2 weeks ago I changed the oil on my 14 hp Command Kohler. I used wd40 which was apparently a big mistake. The engine backfired while it was running and from that point on it stalls when in full throttle or under heavy load. I can still use it at about 75% throttle. My mechanic told me the valve lifters are messed up and he wants $250 to replace them. I ran a search on kohlers and found countless threads with the same problem, but most thought it was a fuel problem. What do you all think? Is that a fair price?