Maintenance & Technical > Kx Frankensteins

KX500 outlaw cart OH YEAH!!!!

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hurtucaine:
Hello All,
Well big bore build did not last long. Started up well, two heat cycles to set rings, then at the track exhaust overheated the top of the new piston right off. No clear root cause. Fuel/oil and jetting was good.

Symptoms.
1. Top of head bolts were leaking coolant after first hot laps ( 5 laps). not using stock acorn cap style head bolts.
Motor temp was reaching operation temp more quickly then in the past. ( up to 150F very quickly!! This is your first clue Mr. Dumbass)
2. Torque of head bolts originally set to 18 lbs then went up to 22 lbs. ( Stopped coolant leak for next heat race.)
3. During that race notice lack of smoke out the tail pipe. (Driver does not know) Within 3 laps it was all over. Motor loss power and over heated, was very hot (190F - 200F) and leaking more coolant out of top of head bolts.

Recover:
NA, cylinder wall is scored and can't bore any larger due to power valve interference. Cylinder needs a new sleeve.





  
Our guess is if the head bolts were leaking coolant then it very well have been sucking air also, made a lean condition and over heated. However: confused by why only exhaust side of piston melted down, Intake side looks well. ? &*^%???

Any suggestions out there?

Lesson learned.
Use new stud bolts when in doubt, use quality gaskets, ( Cometic from now on, not moose fiber) when there is a coolant leak and reaching temp quickly you have a larger problem than you think.

Thanks

sandblaster:
Hey hurtucaine.
I fixed your pics.
If you go to one of the addresses you had listed, you will see "View Original" at the top of the page.
Click on "View Original" then you will see the actual picture.
The address at the top of the original picture is the address you want to post here.

hurtucaine:
So there we are, Big bore pisotn & cylinder toast.

Still have stock cylinder and piston just tore down and surprised by the good condition it is in. Umm .. what to do.... Yea, .. you think... yea sure got a race a no time ( or money) to order new parts...

Ok put the old cylinder and piston back in so we can race. Yea.. that will be just fine.

Ok if we reverse the used piston rings ( top/bottom) and use new wrist pin bearing should work well.

Long story short. back together an ruining well by the 2nd heat race. It was all over.

Lost piston and rod bearing on crank.

( May have over reved motor there at the end.) made a gear change and driver using third gear...oops... Normaly we run 4th gear. So with that mistake our luck had ran out for sure.

But that is only the benign of the story.
After the race late that night at the track licking our wounds with cold beer, someone says hey I remember I have a old CR 500 bike just siting in the corner of my shop with a ruining motor, I let you use it. (it is now Sun 1am and next race is at 6pm.)

Yep, we pulled the CR 500 out the bike, made all the necessary conversion from KX to CR on the kart ( Coolant lines, Clutch, Engine mount and silencer/pipe bracket and some bolt on speed goodies)  and was racing by 6:30 pm that day. it took us all day to get that Kart race ready.

Woo, Hoo, My son did really well that night. We had a tough time trying to get the Kart set up and trying to get faster. It was a whole different technique driving a Honda for my son, but he figured it all out by the Main event. Finished about mid field for the night, Put up some good battles for positions and made some awesome passes to finish the night.

Talk about your emotions and excitements..... the ups and downs!!!. Gota love racing!!!


Detail of KX motor.

We pressure tested the motor before diss-assembly and found no leaks, at tear down, piston was bad and Rod bearing shot.
Guess is debris from previous piston fail may have ended the rod bearing in the lower end of motor. Piston is just badly scored and over heated. Ring all chewed up. Cylinder is reasonably good, going to send it out to be sleeved. so we are stuck with a CR until we can get the KX back together.

Wish us luck.

Brute:
I used to race karts but that is NUTS! Good kind of 'nuts' but still... I will have to check around and see if any of that is going on locally. That would be fun to watch if nothing else.

RoostDaddy:
My guess on what happened to your piston in the "big bore" cylinder is that when you had a coolant leak it went inside and smoked your ring lands on the exhaust side. Probably washed the cylinder of oil and the exhaust is where the heats at. Just my assumption, as I've seen it happen before.

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