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  1. #1

    Join Date
    Nov 2013

    Yam 1100 63M conversion to Yam 1200: Additional Pulse Fitting- Correct Location

    The Yam 65U 1200 cases have three OEM pulse fittings: two on the Generator side and one on the PTO end.

    I recently purchased a locked-up Yam 63M 1100 engine that had seized after only 3 hours of operation per the Seller. The former owner never determined the root cause of the piston seizure.

    Before I spend a thousand dollars rebuilding this engine, I was wondering...

    This particular set of 63M crankcases also has 3 pulse fittings (two "aftermarket" drilled and epoxied in by former owner); however two of these pulse fittings are on the PTO side, exactly opposite of the 65U 1200 OEM pulse fitting arrangement.

    Does it matter where the center carburetor pulse line fitting is located? Which end of the engine is the correct location? Why not install the center pulse fitting in the middle?

    Thank you very much in advance for your feedback.


  2. #2

    Join Date
    Aug 2006
    Generally each carb pulse fitting is connected to the crankcase section that is for that carb/cyl. You do not want to tee all the pulse lines together or you will weaken the signal to the pumps. You also do not want the pulse line to be over 10-12 inches long and you would like them to be "uphill" from the engine to the carb so oil vapor can drain back to the engine.
    So the answer for you would be yes, the center of the engine for the center carb, and on the side the carbs are on.

  3. #3

    Join Date
    Nov 2013
    Thanks for your thoughtful response. Good Technical Tips on the Pulse Line set-ups.

    But why does the Yamaha Factory, for the OEM 1200 65U engine, place two pulse fittings on the Magneto end, one on the PTO end, and none on the Center intake tunnel?

    And does having the reverse pulse fitting locations on my 63M 1100 crankcases (two on the PTO end and one on the Mag end) allow the carbs to work correctly?

    Do all three Pistons hit Top Dead Center 120 degrees out of phase? Or do two pistons hit TDC at the same time? I'm asking this because the 1100 engine seized within 3 hours of running, as stated by former Owner, and I'm wondering if the "backward" Pulse Fitting locations have a role in this Engine failure...

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