Motorcycle Repair: bike goes off at stops, mixture settings, yamaha fzr400

Question
hi chris, I have a yamaha fzr400, 98model and it goes down on me at signals and other stops. Also it is smoking black when I open throttle wide. Is it the carburettor jets or air intake? As is difiicult to open flairings to inspect. Thanks


Answer
Hi Hasnain.
 The first thing that you need to do is pull the spark plugs and see if they are fouled.

If they are, which I suspect, then the bike is running rich.  As long as no one has changed the mixture settings on the bike, then the problem is likely to be that the air filter is clogged.

Take a look at the air filter.  If it is dirty, then replace it.  Yamaha's have a noted tendancy to start running rich if the air filter is overdue for replacement.

There is also the possibility of the floats not closing correctly.  This will also cause the bike to run rich.

And lastly, check your choke.  If it is not opening all the way after warm up, then it will also cause the bike to run rich.

Start there and let me know what you find.
Good luck.
FALCON