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Thread: Yamaha Rz 350 eating pistons

  1. #1
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    Yamaha Rz 350 eating pistons

    Anybody have any experience with Rz350's? Tonight on the way home from work I cooked the left side piston and its not the first time its happened. I was ripping pretty hard when it happened but I wasent holding it wide open for any extended period of time Had this happen a few years ago during a top speed run. Looks like a cold seizure but both times the engine was fully warmed up. Kinda got me baffled Im starting to think the two stroke gods dont like me... Any ideas or common problems that Im over looking??

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    got a pic of the piston?
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    My guess is that your high speed jet is gummed up or at the very least the left carb is running to lean for some reason. Do a rebuild on both of them and set them up using gages.

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    I havent pulled the engine down yet. I took the exhaust off, looked up the exhaust ports and could see the scuff marks on the exhaust side of the piston and the intake side of the cylinder. The engine didnt lock up but started to go a lil tight and I knew what was goin on. Ill pull the carbs down when I get the engine out. Ive had lots of practice working on this bike!

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    lean usually wouldn't cause it to have the scoring.... it sometimes can, but not usually

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    The engine became "tight". I t would indicate the piston grew larger then the cylinder and would cause scuff marks on the piston skirts and the cylinder wall.
    The cause of one piston growing bigger then the other in a prolonged HARD run would indicate to much heat in the one cylinder. My best guess behind the keyboard is that the one carb is set to run much to lean which creates a lot of excessive heat.
    If it still runs, Try ripping down the carbs rebuilding them and setting them up properly and taking it out for another hard run and see what happens. Worst case senario you ll need a new sleeve and piston which you probably already do.
    At the very least, adjust the air fuel high speed adjustment to run richer on the left carb and have at it.
    Good luck, Let us know how it turns out.

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    Pulled everything apart tonight, can see metal transfer on the cylinder on the one side of the exhaust port. Its kind of weird, not the common 4 point seizure. More of a diagonal across at a 45deg angle to the wrist pin. This is the same pattern that was on my last piston that seized in this bike. Im going to take the engine to a guy that has a lot of experence with two stroke street bikes tomorrow for some damage assesment. I checked the carbs to see is they were plugged up with crud to possible indicate a lean condition to no avail. There might have been a slight exhaust leak at the exhaust flange to exhaust port but I dont think that would cause it. Ill get some pics of the piston when I get a chance and post them up.

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    excessive connecting rod play?

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    crank seals.
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    Bad bore, and or ring gap setting a little off. Did the ring(s) spin at all?
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    I agree with fitty... Crank seals.
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    Well just picked up all the parts to get this thing together, just gotta find some spare time to assemble the engine. Shouldnt have been crank seals or anything crank related as the bottom end was fairly fresh, less than 2500 kms on a new wiseco hot rod crank, all new seals and bearings. I think at WOT on a hard acceleration pull the left cylinder went lean enough to build some major heat and seize. I had to get another set of cylinders as mine only had one more overbore size and the sleeve was already super thin around one of the intake ports. Im also trying Pro X pistons instead of wisecos this time as my machinst recommended them over wisecos. Once I get this bad boy back together Im taking it to a well known two stroke guy to help me get the carbs and jetting dialed in perfect. Im also going to install a higher flow water pump and a 2 into 1 air filter set up to keep the engine heat down and make it breath better.

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    if the crankcase and related stuff was all redone then you are for sure dealing with a lean condition on the intake side somewhere, obviously you are on the right track though. if you are using old fuel for some reason that can cause a lean seizure all on its own, (although the scoring is stained with varnish usually)
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    Exhaust leak at the flange can cause air to get sucked back into the cylinder and therefore leaness. Check ALL intake and exhaust connections for leaks. Comments regarding crank seals could be possible as it is the left cylinder. Right side would cause you to suck trani fluid.

    2 corner seizure may be due to out of round cylinder or piston, out of alignment crank or rod. If the cylinder was ported and the flow across the ports is not even then there can be hot and cool spots. Considering it is an RZ have you checked the alignment of the exhaust valves? There can be issues regarding alignment and also worn connections between cylinders which causes the right and left exhaust valves to be open out of alignment. This out of alignment can cause one cylinder to run hotter than the other.

    Have you done plug chops? Was one cylinder reading different than the other? Have you verified the oil pump lines are clear and flowing properly? What about the gaskets? A leaking base gasket is often overlooked on a 2 stroke but can leak and cause seizures. I have also seen porous cylinders and cases but on a bike this old I am sure it isn't unless there is damaged/ cracked cases.

    By the way what carbs, mains, porting are you using? Stock? Mildly tuned and if so by whom? I have seen 35 hp RZ's all the way up to 75hp so there is a big range of issues that can arise from various mods.

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