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Old 07-21-2009, 03:56 AM
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Default 2 rebuild 2 pitted pistons back 2 back

     

Hi 89 RM 125 .. rebuilt top end twice in last month ..both times after 1/2 to an hour of "break in" he 'll come back w no compression ...pull head and both top of piston and head are pitted all to he_ _ !! fuel air mix appear good according to plug ..the last rime aroung we split cases ..everything checked out fine ...looks like something getting into the upper end ..last time it was ciopletly pitted all around outside of piston and head .. crank rod ahs zero play (up and down) even went so far to clean out pipe /carb/air box ect , what is going on here ??any ideas would be greatly appreciated
THANX********HELP
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Old 07-21-2009, 04:22 AM
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Double check for any gaps or poorly sealed areas along your intake and airboot. Any traces of dust will wear away the internals. Dust and dirt is the #1 cause of premature engine wear.

As for the pitting, I would look at detonation/pre-ignition as a possible factor.

Here are some definitions for ya:

Combustion knock. This usually occurs near the end of the combustion process when highly compressed, high-temperature end gases spontaneously ignite, radically increasing the cylinder pressure. This pressure spike moves at the speed of sound in the combustion chamber, and the pressure can cause damage to pistons, cylinder walls, and the head gasket.

Pre-ignition. The onset of combustion before the spark plug fires. This is generally caused by some type of glowing ignition source such as a hot exhaust valve, too-hot spark plug, or carbon residue. Pre-ignition is especially damaging to engine components like pistons and head gaskets, since excessive cylinder pressures can occur even before the piston reaches top dead center (TDC).

Eliminating Pre-Ignition & Detonation - Pounding Pistons

These could occur from a few different things. What ratio of oil/fuel are you using and what type of oil and fuel? Sometimes the engine can pre-ignite because of the combustion chamber being to hot from a very lean mixture or from other problems related with excessive heat. Also, what spark plug you have in there?
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Old 07-21-2009, 06:09 AM
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if the plug looks good, then it's probably only suffereing deto under high load (i.e. full throttle starts and exits of turns) the plug looks good because it's getting good fuel everywhere else. Make sure your low side (pilot jet and or low speed needle is set correctly) you might try lowering the clip on the slide needle. That will give it more fuel lower in the slide movement.
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Old 07-21-2009, 07:26 AM
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I think Aus_rider nailed the problem. It sounds like detonation to me. Too high of compression for the fuel - someone may have altered the head before you. Use a high octane fuel or do some measuring and alter the compression - possibly by using multiple or a thick base gasket - this will alter port timing, though.

Scott
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