I picked up a 1991 Yamaha Warrior that has been sitting for some time. I cleaned and rebuilt the carb, adjusted the valves and replaced the spark plug. It starts and runs but doesn't start that easily hot or cold. I took a hot compression test and without holding the throttle wide open my gauge reads 90 psi, with the throttle open it reads 150. My intstints told me it probably needs a piston but what is with the huge difference in compression readings between throttle and no throttle and why so so high. Carbon deposits??

 

The difference is because with the throttle closed it cant suck in as much air to compress. The more air you can get in there the higher the compression will be. 150 sounds high but not astronomical. Maybe some carbon, maybe someone put in a different piston than stock or maybe you just have real good intake and got a real good spin on it. Based on compression I dont think you need a new piston. Low compression would indicate that not high. 90 is ok with the throttle closed and 150 is plenty also. Im not sure what it should be, 150 is more than I got with a stock piston. Dont remeber the exact number but I would had it been that high.

 

Ok thanks, it must be in the carb then and I will have to go throught it again. any other suggestions? It runs good when it runs but just has a had time starting, on cold start ups I sometimes don't need to use the choke which tells me it is rich.

 

yea 150 is really good i think 114-138 or something is the normal but i cant remember