Bad first ride, a few questions for the pros! See PICS

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biglsells

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Jan 14, 2012
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Grand Rapids, MI
Hello all, fist post.
Quick back story - bought a 1998 vmax 600sx for $200 from some guy in the country this summer, the right cyl was blown, piston ring pin failed and it chewed them up, the sled it otherwise beautiful.

The jug was junk so I bought a used one locally, bought two new piston & ring kits, new top end gasket kit, rebuilt it, cleaned carbs, fired right up, 142psi on both jugs.

Took it out today for my first ride on it as we just got snow here. Made it 2.5 miles and the engine just bogged and shut off. Started it back up, drove fine for about 20 seconds then the same thing, this time it felt rough when I pulled it over so I trailered it back home (I could hear some scraping sound from the LEFT jug when I pulled it over). Took the heads off and left jug off, the piston was melted along the intake side, luckily the cylinder looks fine just have to clean it a bit. My question - what would have caused this? I can fix the piston/ring but dont want this to keep happening.

When I bought this the guy said he was just tired of 2 strokes so I am thinking he was not able to figure out what the problem was. If you look at the pics one is a pic of the bad piston and bad head so you can see coloration, one is of the good piston and head, the exhaust pipe shows that the side that failed is whiter than the side that is fine. Finally, when looking down into the engine on the bad side I thought it looked like it was cracked so I took the whole darn engine out but the case looks just fine from the outside so I am thinking its not cracked.

Oh - one more thing, when you look at the picture of the "bad head" you can see several dings from a ring that must have gotten chewed up before, this is not from today so he must have had these problems before.

What do I do? Any ideas?
I took pictures of the engine during disassembly - here they are
 

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Number one failure of two stroke engines is dirty carbs. Second would be crank seals. I would suggest you check into these before another rebuild happens if you have not done so already.
 
I cleaned the carbs a month ago during the rebuild but since I have the motor out now I'll go ahead and take them apart and do it again. I do have a replacement seal for the PTO side, how do I get the old one out? I'll have to order one for the pull start side. Thanks for the reply stein700sx
 
You have to split the case to get at the seals, if I your you I would completely disassemble clean and inspect the entire engine per the spec in the service manual. You should replace anything that maybe out of spec and for sure I would install all new seals on crankshaft when its apart. The reason you should split the case and completely disassemble a 2 stroke is to make sure you get ALL the piece and foreign material out of the bottom half of the engine. Then I would make sure that you premix oil in your gas the first tank and I would even make sure and jet the carbs on the rich side for the first coulpe of tanks of gas. JMO Good Luck
 
Im working at splitting it right now, I've got all the bolts out its just giving me grief, do I have to pull the flywheel off to do this? It seems like the bottom should separate from the top with the flywheel on. The reason I ask is I have a puller I can use once I get the crank out but its too bulky to get on there while its in the case.
 
Yes, pull the flywheel. Under it is the stator which has to come out before splitting the cases.
 
The damaged side of the engine, bad piston with dings in the head, has no piston wash at all. The other piston shows two small areas of piston wash ( the two elliptical shaped areas on the intake side of the pistons which are bare aluminum. There should be "washed" areas on both pistons, areas which are clean of carbon deposits so the cylinder which was damaged was running lean for sure.

Take the engine completely apart, clean every part, inspect everything, use new gaskets and seals and reassemble it.

Jim
 
Good possibility there was debris from the first time it burnt down. If you didnt split the cases the first time when you put the used jug and new pistons in. I have buddies that still havent learn. Yeah I will admit the alingment of the motor/clutches and a few bucks on seals and a tube of yamaha bond is well worth the warm and fuzzy feeling that I aint walkin!
 
Thanks for the info on the stator, I'll have to borrow a puller tomorrow to get the flywheel off, mine is just too large. There used to be a trick back in my gokart racing days where you could tap the end of the crank with a hammer and it would free the flywheel but I don't know if thats ok to do here.

