I recently bought a prairie 650 that will not start. I was told that the engine got a little warm so I assumed it was low on compression. I checked the compression and it has 50 psi on both cylinders which according to the manual I found online is within the specs of 47-81 psi. I cleaned both of the carbs and replace the rubber boots from the carbs to the intake. I replaced both of the spark plugs as well and it is getting good spark. I did get the motor to start with quite a bit of starting fluid and it stayed running with some throttle. I also set the valve to .006 on the intake and .008 on the exhaust. All of the valve appear to be opening and closing. I also remove the cover from the left hand side of the engine that exposes all of the gears and it all looked good. What am I missing that makes it not start?

Update:
I performed a cylinder leakdown test and it showed that there was an excess leaking past the piston rings. I replaced the piston the piston rings. The machine runs but seems low on power and backfires. I’ve checked and rechecked the timing. I’ve also checked the valve adjustment numerous times. When the machine is running the slide in the rear carb does not move, and the front one moves just fine. I pulled the slide out of the rear carb and the diaphragm seemed to be seated. The one side of the diaphragm looks burnt though (image attached). Is this my issue.