Spark Plug "sign of trouble ahead" ?

Am stripping down my new 98 WR400F to clean her up etc.. I know the jetting isnt right just yet, but when I removed the spark plug I expected light brown, lean running signs as the bike pops and splutters on full throttle (the T/Stop pin is the right length!) But I found the plug to be black and very slightly oily !! I guess this means a head rebuild (piston/rings) or could this be just down to valve clearences ? or perhaps I'm completely wrong and it's down to jetting ?


I just did a top end job on my 99 WR. I found the same type of findings as yourself. I purchased my bike used last spring. I would like you to Make sure it is oil. What I think I found is that my bike was setup very rich (180 MJ) with the wrong Accel Pump settings. This caused excess gasoline in the cylinder to start disolving the carbon on the cylinder head and top of the piston. I think this is the black oily substance that you might be seeing. If I had to make a reccomendation to you I would tell you to remove the head, valves, and cylinder (to check your rings) clean all parts with solvent. Replace your valve stem seals. Between the Valve seals and checking your rings you would eliminate a possible oil problem if you had one. Then assemble. In the end research and spend time in making sure your jetting is correct otherwise you will do all the work for nothing. Good luck.

ORgie,First off in order to properly diagnose your problem.You should first do a compression test and a cylinder leak down test.This will help to pinpoint your problem.This helps determine whether it is valve or piston/ring related.These are the generally accepted test procedures.BUT FIRST,put in a new plug and try that.It is ENTIRELY possible that you are so rich up top you have fouled the plug,based on your description of the problem.Also you make no mention of cleaning the carb. or checking & setting float level.If you clean the carb CLEAN the carb-this would include COMPLETE disassembly of all components including the accel. pump & removal of all jets.IMHO this is cheaper than rebuiding the top end,try the easy stuff first.Hope that helps :)

thanks for the feedback guys.. I've just done straightening the sub frame with a large pole :-) perfect job all straight now, cleaned and sealed the bits of rust, greased as I went. I've put her back together for now with a new plug and tommorrow will strip & clean the carb and find out what jets are in there, go for a ride and check the plug again.. (didnt think of a comp test, cos the exhaust blast could put out a fire, but will do that tommorrow too when i get round to my m8's garage)


Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now