Out for a bimble on the SP4 today, everything fine and dandy except that the bike wouldn't rev beyond about 8500 rpm. No other problems, no funny noises, water temp absolutely fine at 70-75 deg.
If the tank is not open to atmosperics and the pump is running it could cause the tank to partially vacuum. stopping the fuel from going to the fuel lines 4D
I concur with the fuel filter theory. IME a blocked breather is more likely to cause the engine to stop or splutter as it simply eventually fails to get any fuel due to the vacuum that builds in the tank.
I had something similer happen to mine, it just would not rev. I was high up in the Alps at the time. Stoped for fuel a loud hiss as I opened the cap after that It was much better. After that I left the breather off. No probs.