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Piston Reading

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Hi all, 

 

Just wondered if anyone would like to take a look at my piston to let me know any thoughts on running conditions/possible issues etc. The dark spot underneath looks quite large, maybe got a bit hot at one point? also, there is carbon buildup on the crown and some scoring/polished areas on the skirt. 

 

The bottom end is being replaced as the main bearings are totally shot and the crank doesn't seem to be in very good way. Just wondering if you suspect any cylinder damage from the look of the piston?

 

Thanks in advance, 

 

Jim

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Edited by JimboJones

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Looks like your jetting is too lean to me.

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Are you jetted for your premix ratio? I run 32:1 in all my bikes ( which is what all Japanese manufacturers recommend). My last crank bearing failure was back in the late 1970s.

Cylinder damage should be apparent from inspection. You will surely see scratches, but anything you can feel with a fingernail indicates a problem.

Edited by rpt50

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Have the bore measured. That will be the only sure way to know. I threw a couple of piston kits in my bike before I realized my bore was too worn. (even though it looked great)

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Thanks guys. I'm not sure if it's possibly been running lean due to leaking crank seals or not (a quick leak-down test showed leaks all over). I haven't checked the jetting yet but I've been using 32:1 premix ratio. Would standard jetting be OK for this ratio at around sea level in England?

 

Also, I had the bore measured this morning and the size is: Top - 66.456mm, Mid - 66.450mm, Bottom - 66.484mm. From these measurements the bore is oversize. There is no crosshatching visible but also no scratches that you can feel with your fingernail. I guess this means that it should be re-plated?

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Oh, I should add that I haven't owned this bike for very long and when I got it the air filter was in a pretty bad state and since looking at the bottom end found it to be a bit gritty so believe that this is why the bottom end has died (also have no idea of hours on it.)

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Looks like it's sucked some dirt. The piston is due for a change, see the scoring on the skirts? It's time, that's for sure.

 

The black on top of the piston is bad, also the black underneath is not good as well. Looks like it's been running lean, and burning oil under the piston.

 

 

http://www.theultralightplace.com/READING-2-STROKE-PISTONS.html

Edited by Socket

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Def running slightly lean. Not bad though since the rest of the piston looks pretty good but the heat sig on the underside of the piston should only be about the size of the finger nail on your pinky finger.

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What jets are in your carb now? 

 

Your piston looks hammered and your motor sounds like it has had a hard life. eg.  Dirty air, air leaks, poor quality fuel, revved a lot. It can all be fixed up pretty easy. Just hope your transmission hasn't been too badly abused. Check your power valve mechanism for smooth function and any signs of wear.

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Yeah, those are fat jets.  Your needle setup is stock. Since your piston looks terrible, I'd guess you have an air leak, and the motor sucked some dirt. 32:1 is enough oil for normal revs.

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Yeah, the motor was sucking some air (and oil too, i suspect) I guess that makes the whole piston reading a bit pointless. I'll get it back together refreshed and have another look later on. I just wanted to check that I'll be ok with the curent jetting at 32:1 as I dont know the ratio the PO used. 

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Your jetting should be reasonably OK. On a stock motor, at sea level and moderate temps, personally I'd try a smaller pilot and main, and go richer on the needle clip pos. I found that made it crisper off the bottom, more roll on torque at half throttle, and revved out further and cleaner at wide open.   As you say, after the rebuilding steps. :)

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