Kx 125 87 keeps cooking pistons please help

[bazza]

Motul 800 is top stuff.
Start with main jet and do plug chop,get that right first
Check air leak around carb boot?

[Graeme M]

Sounds way lean. Check for air leaks and check that jetting. Any tuning gurus like to comment - I'll assume that a larger carb won't flow a significant amount more air, just slower? So ballpark jetting to start would be same as the original carb?

[80-85 husky]

the needle jet may be too lean limiting the mains effectiveness....

[VINDURO]

Hey guys I've got a 87 kx 125 that keeps cooking the top end of the piston, it only happens when the bike is held wide open for long periods of time (like main straights) the piston ring is wedged into the piston and scoured the inlet and exhaust port at the top of the barrel, I run it 35:1 motul 800, liquid intelligence, it's got a new radiator and waterpump however it has a cast iron sleeve and I'm know they produce a little more heat but didn't think this would be a issue I'm going to get the barrel sent to servo to electro plate the sleeve but wanted to know if there is any underlying problems that I don't know about any info or help would be greatly appreciated thanks

What is the current jetting setup with the Mikuni 38mm flat slide?

May want to pull the carb apart and take note of exactly what size jets are in there.

Have you always used the same type of plug in it? If so what plug have you been running?

Check the barrel and make sure that the ports haven't been damaged.

Cheers

Jock

[oldyzman]

So if you go from 31mm to 38mm, which seems massive for a 125 in any case. then the main jet will have to be massively bigger also as there will be less vacuum (of want for a better expression).
But:
I have just read   http://www.thumpertalk.com/topic/628280-1987-kx125/
it came out with a 35mm carby approx 370main. If you go to 38mm carb then use a 400 or 410 MJ as starting point. _ me think

Also make sure the Jet needle (emulsion tube) is big enough to support the extra size main jet.

Brett

[GMC]

Yeah I just read that stock is 35 as well, I still have the brochure for mine but it doesn't give a lot of detail.
If it was mine I would lose the odd carby and try and find a stock one or just buy a new one from Hatricks, the dollar has dropped a bit of late but a new one should still be under $200 which isn't much when you consider the cost of destroying an engine.

US specs!
Carburetor Mikuni 35mm
Main Jet 370
Pilot Jet 35
Needle Jet R-2
Jet Needle 6dk/3
Cutaway 3.0
Fuel Level 0-2 mm
Float Level 17.1 mm

[oldyzman]

correct advice GMC...

[rocketfrog]

Did the 1988 KX125 run a Kiehin? I have a stock one here that I have no idea how I wound up with, it came from the model which had blue side covers. So what is that 1988 1989? I think it is either a 35 or 36mm.

[GMC]

I think they changed from Mikuni to Kiehin in 88

[FourstrokeForever]

With the bigger carby you will need to increase the size of the main AND pilot jets. Also, check that the float level is right as that could be why there isn't enough fuel (way too lean) on full song. When you add on the pipe you will more than likely need to adjust the carby specs again.

As GMC said, going back to the 35mm Mikuni is probably the best option.

[oldmxracer]

Been following this... For what it's worth, I ran it by my engine guy and he does NOT like sleeves in a plated cylinder because of heat issues. He also recommended a standard carb and also check for oiling holes at the exhaust bridge if staying with the liner.....

[Wusty]

Thanks for everyone's input some good stuff on here I'll start trackin another carby on the side seems like 87 were a one of year 86&88 are both different carbies, it's running a 380 mj so ill try 400 mj and over to see if it comes good plug was a br8es thanks for the support

[Graeme M]

Don't know what was standard on the KX but an 8 plug sounds a bit hot for a 125...

[AdamN]

No air leaks anywhere? Is the piston to cylinder clearance correct when they bored the cylinder? What piston are you running?

Be interested to know what the cause is if/when you get it sorted.

Adam