I own a 1987 yamaha fz 600 air cooled motorbike, its started to blow what looks like a mixture of oil and petrol through the crankcase breather pipe going into the airbox, any ideas what this could be? i think it could either be the piston rings or maybe a too light oil. also would anybody be able to tell me what other engine would also fit into the frame? maybe an fzr or diversion, any help would be much appreciated, thanks.

I have the same bike and the same problem...the floats in 2 of the carberators were bad, and since there is no fuel shut off valve the cylinders and what ever else the fuel could leak into- did. change your crankcase oil and make sure the air lines that run underneath the fuel tank are not pinched or this could also cause the gas in the tank to "flood" out your carbs when cranking it over. also check that it is jetted for the area that you live in, most are factory set for the California area...