2004 kx 250 oily spark plug

[Philthy Phil]

hi guys having trouble with my kx, i bought the bike in pieces from a friend, he stopped it all down as it has a 2nd gear problem, i  fixed the gear and put it together, it's not my first rebuild, it starts fine all gears and engine seem perfect, but it never runs longer than 5 minutes, i found the spark plug has oil on it, don't think its the gear oil.

i run a 40 to 1 oil to fuel mix
the bike ran perfectly before the strip down, so I'm confident the jets are good.
i have checked and cleaned the carb,
i have made sure the air and fuel mixture screw is in the right place according to the manual...
i have made sure the cir clip is in the right place on the needle.

i clean the plug and it wil start first kick and run nicely with good power thru the revs but doesn't last long then it just cuts out much like it would if you run out of fuel, a plug clean and she is good to go....

I'm running a NGKR 9 plug as the manual says,

I'm thinking of trying a hotter plug...


any suggestions

Thanks phil

[Foxx4Beaver]

Start with a leak down test and compression test,then take it from there.

[Philthy Phil]

thank you for your response, will try that...

[Foxx4Beaver]

Don't try running a hotter plug....stick with what's recomended for your bike...hotter is'nt always better,you could do some serious damage to your top end.Did it smoke ALOT when it was running?.Find out what the stock jetting is for it,then look at what you currently have for jetting,possibly you need to go a little leaner....BUT,start with the leak down,and compression tests first before going too far.

[Philthy Phil]

thanks for the reply, yeah thought the same about the hotter plug, a friend of mine said his bike did the same and he put in a hotter plug and it sorted out the problem, but i know of the dangers of doing that so thought i would ask this forum before risking it. the bike didn't smoke much at all, which makes me think the gear oil isn't leaking into the crank, the jetting has been checked and is stock standard...

[Foxx4Beaver]

ok,do you know why the bike was in "pieces" when you bought it?...did PO have problems with it running right?...it's possible the ignition system could be going south too.

[Foxx4Beaver]

Have you checked your float height,you could have it set too low and could be running outa fuel,is the filter clean,all ignition connections good,this one might sound stupid but it happens...is your gas tank vent breathing properly,is the gas fresh with GOOD quality oil?....alot of simple little things can play a role in this.

[The Flyin Hawaiian]

We've been runnining BR8EGs in our 03,04 and 07 KX250s from new with no issues. Also I mix our fuel at 40:1. As the previous poster mentioned check the top end, ignition, float level etc. Dont rule out a crank seal, Im assuming that you did replace the main bearings and seal when you put it back together. Its very easy to roll the lip of the seal over while reassembling.

[pickledkiwi]

What has changed between pre-rebuild and post-rebuild? Piston, seals etc

PK

[Philthy Phil]

piston, rings, newer crank, seals and all bearings, could damaged reed valves cause this?

[Philthy Phil]

air filter is clean and brand new, have recently replaced all the electronics with a new set.
bike was in pieces due to the gear problem from the previous owner and he never got around to putting it back together, fuel i drained the fuel and replaced it with fresh fuel and using castrol TTS two stoke oil. will check the gas tank vent, and have checked the float height, i have a feeling it might be something silly, but its seems to have me stumped....

thanks for all the help and keep it coming.

[Motorrad]

coil well grounded?

[pickledkiwi]

I had a very similar problem last week with my 99 KX250. Would run for a couple of minutes then stop. Checked all the electrics, changed stators, Coil, plug etc.

Turned out that it was cold seizing.

I had replaced the piston. Same brand, same size but the new piston was bigger than the old one. 68.56mm cf 68.45mm!

I have never had any grief with Wiseco before as their pistons are very consistent however I should have measured it before installation.

I now have a munted piston and a bore covered in aluminium.

Remove your pipe and have a look inside.


Cheers
PK

[Philthy Phil]

Thanks PK, checked the piston she still good, going to double check float height and clean the carb, and jets, and see how it goes. i have checked the coil is correctly grounded, thanks Motorad.
if the problem still continues then ill go with a BP8EG thanks the flying haiwaiian.

will keep you posted as i get on.

thanks again for all the help.

[kwakman]

my 500 would run ten minutes then die.chased the cause for ages. mullered cdi, gives up when it warms up. maybe a rock to look under? K.