Need help diagnosing pistions after seizure

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kaw550

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Feb 3, 2004
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The sled is a 91 exciter .40 over.
I pulled off the head.
The first thing I noticed one side had a small amount of carbon covering the pistion the PTO side had much less. You could still see the original gray color.

Then I noticed the edge of the pistion near the exhaust port looked like it had crumbled away. I assume it melted.
The cylinders were not bad. just a few scratches on each.

The sled was only on the second tank of gas after an engine rebuild. It was running great until it was shut off.

Can anyone tell me
1) Why one side had more carbon? I believe both were firing.
2) what would cause it to melt?


TO give a little history on the sled:
A friend bought it with a blown motor. Had the cylinders bored and installed wiseco pistons. Well the cylinders were nikasil so it seized in about 3 miles.

It was given to me in the fall. I changed the seals, cylinders, pistons and gaskets. It was the third time out probably only about 60 miles on it. THe cylinders are from an earlier exciter (no nikasil).
 
obviously a lean burn......pop off the carbs and clean really good and also check the reeds but also check the carb boots cuz they tend to crack from vibrations.....when you check the carbs see if they are in sync and if the needles are in the 3rd position....also main and pilot jets........hope this helps......and i know how you feel i just melted my piston on my bender big bore.........
 
what did the plugs look like ? there are no reeds on the exciters they are piston port .... did u use extra oil in your gas tank for at least three tanks checking the plugs is the best way to tell what happened ,maybe your oil pump wasnt working or had air in lines ...thats another good reason to put oil in the gas tank id rather foul out plugs then burn your motor down... start out with rich jetting then come down from there to get the right color plugs (light tan) i even suggest using dial a jets youll tweek alot more power out if you have aftermarket pipes pipe porting job etc... and cure lean conditions..... i hope this helped :D Richie
 
The PTO side will run hotter than the MAG side. There should be a fatter jet in that carb, if not put one in :) . Air in the oil line will melt a piston every time. Wiseco pistons are verrrrrrry picky to warm up and overall abuse. I burned my Exciter motor down three times with the Wisecos and will never use them again. Use stock, they will last. Good luck. :D
 
Reason for meltdown?

The plugs were covered in a light black ash. One side was a little gray.
I beleive the jets are the stock size. I had replaced the ones that were in at the start of the year.

SHould I go up one size just to be safe? Could I just adjust the airscrew and the hight of the needle for a richer mixture?

The sled does have an aftermarket exhaust. Would aftermarket exhaust and stock jets cause this? If thought it would as if it had a stock exhaust.
 
Sounds like it ran to lean, If you are running aftermarket pipes you should of went up on jet size. You should also mix oil with your gas. Always be on the safe side.
 
Is it the pilot jet or the main jet?
I had mixed the gas at 50:1


I have heard that mixing oil can actually lean out the fuel mixture. In fact i read it on this web site.
 
Pictures of the pistons

I have attached a few photos.
The are taken from the seat facing forward.
The piston on the left measured 60psi. You can see it is melted/broken away along the top front and the ring has lodged into the back of the pistion.


The right side is melted along the top front.

Both have some scoring on the sides.

It hs an aftermarket exhaust and is .040 over.
The main jets are 310's should I go to320's or 330"s just to be safe?
Will the new jets make it hard to tune?
 
Does it have a thermostat and does it work ? I am wondering if you had a head gasket leak . If it was lean the pistons shouldn't
be black .
 
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