I have a 00' YZ426 with an aftermarket decompression cam. The bike has always been bullet proof and kicked over with very little effort in the coldest weather. Recently, the bike has been hard kick over at which point I took it to a new mechanic. He said a preliminary check of the motor showed low compression. He said I should have approx 150lbs but was only showing 75. He said I needed a new piston/rings and would check the valves while he was in there. My question is would the aftermarket cam give him a false reading on the accurate compression. If I do need a piston and valves, would should that run me?