Running really rough

I took the 900 for a quick run last sunday. Below 4000rpm there was a lot of backfire and rough running, above 4000 it was strong and at 7000 it pulled like a train. Q would a leak between the headre and cylinder head cause the backfire and rough running? I noticed a trace of oil on the exhaust header flange on the rear cylinder is this a valve seal or worn guide

pull the plugs. what color are they? should be tan like toast (w/o butter). bad tank of fuel, kinked tank vent line? check the easy stuff 1st.

Plugs are new NGK DR8ea, I drained the tank and put in fresh gas and there is a full charge on the battery

It really sounds like the symptoms of clogged (restricted) Pilot jets. If it had sat with un(Sta-Bil)treated gas in the bowls for any length of time, they can gum up. They're the smallest fuel soaked orifices, and the first to gum over.

Can't speak for the oil leak.

Thanks Will I should have thought of that. Are you the same Will I met at a US Desmo trackday at CMP in 03?

That would be me.