Kx 125 87 keeps cooking pistons please help

[Wusty]

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

[Nathan S]

Tried a larger main jet?

[Tossa]

Tried a larger main jet?

I'm with Nathan sounds like it's leaning out when held at top revs for long periods

[oldyzman]

Also consider if the fuel is flowing at a fast enough rate to the carby, blocked line or breather...

[OverTheHill]

not something silly going on with the ignition advance!!. A local lad had an "87 or thereabouts new back in the day & it broke two conrods before the importer decided the cdi box was doing crazy things & replaced it plus everything else that was damaged [twice] & was ok from then on. Mind you, by memory each time it went bang he was on full song in the air so don't know if the crook box was ever proven or if it was a known issue with that year [around "87]. Had an "86 new myself & still rate it among the best, if not the best most reliable new bike i've ever had--was thrashed by me and the kids & one gun when his bike gave up on the day [that was an "87 by the way], different to the conrod bike. Anyway,  [you]put a genuine piston & ring in & richen it up a bit--providing your cylinder is still ok & not severely worn around the ports. Cheers.

[Wusty]

Thanks guys for the input yeah I changed the main jet in it to a larger 1 the first time it let go it mighten be enough may need to go larger again? It has a slightly larger then standard carby so uz could be on to something I'll check the Cdi out aswell thanks fellas it's been a hoot to ride except for that

[Iain Cameron]

Have you raised the compression . I had a similar problem turned out the pilot jet had to go up as well . Iain

[Lozza]

Don't bother plating the sleeve unless you can't get any oversize pistons. That won't make any difference to the heat path. Can you post a pic of the piston and cylinder?

[GMC]

I had an 87, it was a hoot to ride as well and mine didn't have any major drama's and I ran it in the 4 day plus the Vic Enduro series, club MX, and trailrides all year long.

I would be a bit suss about the carb, I was going to say first check what the standard jetting should be but that idea goes out the window if it has a non stock carby.

As Lozza says no point plating a cast iron sleeve if you can easily rebore it. A rebore will be much cheaper than plating

[Slakewell]

What colour is the spark plug afterwards?

[shelpi]

What colour is the spark plug afterwards?

bingo, always do the plug drop, and go read dispelling the mystery between belray MC1 and other 2T oils, sorry to dumb to link so use the search up and to the right

[MaxPower]

How long is it held on  for before you feel the sieze? Is it a strait on a mx track or a strait bombing around  off road?
Stock exhaust? Pump fuel? Is the plug white? Is the track sand ? Remove the stator cover. Is there gas pooled in the bottom? Grab the rotor and try to shake it up and down. Does it have play or is it tight? Does it seem to smoke at lower revs? Did you have the bike for a lenght of time and it just begin to happen ? If this is a new bike to you. Check the main jet against specs. Maybe someone before you that was much slower leaned the main from stock.

[yamaico]

Does it seize when the throttle is still wide open or does it seize when the throttle has been shut off immediately after it has been wide open?

[Graeme M]

"Slightly larger than standard carby".

Soooo.....  not the standard carb huh? How good might the jetting be for this application? If the jetting's way too lean for this use going one size larger won't help much. I'd be starting right there and work slowly towards getting the jetting right.

[Wusty]

Soo I dove a little deeper, the original carby is a mikuni 31 mm round and I have a mikuni 38 mm flat slide on it I've only had the bike 6 months and started racing in qld the past few races, last race was conondale classic and only got 2 laps in, got to the end of main straight when it lost power and stalled I didn't chop the throttle, just had a look at the colour of the plug and well there is no electrode left! Plug greyish white what's left of it, It has a stock expansion chamber, just got a pro circuit expansion chamber that I have yet to fit, pro circuit silencer, runs premium 98, the compression hasn't been changed as far as I am aware, pulled stator cover off and no fuel residue and rotor has no unusual play I will post pics of piston plug and barrel tomorrow thanks heaps for your input guys