450 missfire?

Hi there, I'm the lucky owner of a brand new 250 with no problems what so ever, where as my cousin has a rather sick '03 450. I have been asked to have a look cause he can't tell one end of a screw driver from the other.

Now then the 450 has developed a splutter or a missfire on 1/4 ish throttle, if you hold it there it just coughs along, when you open up she clears straight away and wants to lift the frount wheel, I'm going to have a look at the jets but I'm not sure which one to pay attention too or is there anything else worth looking at, I've changed the plug and cleaned and re-oiled the air filter The bike is bog standard, he recons there's plenty for him with out messing.

Any other ideas will be greatfully accepted,

Cheers, Sid

if you open the throttle all the way from an idle it will do that

also the tps does weird things

could be a few things but first...the needle could be too rich....lean it by raising the clip 1 or 2 spots(flatend on top)

it happens if you hold it there, real pain when yuor in th twisty stuff. This problem has only syarted recently, why would the bike have started running richer? Is this normal asa bike gets older?

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