Center cylinder lost all compression - your guess?

A couple of bucks said:
Positivity ensure that the center exhaust gasket is not leaking AT ALL!!!!!
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I replaced all gaskets with new ones and followed the tightening torque scheme.

When I turn off choke, it revs up and one cylinder stops running after a couple seconds and it sounds like the other two are running synchronized ( dadadada-dam-dam-dam-dam-dam....)
 
Time to strip the carbs and ensure every jet and passage is clean. There should be a step by step instructional page here somewhere if you need it.
 
Today I finally removed the carbs and followed the step by step instructions found here at TY. Re-assembled everything and guess what, it runs great.

Thanks for all input along the way!
 
Unfortunately I have to bump this post,

Finally winter came to my parts of the world and time to start up the Viper again.
I noticed it was harder to start then before rebuild. It usually started on three pulls regardless of temperature. Now it took 6-7 pulls and I had to maneuver the choke and throttle to keep it running first minute.
Scared of another burn down I had adjusted the fuel screw to 2 turns out from my previous owner setting of 1½ turns out.

First couple of rides with easy cruising trail riding all was fine.
But yesterday after it became warm it just would not idle. It wanted to die and backfire all the time.
I usually have to give half throttle when warm and it starts right up. Now I believe I must almost give full throttle when warm.
I've been gentle on the throttle because of the new pistons etc.
Plugs were looking somewhat greish and towards the lean side.
Yesterday I was out of ideas (been cleaning carbs for like 4 times and adjusted idle) and thought I have to check compression again and guess what the left side cylinder shows close to no compression now.
The other two is down by around 30 percent from usual figures.

I'm hoping for a gasket problem right now cause if another piston melted this sled will go to the dump for sure.

What's your thoughts, seems kind of unreal this happening twice in a row..
 
I pulled the top off to inspect the damages.
The low compression cylinder (clutch side) had melted rings and the nikasil is severally damaged.

Top of the pistons looks ok and also the plugs.

I do not want to invest in another replated cylinder without knowing why this happened.

Ok, when rebuilding I didnt flush or rebuild low end crankcase. So perhaps I had it coming. Could a bad crankseal or debris from previous burn down cause this?

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sxviper-s said:
Looks to me that piston rings were hitting the power valve. The power valve looks to be in a bit to far.
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I think it's the angle of the shot. Here another photo more straight above.

piston_top.png

I think the valve could have been hit though, but I suppose that is after the meltdown. The other way around is probably not possible by design (?)
 
I've bought a brand new SRX powervalve a few years ago and it was too long to install in the cylinder, factory flawed, I kept it and used a dremel to adjust the shape of the valve myself. As for you condition that is LEAN, pistons are clean and white, no wash on them at all, spark plug is white as well. Either your carbs are way out to lunch which I doubt, or a bad crank seal or seals or cracked intake boot.
 
pete_man26 said:
I've bought a brand new SRX powervalve a few years ago and it was too long to install in the cylinder, factory flawed, I kept it and used a dremel to adjust the shape of the valve myself. As for you condition that is LEAN, pistons are clean and white, no wash on them at all, spark plug is white as well. Either your carbs are way out to lunch which I doubt, or a bad crank seal or seals or cracked intake boot.
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In one of the photos it looks more white but that's because of the camera flash.
In another photo showing the full top I'd say it's brown. Same for the plugs, they looked more brown than white but perhaps that's not dark enough even for new pistons?

From the posts so far it seems there is no other likely cause than a bad crank seal, so I'll have to figure out if it's worth the effort...

The compression was down on the other 2 cylinders as well although the cylinders looked ok but do you guys think I need new pistons/rings for them also?
 
I have now pulled the engine and taken a closer look. It took a year to find the motivation.

This time it was the left/clutch side piston and rings that's melted on exhaust side (same side as before but different cylinder).

The other 2 pistons/cylinders are looking good. However the compression test showed they were down by 30-40% from normal figures so my question is can the bad cylinder/piston with close to zero compression influence test results on the other two?

The plan right now for this summer project is to clean everything, split the case, clean case and replace seals, go through oil pump, water pump, fuel pump, tank filter, carbs of course and raise needles, richen pilot and run some premix until I trust the oil pump.

Most likely I'll use the old original OEM piston (if within spec) with new aftermarket rings.
Overall the original parts feels more solid and higher quality wrt rings and gaskets. The SPI rings feels thinner and softer, the Winderosa gaskets broke apart while dismantling after just a couple of miles riding. The old saved OEM ones looks much better and are still in one piece.
 
Some pics. Looks like the clutch side was running lean as several already stated.

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Can't see anything that would suggest permanent damage to center or recoil side.
All pistons have only a couple of miles easy riding in them an the center cylinder is newly re-plated also.

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Looks better when cleaned up

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Any insight on my question above is appreciated: Can the the bad cylinder affect the compression test results on the other two cylinders that looks ok? Or is the test results valid, are the other pistons garbage also?

Thanks
 
MURDER YAMAHA said:
Bad cylinder won’t affect other compression readings.
What did the other two read?
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Ok, I was afraid that was the case.
The "good" ones are down from 130/120 to 90.
The bad one gave almost no reading on the meter at all.
The first burn down looked much worse but still gave around 50psi reading while the undamaged were still at 120/130.

Do you think I can get away with just new rings on the bad ones?
 
Well, if I said yes or no, it would only be a guess.
The sure fire way to answer that question is to measure the piston.
Then, measure the inside of the cylinder.
Yamaha has specs on them that they need to fall in, otherwise if you run them, you may get another failure.
Pistons wear quicker than cylinders generally speaking.

You sure your compression gauge is still reading correctly ?
 
MURDER YAMAHA said:
Well, if I said yes or no, it would only be a guess.
The sure fire way to answer that question is to measure the piston.
Then, measure the inside of the cylinder.
Yamaha has specs on them that they need to fall in, otherwise if you run them, you may get another failure.
Pistons wear quicker than cylinders generally speaking.

You sure your compression gauge is still reading correctly ?
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The coating is wearing a little but still most of it is there. I wonder how thick that layer is?

I used the gauge on my other sled as part of yearly service/check up and those figures were spot on from last season.
 
Each Cylinder is separate from the others that is how two stokes work .

What kind of oil do you use? Yamahalube is Crap I like Klotz or any TCW3 Spec fully synthetic oil ..

Do you have a Temperature gauge? If you are running on bare ice you may not get any cooling.. My 2014 Skidoo 800 E-tec was overheating on a solid ice trail going an easy 40 mph.. I still have not installed my Ice Scratchers.. The only way I could cool was to go off trail and hit some deep snow spots in shady areas. It had gotten really warm and most of the snow melted then got really cold .. It was 10F out and my sled was overheating . My Skidoo has a Temp gauge.. I added a Temp gauge to my Yamaha Apex..
I have seen these sell for close to $100 it's not exactly the same as mine,.. Mine has a clock in it which I do not need
https://shop.kencomotorcycle.com/da...ater-temp-gauge-detail.htm?productid=18164639
 
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