Blowby - something to consider

R

Rockenstein

New Member
If you run into a situation where your engine starts to loose power or won't idle properly anymore something easy and quick to check off your trouble shooting list is blowby. Blowby happens when the piston rings don't seal the cylinder properly and hot combustion gases blow past the rings down the sides of the piston. An easy way to check to see if this is the cause of your woes is to pull your muffler off, roll the engine over and looking in the exhaust port inspect the area of your piston below the rings, if the area below your rings looks anything like what is in the attached pictures you've got blowby happening. I'll add that in most cases the exhaust side is the side that fails first which makes sense because it's the hottest and driest side. The only fix for blowby is to re-ring your piston and very very gently hone the cylinder...keep in mind though that every situation is unique. You'll have to replace the piston, rings and cylinder if you've worn it all out real good. Good thing is that rings are cheap, at the first sign of any blowby re-ring your piston and run the new rings in gently for a few rides. Providing you keep on top of things and use a good quality premix oil it's not uncommon to re-ring any small displacement 2 stroke 2-3 times before a new piston and cylinder is needed. The toluene based carb cleaners in a spray can will aid greatly in cleaning and removing carbon and other deposits from a piston you need to re-ring.


Cheers...

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Good info Rockenstein. All the good information lately is making this site even better then it was.
 
Thanks reg...

For reference here's a pic of a piston with rings that are sealing well. Important part is below the rings...this area should stay reasonably clean for the life of the piston.

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A couple of things- I wouldn't hone the cylinder unless you really know what you are doing...the plating on these cylinders is very thin and can be ruined very easily.

Also, the best way to break in or seat rings in not to run them gently, but to run them fairly hard right away, with various throttle settings and rpm, never run it so low it is lugging.
 
I agree, don't hone if you don't know how to do it...take it to a shop, a friend that's experienced in such things or find some old cylinders to practice on.
 
The only one I know of to be sleeved are some of the 48cc engines. The 80 are nikasil type or "chromed" and it can't be bored, and shouldn't really be honed, except by "the experts".
 
Just to be clear I'm not conveying honing for a "new" finish or a cross hatch...more what I'm trying to say is that you don't want any sharp edges in the cylinder bore that would imprint on a fresh set of rings. You should be able to knock down any serious edges by hand with a little oil and a strip of 600 paper. I've used a bottle brush wrapped with a rag and dabbed with metal polish to clean up a plated bore, looked amazing after it was washed out :)
 
marvle mystery oil will help rings to reseal also.. just 1 thing to try b4 tearing down for a re ring.. follow directions on bottle for 2 stroke
 
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