My brother just bought a used 08 yz450f the guy he got it from said he had to do head work because it jumped timing. He test rode it and it looked like it ran fine got through all the gears no prob. But we went to the dunes and it was a little poppy slowing down, he let me take it for a spin and it felt super dead on the low end, mid and top were powerful though. Do these bikes have problems with valves like the crf's? Is it normal for this bike?