Hi there - I have an air cooled KDX hybrid, the motor is in a converted '89 KX125 chassis. There are many variables on this bike so I'm finding this persistent problem hard to diagnose.
The motor is fresh - new nikasil, piston, mains and seals. (98 fuel , 40:1 fully synthetic premix)
But it's always made a metallic clanking noise (like a quack sound) when shutting off from full throttle under load. Riding sand trails where I live it's very noticeable.
The sound only happens in gear, when the drive/momentum is pushing the piston. It'll rev out without clanking, and won't make the sound if if I pull in the clutch on deceleration.
I'm guessing it's firing unburnt gas in the header/main pipe. I don't know why.
Some areas of suspicion:
- FMF main pipe (as specced for the engine). It doesn't seat all the way in at the cylinder, possibly both
o-rings not sealing and allowing blow by under load.
- Incorrect ignition timing? (timing set wrong, failing CDI, bad plug cap?)
- Is compression too high, or wrong squish for the carb. The head looks standard, But I think the deck has been ground at some point. Compression is what I'll test next.
- Fuel - Has a Lectron which seems nicely dialled in all the way through to peak revs. It doesn't idle though, with idle screw fully in.
- Non standard air box for the engine (from '89 KX125).
Thanks for any thoughts, diagnosing it is driving me crazy. Been a long term issue since the build.
