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  1. #1

    Why does the #3 piston fry besides not getting oil from the carb (this is on a premix

    This question is a bit premature but with the arrival of my new busted SUV this Wed, I wanted to see if anyone else has a collection of knowledge on why pistons burn up when a ski is on pre-mix.

    One thought I have is that since this is a tripple carb setup (1200 non-pv yamaha SUV) that one of the carbs may have become clogged or otherwise unusable so then it would stop serving the fuel/oil mix to that piston. The other 2 pistons would continue to push over the crank and eventually the 3rd would dry up, rub melted aluminum all over the cylinder wall and eventually lock up like a proper welding job.

    Any other thoughts on what could lock an engine on pre-mix before I get my hands too dirty

  2. #2

    Join Date
    Aug 2006
    Your thought is a possibility, but so is an air leak causing a lean seizure. An air leak at the intake, cylinder base gasket, or the rear main seal could be a leak source also. A good practice is to leak check the engine before pulling it apart to fix.

  3. #3
    Quote Originally Posted by LT1GMC View Post
    A good practice is to leak check the engine before pulling it apart to fix.

    Without looking back at some previous reference material on this site, it may have been you LT1 who strongly recommended doing a leak check before installing. I know plenty of very smart folks on this forum either agree, generated that post or at least chimed in with a +1 on it. When we did my bud's 1200 top end a few weeks back, I wish I had crossed paths on a post that recommended doing the leak-down test before ripping it apart. If we had done that, we would have saved about $300 and a TON of time doing my first top end job. That was a block that had a crack in the bottom of it below the mounting cradle in a spot that we couldn't see without ripping it apart. The engine looked so dang clean that I never though to look for a leak. will be done this time on this blown engine! I'm hoping to be able to do it by just pulling the carbs and exhaust off down to the muffler assembly and blocking off those with rubber sheeting (shower pan rubber material from Lowe's). The last time I put a thin layer of RTV on the metal surfaces before I layed the rubber on and it seemed to work well to seal off the engine for the test. Unfortunately I did not get to find out if there were any other leaks due to the air pouring out of the hole in the bottom of the case

    You mention "rear main seal" as a possible leak source for air. Are you talking about the area by the intermediate housing near the rubber coupling or the other side at the flywheel end seal?

    BTW, if I get inside this thing, is it easy to figure out if the crank bearings are contaminated and or out of spec or is this something best suited for a shop used to doing this type of inspection? I have the manual but don't know if I have the brains to decide if it is in good shape for a top end rebuild. Again...getting ahead of myself but my general interest in these things keeps me curious.

  4. #4

    Join Date
    Aug 2006
    I was referring to the PTO/Coupler end seal, but the front seal can leak as well. Once you put some pressure on the engine, you can use soapy water and watch for bubbling to see exactly where the leak may be. That is if the engine does not hold pressure for 10 min or so. If it holds pressure, then you are good.
    As far as crank bearings, I am no expert but if there are metal particles down in them and the engine has run that way, I don't think I would trust it. It just cost too much to build an entire engine and take a chance on the crank. There are specs on side clearance on the rods and checks listed in the manual.

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