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Discussion Starter · #1 ·
Im new to the motorcycle scene and I just picked up a 91 Ninja 250 to fix up. I have fuel in the crankcase when i drain it and it seems like its running on 1 cylinder most of the time, there is also quite a bit of oil backing up into the airbox. anyone have experience or suggestions. thanks in advance.
 

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Its called dilution

Hi:
Fuel in the crankcase is called DILUTION and is not a good thing. It thinens out the oil and makes it useless. It is caused by fuel getting past the rings and into the crankcase. It can be caused by a number of problems. One could be a stuck float in the carburator allowing fuel to gravity feed into the cylinder while not running and leaking past the rings to the crankcase. Reading your description it sounds like you may have a cylinder that is not firing. (ie-it seems to be running on one cylinder all the time) if that is the case, then the one cylinder that is not firing is still getting fuel to it but the fuel is not burning. Instead RAW FUEL is being sucked into the cumbustion chamber as it should be but not being burned off, thus the unburned fuel is going past the rings and into the crankcase. Possibly a bad spark plug, wire, coil, etc. or maybe the plug is firing but the cylinder in question has no compression due to broken or worn rings. Check for spark first in that cylinder. If it has spark, check compression in that cylinder. It could be a burnt or unadjusted valve. The oil in the air box is consistant with blowby, a condition consistant with worn rings, but it could also be from a burnt or bent intake valve allowing compression to back up into the air intake system. Oil level could be overfull due to the fuel leak. Get the dead cylinder to fire then change the oil and keep an eye on it. But dont run it with DILUTED OIL
 

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Discussion Starter · #3 ·
well, I checked a few things out, there is spark getting to both cylinders and compression is right around 120 on both cylinders as well. With that being said i guess its narrowed down to carb problems or valve problems. thanks for the help and any other input would be appreciated.
 

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Fuel in oil

If you have compression, then the valves should be o.k. It could just be a carb thats leaking fuel into the cylinder while the bike is sitting, fouling the plug and loading the crankcase up with fuel. If there is a fuel shutoff, be sure its shutting off. Ive seen them drip slightly when in the off position.
 
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