Have a 670 that will start and run for about 20 -30 seconds then die out. I don't know the history of the saw as it was purchased as salvage. Have cleaned and then rebuilt the carb. Runs the same after cleaning and rebuilding. Starts normaly, choke and throttle lock, 2-3 pulls it will pop. then un choke and it will crank and run. Good throttle response as soon as it starts and then seems to get weaker and then will quit. Will pop but not start back up. Will repeat next day. I had an atv once did that and it was low compression. Start cold then die and not start back when warm. Checked compression after it quit and had 120 psi. Have not checked it cold. What should compression be on this saw?