I'm hoping one of you can help me with this problem - it's got me pulling my hair out.

My GF has just bought herself an RS1800, but it doesn't seem to rev properly. It pulls like a train until you get to about 4000rpm, and then all of the power drops off suddenly. It doesn't misfire it just seems to loose power unless you change up. If you keep the pedal down it will hit 6000rpm, but its a real struggle.

I've checked the cat and it doesn't seem to be blocked. The fuel filter looks new and the cam timing seems to be OK. The air filter was a bit grubby, but I'm guessing it would have to be filthy to cause a problem like this. I've also checked the plugs and they are in good condition and correctly gapped. The MAF is clean and looks quite new.

I've connected it to fault code reader and no errors are reported.

Has anyone got any idea what might be wrong with it before I find myself reaching for the hammer...

Cheers
Mark