engine.

    Here is a quick rundown on timing the Rotary valve on the 787 engine.

    get a TDC gauge and bring the MAG piston up to TDC.
    DO NOT ROTATE CRANK!!!

    For opening mark, first align 360 line of degree
    wheel with BOTTOM of MAG side inlet
    port. Then, find 147 line on degree wheel and
    mark crankcase at this point.

    For closing mark, first align 360 line of degree
    wheel with TOP of MAG side inlet port. Then,
    find 65 line on degree wheel and mark crankcase
    at this point.

    Remove degree wheel.
    Position rotary valve on shaft splines to have
    edges as close as possible to these marks with
    the MAG piston at TDC.
    NOTE: Rotary valve is asymmetrical. Therefore,
    try flipping it over, then reinstall on splines to obtain
    best installation position.
    Apply BOMBARDIER-ROTAX injection oil on rotary
    valve before reassembling rotary valve cover.
    Remove TDC gauge.

  3. #3
    thats the best explination thank you so much.
    One question is you wrote in caps do not rotate crank. Is this after I get it to TDC? Don't I have to rotate it to get it to TDC? Sorry if thats obvious and i'm missing it.

  4. #4
    Quote Originally Posted by pnkrckchiro View Post
    thats the best explination thank you so much.
    One question is you wrote in caps do not rotate crank. Is this after I get it to TDC? Don't I have to rotate it to get it to TDC? Sorry if thats obvious and i'm missing it.
    find TDC on the Mag piston, and leave it there, do not rotate the crank after you found TDC, correct...
    make your marks, I used a straight edge to carry the degree mark to the cases by keeping the edge of the straight edge on the center of the rotary shaft and the end on the specific degree, make a mark with a fine pen, or scratch awl... good luck!...

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