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I have a 2003 ZR 900 that I've rebuilt multiple times in the last few winters for cruising steady speeds and one piston will seize and score the piston and cylinder wall like its not getting oil. I've replaced oil pump, check valves, oil lines. Slides in the carbs were replaced, crankshaft replaced, obviously crank seals replaced at the time also. Since everything has been replaced, I've still had one piston seize up on each side, mag and pto. Kind of at a loss......
 

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well 2003 900 criusing speed usually goes lean.
what needle?
what is your elevation?
piped or stock?
open airbox?
what clip is your needle on from the top?
what size main jet?
what size piolt jet.
on your cdi box, what is number stamped on it?
 

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Discussion Starter #3
Current set up, which is the way it's been for 4-5 years, is
450 main
45 pilot

needle in the middle, don't remember off the top of my head what size
stock air box, funnels still in
stock pipe.
Running sea level
Don't know what number is on the cdi box, but is original to sled
 

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well
how is piston wash on the good piston?
specs for you to start with
50 pilot jet
mains 450/460 pto/mag 20-40 degrees out
460/470 0-20 degrees
needles 9DFH9-59-4
clip in #4 position.
there is a cdi that advances timing, it is good for high elevation.
need to make sure you do not have that
 

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I would expect the edge at the top to be melted and rounded over, if it was a lean burndown, but the top of the piston does not look too bad in that picture. If the cylinder is getting fuel, it should be getting oil. What does the intake side of the piston look like? Is it always the same side that is siezeing? What is the cooling system like? Is the coolant circulating well and properly? Do you have an IR temp gun that you can use to measure cylinder head temps to see if that side runs hotter?
 

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I would expect the edge at the top to be melted and rounded over, if it was a lean burndown, but the top of the piston does not look too bad in that picture. If the cylinder is getting fuel, it should be getting oil. What does the intake side of the piston look like? Is it always the same side that is siezeing? What is the cooling system like? Is the coolant circulating well and properly? Do you have an IR temp gun that you can use to measure cylinder head temps to see if that side runs hotter?
Intake side stuck just like that. That piston was two meltdowns ago, last time was mag side
 

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So, the seizing piston moves form side to side. That might lead me more to cooling and think the engine is overheating, but if both faces of the piston, intake and exhaust side, are sticking as shown it is not quite the typical "four corner" seizure from a rapidly expanding piston in a slowly expanding bore.

What do the plugs and piston wash look like? It can't hurt to start off with a richer tune and work your way back down to a proper jetting set up. Some new main jets are going to be a lot cheaper than new pistons and cylinders, and probably easier to change out. Make sure the cooling system is functioning properly, make sure the oil is injecting at a good rate and throw some more fuel at it. Try a temp gun to tell you if one cylinder is getting out of line with the other, monitor plugs and piston wash and hopefully you can figure out how to make that engine happy and stop eating pistons.
 
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