OzVMX Forum
Clubroom => Tech Talk => Topic started by: Wusty on August 12, 2014, 08:04:10 pm
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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
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Tried a larger main jet?
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Tried a larger main jet?
I'm with Nathan sounds like it's leaning out when held at top revs for long periods
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Also consider if the fuel is flowing at a fast enough rate to the carby, blocked line or breather...
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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.
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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
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Have you raised the compression . I had a similar problem turned out the pilot jet had to go up as well . Iain
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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?
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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
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What colour is the spark plug afterwards?
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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 ;)
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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.
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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?
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"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.
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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
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Motul 800 is top stuff.
Start with main jet and do plug chop,get that right first
Check air leak around carb boot?
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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?
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the needle jet may be too lean limiting the mains effectiveness....
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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
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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
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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
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correct advice GMC...
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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.
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I think they changed from Mikuni to Kiehin in 88
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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.
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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.....
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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
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Don't know what was standard on the KX but an 8 plug sounds a bit hot for a 125...
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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