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Yamaha Tech Originally the Yamaha XT600 Tech Forum, due to demand it now includes all Yamaha's technical / mechanical / repair / preparation questions.
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  #1  
Old 4 Jan 2009
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Have I made a big mistake???

I have recently completed another XT600 engine re-build.

It's an early kicker engine with decompressor.
I thought I'd do a 'special' as I've built a few 'standard' engines without problems.

A basic run down of the spec is:-

Standard bottom end with Carrillo rod
Standard bore cylinder
High compression piston
Gas flowed head

All this may seem OK - but I think there might be a small conflict.
High compression piston + decomression lever = bent valve

Has anyone done this before?
I am dreading stripping it down as the valves were flowed aswell and won't be cheap to replace.
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Old 5 Jan 2009
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Valve timing?

Double check the valve timing, rotate the crank with a socket through the central hole in the clutch cover, then sight through the 12mm plugged hole to see the crank timing marks, the piston TDC is the last mark as you rotate anti clockwise, which you can confirm with a probe down the plug hole resting on the piston. With the tappet covers off, check that, for both TDC positions that the tappets have clearance, ie the valves are closed. The cam might be a tooth out with respect to the crank.

Last edited by tenere_rider; 6 Jan 2009 at 07:05.
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Old 5 Jan 2009
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That was the first thing I did - then did again to make sure I didn't miss anything the first time.

I'll post some pics of the parts - give me your opinions.
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Old 5 Jan 2009
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OK here goes......




This is the Carrillo rod - no worries about damaging that.





This is the high compression piston - check the RAISED crown (this is what I think may have hit the valve)





Normal head and ported head - can you tell which is which???





Another view of the ported head



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Old 5 Jan 2009
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Hi topped piston vs decompression lever

OK, try this, it should work... smear the piston valve clearances with plasticene, coat the valve faces with oil, crank the motor, with the kick starter through a cycle, measure the exhaust valve imprint depth. Then calculate how much you need to machine the piston crown. Or use a thicker head gasket made of copper and lose some compression. Or not use the auto compression lever, use the electric strarter only. You might be able to adjust the decomp cam to open the valves just off the seats enough to enable starting but without touching the piston at TDC. Did you hear a "bang bang bang" as the valves touched the crown when cranking?

Last edited by tenere_rider; 6 Jan 2009 at 00:16.
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Old 5 Jan 2009
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Hard to tell if there is a noise when you are kicking the life of of the bike.
There's fuel and a spark but there is a lack of compression compared to normal.

I guess this buld error was just an oversight on my part - or getting over confident

I'll be doing a stripdown soon so more info to come.
I'll be re-building without the de-comressor this time.
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Last edited by Tenere Tom; 5 Jan 2009 at 13:44. Reason: typo
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Old 6 Jan 2009
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Lack of compression

If as you say, the valves are bent, they won't seat and no compression. The decompressor lever is pulled by the cable only at the start of the kick, just to get some momentum, and probably not enough stroke to contact the piston. Many TT600s went hi comp dome top pistons for racing... and they were kick start only... By the way, if you haven't already done this, helicoil (stainless steel insert thread repair) the threads in the head for the front and back short studs, the ones with the acorn nuts between the ports... and perhaps up the a foot pound or two torque on all head studs, like the VW engine hi comp builders do. Has the head gasket surface been machined as well? Might be too much.
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Old 9 Jan 2009
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If you think you're loosing compression, you could do a leakdown check. You will need compressed air. It will have to go through the spark plug hole, so you might have to make something there.
Put about 80 PSI in after the engine is set at top dead center. Listen to the exhuast pipe, the carbs and someplace open on the crankcase. That will tell you where the compression is going.
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Old 9 Jan 2009
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It's a bent valve - decompressors and high compression pistons don't mix
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