98 VMax 500 problem

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blueracer17

New member
Joined
Jan 4, 2010
Messages
2
Age
48
Location
Wisconsin
I bought my son a Vmax 500 for christmas back in December. He has ridden it around the yard a bunch but that's it. When i bought it compression was at 120 lb per side, 4400 miles, original. I took it out last week down a snow covered road and opened it up in short bursts, no more than a 1/2 mile wide open throttle. It ran great. Coming back cruising at 45 it shut down. I limped it home, tore it down and found a seized piston on the exhaust side. Put in a new piston/ring set, gasket set, set the ring end gap to spec, cleaned off the aluminum from the cylinder wall and checked compression before starting it and it was at 120 on both sides. Started right up, let it warm up, took it for a ride around the yard and down the road in front of my house. It ran great, only ran it for about 5-10 minutes.

This Sunday I took it for a ride down the road again (same road it seized up on me last time). This time, I let it warm up, drove it down the road at no more than 45 mph, and not more than 7000 RPM, mostly 5500-6000. Slowed down to turn onto a different road and it seized up again. Limped it home again (2 miles), tore it down to find a melted piston on the same side as last time (mag) but it was on the intake and exhaust side this time. Looking at pictures online it looks like a lack of lubrication. The oiling system was not disturbed, lines are intact. I tore the carbs down today, the pilot jet looked a little dirty.

So, I will get a different cylinder this time, and do a leak down test before I start it up (should have done it the first time but it gave no indications of a leaky crank seal). Any thoughts on what would cause this cylinder to go down twice in a row? PTO cylinder and piston look good. (original and new one with 4 miles on it). Thanks.
Sorry for the long post.
Pics are of original pistons, new pistons look the same, besides being seized on both sides.
 

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super1c said:
You went through carbs? Crank seals? Boot leaks? What do top of pistons look like?
Click to expand...
Top of pistons look good. I have since gone through the carbs, pilot jet seemed dirty. Intake boots are good, I have not done a leak down test to confirm if crank seals are good. It does not seem to hunt for an idle, and starts easily with the pull start using some choke. No lean bog or hesitation whatsoever. I have worked on a lot of sleds and built a lot of snowmobile engines, this one is kind of a stumper. I think I will tear it down and put new crank seals in it, put it together with a new/used cylinder, leak down test it and move on from there. Likely will be this summer, I'm not going to dump a bunch of money into it at the end of the season. Thanks for your thoughts.
 
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