saving a 1980 KZ750 twin

teazer

Active Member
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Perversely enough, low crown pistons are better for flame propagation. I like the flattest chamber and crown combo I can get away with and that allows timing to be backed down a long way in some instances.

There's a fair amount of carbon for not a lot of miles that could be cleaned out with a wire brush. The side electrodes look like they have run a little hot but are not showing much sign of distress. Try colder plugs and a touch less advance and see how she responds.

You are right that on a 4 stroke we measure static compression and valve timing and pipe design can change that in a running motor.

On a long stroke motor with big old fashioned pistons, actual squish on the exhaust stroke (ie when nothing is being squished) actual running clearance at high revs will be getting close to zero. Unlike a 2 smoke that never has an uncompressed stroke, a 4 stroke has a lazy stroke where the rod can grow as it's not cushioned by compressing gas.
 

der_nanno

Faster!
Pretty much spot on what Teazer said: You want an as flat piston you can get away with. Squish is ideally a band (ring) all round the combustion chamber and then some room to escape towards the plug. (Modern combustion chamber design favours roof-shaped combustion chambers mainly because they are cheap and you can adjust compression ratio as you need it, without changing tooling too much.) I run an all out 5mm squishband on a 95mm combustion chamber on my TR1. IIRC yours is about 80-85mm bore so you can quite easily aim for the same overall specs, but don't go wider than 5mm or you might run into issues of creating hotspots in the squishband, because it can't be evacuated fast enough at mid rpms.

If you want to see a piston that does exactly this to perfection, look at a standard XT600 piston (plenty of others out there as well, but these I know personally.
 

doc_rot

Oh the usual... I bowl, I drive around...
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sandmanred said:
Congratulations on September BOTM! It's a well deserved win. Really great job on the build.
Thanks man. I appreciate it. I dig your bike as well.
 

doc_rot

Oh the usual... I bowl, I drive around...
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getting a move on this. I decided to bite the bullet and get some custom JE pistons in 86mm bore. this will put the bike at 906cc. If you would like to get in on a limited run of pistons and sleeves let me know soon as I will be pulling the trigger on this in the near future.
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