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Thread: Cylinder honing

  1. #1

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    Cylinder honing

    Last fall the pto piston holed on my SLT780 so over the winter I rebuilt the carbs and top end and went .25mm over on all the pistons.
    Well after about 10 min. the pto piston holed, so I went up to a .5mm over piston and re-rebuilt the carb. for the pto cylinder.
    This time rode for about 2 hours and everything seemed ok. when I got home I pulled the heads and found the pto had almost burned through again. Now I've replaced the cranks seals, resealed the crankcase, checked the crank index and did the vacuum test on the crank case everything is good except I have to go 1mm over on the pto cylinder.
    My question is:
    Is it alright to have that big of a between cylinders? the mag and center will be .25mm over and the pto will be 1mm over.
    This is my wife's ski and she never pushes it.


  2. #2
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    I'm sorry you learned the hard way why we recommend a leak down test after a rebuild. Prob would have saved you some $$$ and frusteration if you would have done that after the first piston replacement, huh?

    But to answer your Q, it's not recommended to run it like that, but you may be able to get away with it. Pretty sure guys have tried it.

    Hopefully someone else will chime in on this one.

  3. #3

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    Oh well live and learn.
    Turned out to be the carb, seems it was leaking at the inlet cover o-ring. When I checked the pop-off it wouldn't pump up, so I put it in a bucket of water the applied pressure and air leaked out around the plate. I'm not sure why I tried a new o-ring and it still leaked so I applied a little gasket sealer and that fix it.

  4. #4
    Buck3yeRideR's Avatar
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    I definitely do not want this to happen on my rebuilds. So i should perform a leak down test, pop off test, and carb leak test?

  5. #5
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    Quote Originally Posted by Buck3yeRideR View Post
    I definitely do not want this to happen on my rebuilds. So i should perform a leak down test, pop off test, and carb leak test?
    Yes!

  6. #6

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    I got a replacement cylinder and piston from SBT and went through the engine again, crankcase vacuum test, carb pop off and crankshaft index which was about .5 degrees difference from the center to pto and 1.5 degrees from mag to pto. I think that ok? everything else was good. I also rebuilt the fuel pump and bypassed the fuel shutoff. So I went out on a test ride. This time I didn't ride long when I got back home I pulled the heads and the pto is still very lean. I did adjust the carb richer but it didn't seem to make any difference at least this time there wasn't any major damage. Is it possible for the reeds to cause it to run lean?
    If anyone has any suggestions it would be appreciated.
    This has become a pain in the a**, but like I told my wife, who is getting a little upset about the cost. It's now a quest to repair this and I'm not going to stop until it's perfect.

  7. #7
    Polarisitis loonatik's Avatar
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    Where do you ride? Wanna to get together sometimes?

  8. #8
    it would make sense if it advanced your ignition far enough on the 1 cylinder to cause preignition. I gotta imagine the distance should be equal between all cylinders and that the .5 to 1.5 might be off. Also, maybe I missed it but have you done a leak down test? Have you converted to premix? If so the oil injection port is post carb and you could be sucking air in from there, but that's unlikely if you still have injection.

  9. #9
    I know it's a pride thing and I've been in the same boat as you with chasing your tail around to try and fix it but sometimes you just gotta cut your loss and stop bandaiding and either get a entire new motor or sell the ski and get something else that you know will run. It's hard to admit defeat but sometimes you are better off.

  10. #10

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    Yes I did a leak down, only lost 1in vacuum in 30 min.
    I'm using oil injection. I'm going to check the reeds tonight if they look good I may just get a rebuilt engine from SBT.

    loonatik, I ride in New Port Richey usually out of Green Key and go out to Anclote Key.

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