Xj900s missing on cylinder 4

Greetings all
I am a newbie to this site and also the XJ, although I have owned a variety of bikes throughout my life. Reasonably handy but no mechanic...never stripped a carby other than an Amal concentric on my Beesa.

I purchased the 1998 xj900s as a commuter (and toy). It is a ex-police bike with nearly 100,000km on the clock. The motor feels strong although it has developed a misfire on cylinder 4. Initially this only occurred under load however has progressed to a persistent misfire.

I have replaced the plugs and put carby cleaner through the carby's which brought about a slight temporary improvement. I recently replaced the fuel hoses and filter which seemed to fix the problem ..........for about a day.

The misfire is definitely on cylinder 4 (I have swapped plugs, caps and leads without change). The plug develops an oily buildup compared to others.

Of interest is that while removing leads while the motor is running, I get electric shocks...is this kind of current leakage from the leads normal?

Also, there has consistently been a strong smell of fuel with a slight leak apparent from the carby for cylinder 4. When starting, you need to give the throttle a couple of good pumps while firing and then it farts into life.

I am not sure whether this is a spark /electrical problem with the fuel leak being due to some sort of back up in the carby due to the misfire, or a fuel problem causing the misfire or both.

The only other symptom which may be unrelated is that on occasions when starting, the starter will not turn over at all ; after a couple of attempts however, it will cooperate.

The battery shows a good reading when tested. The resistance across the coils is the same in each although slightly higher than the manual suggests (around 27ohms if I remember correctly).

Any suggestions as to where to start?

 

Do a compression test

 

not firing on Cyl 4 is possibly indicating time to do a thorough cleaning on the carbs. If it were missing on 1 and 4, I'd say a coil, but if only on a single cyle, it's not coil related. Smell of fuel is because it's not getting burned....its raw fuel going through.

Now, before you jump right into the carbs, I will ask "when was the last time you checked the shim clearances?" It is quite likely that if you never checked them, you may have some valve issues instead, which would cause the same thing

Starter issue could be indicating that it's time to at least check the brushes on the starter.

Dave F

 

Welcome to the forum. Probably float level. Sounds like you need a starter rebuild too

 

"carby"?

 

CARBY....... Australian slang for CARBURETORS or CARBS here in North America