Please Diagnose .... (Pics)

MURDER YAMAHA

MURDER YAMAHA

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So, had the motor drop a skirt on the exhaust side last weekend, and the other piston was cracked as well.
Understandable, as the cylinders are old, and out of normal wear spec at this point. Sending them away to get re-plated.20170121_214851.jpg20170121_201214.jpg

What I cant wrap my head around, is the reeds.
Right before winter, I installed a set of SRX reeds w/notch. They were perfect when I installed them.
Picture is after about 50 miles or less of riding.
Both sets of reeds had the damage on the bottom, but nothing on the top side.


My sled is as follows:

1997 XTC 600
Bender Porting
Single Layer head gasket
21 tooth top sprocket
Bender silencer w/stock pipe
Fairly stock clutching
 
Are you running spacers with these reed cages? My guess is they are comming too close to piston. I ran viper reed cages on my 98xt 600 and I can't remember the rule with these cages on a ol' 600 motor. See if I can find it in my note but I bet your pistons were hitting the reeds on the upstroke at some point.
 
Nope, cause only lost the skirt on one piston ...

And it can't be the piston hitting it, because the center of the cage would have gotten it the worst.
 
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looks like the pistons were well worn from not having rings put on them in a timely manner over its life, they rock in the bore and break off the skirts. The reason the reed petals would be damaged is the blow by that comes back up from the sloppy fitting piston/rings, also why youll notice its the end bottom ones that took the damage and not the center, because all your pulse is from the outsides.
 
Well, the rings had maybe 50 miles on them, just had cylinders honed before winter and new rings put in then.
But, the bore had gotten too big, compression tester said both cylinders at 128 psi even after losing the skirt.
You think that the bore being too big caused the reed damage? Or, do I need to be looking for another culprit ?
 
once the pistons are worn beyond the spec, the skirt to wall clearance is too big, and even with new rings will not be stable in the bore. The bore will and likely is now tapered due to the miles that were put on with the worn pistons rubbing the skirts excessively. Youll likely need to send the cylinders out to strip, and replate and hone to get true again and then new pistons and rings, new reed petals and you should be good to go again for a few thousand miles.

see this everyday, I know people think its great that "they have 8000+ miles on their sled, never did anything to it and it runs just great", but really its wearing and losing power slowly over the process and they don't notice it. Things like low on rpm and harder starting, etc. because it happens over time. 2 stroke engines are not in cars today because even though they make awesome power per displacement and weight, they have 1 downfall, piston and ring life is much shorter because its the heart of the engine and it sees 2 times more work then does a 4 stroke.
 
97XTC600 said:
Well, the rings had maybe 50 miles on them, just had cylinders honed before winter and new rings put in then.
But, the bore had gotten too big, compression tester said both cylinders at 128 psi even after losing the skirt.
You think that the bore being too big caused the reed damage? Or, do I need to be looking for another culprit ?
Click to expand...

yes, you can still have compression, as to have compression only means it holds the seal for a short bit(above the exhaust port), but that's why a compression test really isn't that great of a tool to tell you the condition of the engine. A leak down test tells you the condition of the topend, as you can tell a lot more by how long can the engine maintain that pressure. Your engine as an example if on a leakdown test, would lose the pressure rather quickly. Yamaha uses a cast piston in most and powder forged in other snowmobiles engines, they use tight skirt to wall clearances like .0025-.0028" as the pistons don't expand greatly and they stay stable in the bore improving ring seal.

a compression test only gives you what it can build and doesn't show you the amount that leaks past the rings and piston. its still a good tool but far from the absolute.
 
Thanks for the info !
I am sending the ported jugs out to US Chrome to have replated along with 2 new Yamaha pistons.
Is there a certain spec I should give them for clearance for best performance? Or just let them do what they think?
Also, what should I do with the crank ? I don't believe its ever been touched, about 3,800 miles now...
I assume at least new seals ?
 
97XTC600 said:
Thanks for the info !
I am sending the ported jugs out to US Chrome to have replated along with 2 new Yamaha pistons.
Is there a certain spec I should give them for clearance for best performance? Or just let them do what they think?
Also, what should I do with the crank ? I don't believe its ever been touched, about 3,800 miles now...
I assume at least new seals ?
Click to expand...

no problem.

ya, if your gonna have it down I would for sure throw in new crank seals and make sure all the crank bearings spin smooth. Crank seals mostly due to age not mileage. Since its a twin check for bearing chatter in the cases,once you deem the bearings good if there is some chatter marks(common on twins due to vibration), I use a green Loctite on the outside of the bearings race to case when you assemble it, let dry good overnight before oiling the bearings, if 2-3 days... better ye,t on drying time.

just have the cylinders redone to stock specs and use stock oem Yamaha pistons and rings, youll have no issues!
 
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