Bike has started running rich?

Hello chaps,

I think my bike has started running rich but I don't know why. Nothing has changed since I got the bike in May.

Under hard acceleration with over 50% throttle, usually in gear 3 or above, about half way up the rev range the bike hesitates, only slightly, but noticeable. It will be gaining speed then just die a little for a split second - same part of the rev range. It's not a flat spot - it's a fuelling glitch.

The previous owner had put a K&N air filter in so I cleaned it and oiled it making sure I didn't over-oil it and changed the plug.

Also, there is a sooty patch on my rear plastics by the exhaust exit. Also the plug is pretty black.



I'm pretty sure the bike wasn't doing this when I got it.

The only thing I can think that has changed is the air temperature but surely that wouldn't affect the fuelling this much?

Any pointers as to where to start looking? It's a 2005 Honda XR400 by the way.


Air temperature and humidity can really play with the way a 4 stroke runs.

With that being said, the first thing I would check, besides the obvious (air filter etc...) is to make sure that the main jet hasn't fallen out.

Don't laugh, I have had that happen on 3 occasions. On all three occasions, the bike had been previously re-jetted by another person. They apparently did not tighten the jet enough. Check this post: Main Jets falling out

All you have to do is remove the drain plug at the bottom of the float bowl and check.

Good Luck!

Hey dmrogers thanks for the info I'll keep that in mind for the future.

In my case it seems the pilot screw had somehow worked its way loose - wound it back in and back out to factory setting (2 1/4 turns) and hey presto - the glitch has gone and bike is more responsive and goes better.


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