Spark plugs going sooty, Causing engine to cut out.

[MRFINCH]

Hi, hope somebody can help. I was out today on the XS400 and as I slowed at traffic lights the bike cut out. I managed to get it going but a mile or so down the road it did it again. When I got home and removed the plugs they were really sooty and black, the left one slightly more than the right. This is after giving them a really good clean with sandpaper the day before! I think this means the fuel/air mixture is too rich, does anybody know how to sort this. Any input is greatly appreciated!

 

[Sesty]

check your float height. When i first got mine it, i noticed the revs would drop really low and it would want to die when i came to normal speed stops, like at traffic lights.

My float heights were way off

 

[Gramatron]

Yes, those float settings are super sensitive. The left being worse than the right may be related to the side stand lean of the bike. If you rule out the floats, move on to the jet/needle settings.

 

[Unkle Crusty]

What heat range plugs are you using. Are they what the book says. Sandpaper can be a bit rough on plugs. Might be a good idea to install new ones. Float height is always critical. Was it running well before? Floats can change, but jets do not.
Is the spark good. If the spark is diminishing, that could be a problem.
How is the compression?

Unkle Crusty