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Yong Heng oil capacity

That's what the manual says alright. That would be around 17oz.

But it's not right. 

I believe it's more like 10oz or 12oz at max as that is all I can get into mine before it goes over the fill dot in the indicator.

I just drained mine it was just a bit overfilled and got a little over 10 oz out of it. There may be a couple of oz in there still as you can get it all out.


 
Mine call for 9oz.

You don't want too much oil because the crankshaft will beat air into the oil making it less effective. It also slings too much oil onto the cylinder walls which will cause more oil to get past the first stage piston rings which will in turn let oil reach the second stage piston and you don't want that. Same applies to all engines other than the second stage part of course.