Thanks RX1JIM for the info on the piston wash. How do I make the left carb less lean?

yes, unfortunately I did not split the cases prior to rebuild when I bought it, lesson learned the hard way, doign it right this time. - Devilin, what is yamaha bond? Is that something I need to use in addition to new gaskets?
 
Yamabond is the sealant that yamaha recomends and sells! Some guys swear to use it. Others will use what ever kind of permatex or RTV sealant they have laying around. Goes between the 2 case halves primarily. When ya get er all back togther and fired up for the first time spray starting fluid at/on carb boots,ptoand mag seals. If ya got a leak the idle will tell you. I would be leaning towards a bad seal or dirty carbs burnt down 1st time, same seal or debris from 1st lead to 2nd. I burnt down my 00 sx700r because I was in a hurry cleaning carbs and skipped the needle and seat. Sled idled great after quickie cleaning and burnt down 12 miles later! Might have left me gunshy on the "safe" then sorry side of things. Did you pull motor right out of sled or still in engine bay?
 
Devilin - dragged the sled home after it cooked (it actually still ran, but not well) and yanked the engine right out, its actually sitting on the floor by the fireplace so I can work on it and watch football :) I'll rip the carbs apart tomorrow while I wait for the gasket kit and piston/ring to come in the mail and once it's all back together I'll try the starting fluid trick by the carbs and see what happens, any fluctuation = leak somewhere I am gathering.

mtnmax - will grab some bond and yes, found a manual online I am ordering, thanks!
 
You need to check the jetting in the carbs and compare it to the stock specs if your sled is stock. The piston wash has to be examined across the rpm/load range since different carb circuits are used: fuel screws, pilot jets, main jets and needle position. Check out all of these against the stock specs.

The mixture will be lean if the carbs are dirty, varnish/residue in the jets reduces their size thus a leaner mixture. Is the airbox on the engine and is it stock?

Yamabond (or Hondabond) is the ONLY sealer that is to be used to seal the crankcase halves to each other. DO NOT use any other sealer.

Jim
 
Where can I find the specs to compare the carb setup to? I'm going to tear down the carbs again tomorrow again so I'll compare it to spec, and yes everything is stock on this snowmobile aside from new pistons/rings.

rx1jim - I was going to buy a "full engine rebuild gasket kit" as it comes with a bottom end gasket, wouldnt that take the place of this yamabond or do I put the yamabond on both faces of the bottom end and sandwhich the gasket in this?

Airbox is on the sled as well.
 
The technical section of this site lists the stock specs for all the yamaha sleds: carb settings, clutch setting, etc..

The Yamabond (or Hondabond or Threebond) is a sealant that is used to seal the crankcase halves together. There is no gasket between the halves. I put a coating on only one half of the crankcase. Put just enough, don't put too much on it or else you'll have a lot of excess material that can cause problems. You'll want to make sure both surfaces are very clean.
 
Here are two pics of the actual piston, first is intake side the second exhaust.
it just seems like the left side had no lube! Anyway let me know what you think so I can chase down the cause of this thing continually failing.

Thanks RX1, I thought I needed an actual gasket for the case..
 

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The trick is to translate what sled is telling you. You asked on an earlier post "less lean" = richen. Too rich = excess fuel. Lean = to little fuel. Rich will have blacker smoke, burn your eyes, foul plugs and rob performance. Lean goes BOOM! Lean conditions are usually caused by dirty or wrong settings in carb or air leak. Whan you see signs of either you objective is to figure out why? Theres a nice write up in tech section about "piston wash" and "reading plugs" with pics and how too's. With the late snow I understand everyones frustration to RIDE! With pistons being newer it may be harder to read wash. When reading plugs you want to use used plugsas they are easier to read than new. You may end up spending the afternoon checking and rechecking, adjusting and checking again. NOT the fun part of sledding but well worth the longevity of your ride.
 
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