KX60 blowing big ends! What do I do with this engine!?

[easykiwi_rider]

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.

[DoldGuy]

Kiwi,

Those motors are tough, I would think your washer in the exhaust is most likely the culprit as its going to keep too much heat in the motor & from your description of it going lean on the exhaust side goes with that theory.

DoldGuy

[Foxx4Beaver]

I agree with DoldGuy....those motors run at such an RPM, they need to breath. 25:1 is way too heavy on the oil if you're not jetted for it too...you're actually leaning it out, making it run hotter.
The other thing is the aftermarket cranks are absolute garbage...period!!...find an OEM crank.
A buddy of mine was going through a Hot-Rod crank every 3-4 races on his sons RM85...and his YZ250 was only getting 6 races per crank.
Hot-Rod cranks and Wiseco cranks are just junk, and are not meant to last.


Part# for an OEM crank is 13031-1169....Rocky Mountain ATV has them listed for $201.05...which is a fair price, but...
I would shop around a bit and see if you can find one any cheaper.

[sandblaster]

+2... Get rid of the exhaust washer and see if you can find a throttle with a set lock such as on a pw50.
I'm not a big fan of those things but it's by far better then smokin the motor.

[easykiwi_rider]

Thanks heaps guys; some good information there.

Looks like, in trying to protect the motor from my son's revving, I may have made it worse 

The piston is black underneath; so getting hot probably from getting lean.  Plug looks OK though.

 I added the exhaust restrictor based on other experiences on other forums; but also based upon what they do with Peewees (his previous was an '88 PW50). (Interestingly, this article talks to the need to have jetting increased when a restrictor in included).

 Eric Gorr's excellent "Motocross and Offroad Performance Handbook" also shows a photo (p. 102) of a blocked exhaust inlet with the caption:

 "...carbon buildup [same as restrictor washer] has reduced the diameter of the pipe, which can make the bike lose power [actually what I was after] and run hotter [defo not what I was after...].

 So it's looking to me then like we had the perfect storm;
 1/. Hot day
 2/. Soft sand
 3/. Exhaust restrictor leaning out mix
 4/. Restrictor comes out allowing young fella to give it full herbs and finish the job.

The other thing Eric Gorr mentions is the chance that casting plugs on the crankcase (magneto side) can leak (p.181).  Haven't checked this yet; but even leaner if that is the case...  Anyone else experienced this?

Not that familiar with smaller motors, but it didn't seem to last long though  .  I should have mentioned I've also got a torque spacer and flywheel weight on their too, but don't expect that would have made much difference...

It's a great bike when it's going.  Good bottom power with those mods and heaps up top.  Light and fast for the sand; I love my 2-ts when it comes to gliding over soft sand...   

[easykiwi_rider]

So I cleaned the left side of the crankcase up under the magneto with a wire brush.

Pics showing moulding mark failures as predicted by the Guru Gorr.
 
I pasted epoxy resin over them.

[easykiwi_rider]

Hi all,


I thought I'd give you all an update (I hate it when people don't update!) so we can share the lesson.

So, I took the good advice offered and decided to rebuild (again) but this time without the washer.  And also without the carb spaced (for good measure).  And also the case cracks plugged.

It went together a dream and started easy.  Bore a little scratched from the imploding crank; but still starts OK.  Might resleeve is as Smacoroni suggests when it needs a new piston.

The end result?

Running 32:1, we've got 8 h on it and counting.  The young fella's learning about throttle control (she flies when it hits top end!) but having a whale of a time.

So I'd have to say exhaust restriction #1 problem.  Cracked case #2.  Put them together and buy shares in cranks...

[Foxx4Beaver]

Excellent!!...roost on!!