Jasle

Sponsoring Member
Nov 27, 2001
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OK Trying to figure out what happened to this motor. New topend with about 10 minutes ride time. Bike was not rich previous to this topend. if you notice it burn off the top to the 1st ring on the exhaust side. also the tip of the plug burnt off. but the piston top and insert are bblack with carbon and everything was oily. some slight presence of detonation around the outside of the piston. squish was set at .028" it was ridden in some crazy deep sand though. Pics are as close as I can get without beeing blurry. oil was motorex cross power 2T. premium pump gas.
 

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Rich Rohrich

Moderator / BioHazard
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Jul 27, 1999
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The usual culprit when you burn off the ground strap and get detonation damage on the exhaust side is overly advanced ignition timing for the load or excessive heat. If the cooling system wasn't working correctly you could see similar issues from overheating.

If you pay close attention to it you'll find the plug ground strap is a pretty good indicator of timing and running temps. The underside of the piston crown will also give you some clues.

Is the ceramic on the nose of the plug cracked?
 

Jasle

Sponsoring Member
Nov 27, 2001
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yes the ceramic is broken. And yes it is a champion plug. 8339 and there is no direct cross from NGK. The BP9EG is close but the insert was designed around the tip of that particular champion plug. The B9eg plug is at least .030 shorter. I'd have to machine the insert to use them and keep head volume the same. But we race AMA stock rules so its a no no...
And been using those champion plugs exclusivley(in these bikes anyway) for about 3 years and having very good luck with them. Cannot say the same for the NGK. Had the tip of one break off. Almost brand new plug and it did not burn off...But thats another topic.

But not a japanese bike. American bike...Look close you'll see the clutch. should give it away.

I did find the water pump seal leaking throught he weep hole but still had fluid. Looking like its not holding pressure though. drips once warmed up. I'll check the timing ASAP...Thanks.
 

Rich Rohrich

Moderator / BioHazard
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Jul 27, 1999
22,838
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Jasle said:
yes the ceramic is broken.

Usually when the ceramic breaks it's from the high cylinder pressure shocks from detonation, and excessive heat would put that process in motion in a hurry. If the cooling system is compromised it looks like you have at least one of the causes nailed down. I would still look closely at the ignition timing to play safe.

As you already seem to know, the plug brand had nothing to do with it. :coocoo:
 

jason33

Member
Oct 21, 2006
655
0
what bike is it
new top end- did you change the timing? you didn't say you did-
a topend doesn't req a timing change-
i have had bike get super hot from lack of water and never seen that happen
the wrong plug could be a cause-
preignition- bad fuel,wrong timing
did you use a fuel additive?
i can't believe it didn't do any thing else to the piston- no hole in it?
 

Jasle

Sponsoring Member
Nov 27, 2001
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checked the timing. it was .035" BTDC. Pretty much spot on for where the mfg specs the timing at. Still had coolant but it was the first outing in deep dune type sand.
 

whenfoxforks-ruled

Old MX Racer
~SPONSOR~
Oct 19, 2006
8,129
2
Merrillville,Indiana
Cold seizure on the bridging ended in detonation? Or overheat and seize on the bridging? The champion statement is an undocumented fable exclusive to jap bikes! Did the wire tip leave a timing clue or when it fragged wipe it out?
 
Last edited:

kkencr250

Member
Sep 25, 2006
27
0
New top end not broken in, straight to deep dune sand, prolly wasn't the best idea. You want to make sure the motor is broken in properly and everything is working before you put it under a heavy load like sand. Sorry to hear it man, at least it's only the top end.
 
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