Piston failure

1980SRX440 said:
I was talikng to my buddy who gave me the sled and he mentiond that the idle was sometime erratic (hang alittle high the drop down toa normal idle), I did not notice this, but then again I did not let the sled sit and idle since I just did the top end.
Still sounds like it could be a carb or boot leak issue. If it was a labyrinth seal, why would it only leak to one cyl and burn just that one down?
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toydoc said:
1980SRX440 said:
I was talikng to my buddy who gave me the sled and he mentiond that the idle was sometime erratic (hang alittle high the drop down toa normal idle), I did not notice this, but then again I did not let the sled sit and idle since I just did the top end.
Still sounds like it could be a carb or boot leak issue. If it was a labyrinth seal, why would it only leak to one cyl and burn just that one down?
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That's why I was asking, I wasn't sure how this seal worked. Just about everything else I have or have had has been a Ski-Doo or Sea-Doo. They have a seal on each side of the rotary valve shaft, so by pressurizing that cavity I can single out which side of the rotary valve seal is blown. I guess there is only one Labyrinth seal to seal the crankcases. This is my first Yammie in my collection, I know a great deal into how to troubleshoot and pressure check a bombardier but not a Yammie (yet). Sorry to be so detailed but the devil is in the details. Your help is greatly appreciated.

I'll remount the carb flanges and turn some plugs on the lathe to fit the carb holes and then pressure check the flanges for leaks too. Right now I removed the flanges and made plates to fit on the intake adapters for my previous pressure testing.
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All good suggestions I think. I would doublecheck and triple check the jug with telescopic gauges or a CMM. Confirm ring gap and also wouldn't hurt to smear some grease around the carb boots when reinstalling. Erratic idle could be a crank seal as well. I had the same issue with a 83 Vmax 540 about 15 years ago. Ended up changing seals and went back to Yamaha pistons Did you deglaze the cylinders with a light hone? I use a set of stones and varsol on the mill for about a minute when installing new pistons even with coated bores chrome nikasil or electrofusion (Kawasaki)
 
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