Hi all,
I'm hoping someone can shed some light on a very frustrating problem.
I recently bought a 1996 KX60 for my son. The engine had been "completely rebuild" except for the bottom end which the seller said the mechanic reckoned looked tight. Very tidy bike all around. Fair enough I though; I can see lots of work done on it and even the hone marks still on the bore. He was selling as too powerful for his 5 yr old (funny that)! My son is 9 so I fitted a Moose carb spaced to move the powervalve down, a 12 oz flywheel weight and a washer with 1/2" opening gin the exhaust. Gave it a nice torquey engine and no snappy powerband; most of you will know what these pocket rockets are like! He's still learning to ride a gear and clutch bike coming from a PeeWee.
Because the motor only has a few hours on it I mixed it 25:1 with semi-synthetic oil and took my son for a ride. He got around 1 km on it and it seized.
I took it apart that night; completely blown big end.
Long story short, I fitted a whole new Wiseco crank and new main bearings and we went for another ride over a weekend camping. Hot day (30oC, 90oF), still learning the clutch in some, soft sand so lots of stalling. A bit of coolant overflow but not much lost. Presumably normal as no expansion chamber.
We go down a firebreak and he is revving it hard; then it suddenly stalls. Won't start. Onto a trailer and back to camp where we strip it down. Spark and fuel; must be compression which doesn't look great. No idea why with a new top end though. :thumbsdn:
Take the barrel off tonight; the big end is gone. On the new Wiseco crank. Again. :banghead:
Bore is scored, piston ring jammed.
We've managed to write-off both bottom and top ends this time.
So, two questions:
1.What the heck happened? I'm loathe to rebuild a motor that gets 10 miles per crank.
2.Rebuild or fit a used motor? Same price probably...
I've got experience with 2t's had a KDX for years and now ride a KTM EXC 250. I know these things eat top-ends; but not big ends!
First up; the best in all it's blown up glory. A sad sight indeed. :sad-smile^clap
http://www.miwer.org/downloads/KX1.JPG The crank complete with blueing; it got warm. Presumably; this was before it failed and not due to it seizing then blueing?
http://www.miwer.org/downloads/KX2.JPG Top of piston showing damage from roller pins dancing around and crushing the top of the piston down sticking the ring. Goodbye compression.
http://www.miwer.org/downloads/KX3.JPG Exhaust side of piston showing scoring (deep) and blueing (mainly this side) on crank. Top end had been resleeved by previous owner. Although I prefer Nikasil...
http://www.miwer.org/downloads/KX4.JPG Scoring on piston on exhaust side. Evidence of lots of blow-by which isn't surprising given state of the rings. Curiously though; no damage to inlet side of piston...? :confused:
http://www.miwer.org/downloads/KX5.JPG Underside of piston showing burning; clear indicator of running lean and over heating I guess...
http://www.miwer.org/downloads/KX6.JPG Any ideas on why the bearing is blowing and the best way to get the bike running again greatly appreciated guys.