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  1. #1

    Low compression on #2 cylinder on 787

    Hi, I just got my rebuilt supercharger installed on my GTX SC, thanks to Jerry's great work, and my buddy was riding my 787, while I tested the SC, when the 787 quit. It tried to start, but was obvious it was running on one cylinder. I have oil in the bilge, and the back (#2) cylinder seems very low on compression. The engine turns by hand smoothly, but very little compression compared to the front cylinder (but there is some compression and suction as I turn it over by hand). I'm thinking a seal or gasket let go, but not sure where to start. Anyone have any ideas? Thanks, Mike

  2. #2


    I was hoping someone had an idea... Do these engines ever "blow a head gasket?" There is distinctly something not holding much compression. I could be something like broken rings, but is that commmon? Is there a seal that blows that will give this kind of symptom? Since one side is fine, I don't think it's the rotary valve. Oil in the bilge is another indication, I'm just not sure what or if it's related. Isn't there some kind of check valve in the oil injection system that can cause a problem, but that sounds like it would affect both sides. Once the piston reaches the top ports, then a leak at a gasket or seal shouldn't effect compression unless it's the head gasket or in that area. So, I'm leaning toward pulling the head first. Any ideas here? Thanks, Mike

  3. #3

    Not much going on in the 2-stroke forum?

    Hey, Well I pulled the head and the back cylinder seems to be scored. It's not too bad, but in need of fixing. Anyone know of any deals on 787 cylinders and pistons? Thank, Mike

  4. #4
    785xp's Avatar
    Join Date
    Feb 2006
    You can bore out the cylinder you have. I am sure someone here has a connection for pistons. OEM and Pro-X are the ones most go with for the 785.

    Pull the filters out of the carburators to see if that was the cause. If you have the grey fuel lines, change them.

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