Hi I have a 99 sherco 250 having problems with it not revving out. I'm new to trials riding so was just looking for some help. It goes all spluttery at high revs. Any help will be great. Thank you.

Sounds like a fuel blockage , either on tank tap filter or in carb its self

could be spark plug.

I will take the carb off tomorrow and give it a good clean then and get a new plug. I already put a new fuel filter in line.

Shouldn't need a inline filter, the dellorto fitted standard has 1 behind the cover where the fuel line goes in

When you say spluttery what exactly do you mean.

If its fourstroking that usually means rich mixture, possibly clogged air filter

If it splutters as in intermittently dying out that is more like fuel shortage.

Make sure fuel line doesn't kink when warm. I fitted inline filter with new tube that moved when hot and blocked supply.

Replace plug with NGK BP5ES gapped to 0.6mm and report back. Buy two, they are cheap!