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Bike has started running rich?

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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.

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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.

Cheers.

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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!

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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.

Cheers.

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