98 Wr400f Problems, HELP!

I have a problem with my bike. For no reason a few weeks after I put on a fmf powercore IV with the quiet insert the bike starts acting wierd. It's kind of hard to start now and when it warms up it backfires a lot and won't idle. it just stalls. Before this it was jetted perfectly but by the previous owner. It started on 1-2 kicks and made loads of power! Any idea what happened?

You're too lean with the new pipe. Time to re-jet.

ya it needs rejetted

not sure what the problem is but I have a 98 400yz w/ power core IV and power bomb header and it works ridicuolosly great far better than the old stocker..., well I would hope so, good luck

Ok, so why would it change a few weeks after he installed the pipe??? In all likelyhood, it's a plugged pilot jet.

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