I recently inherited a 2003 Mule 3000 with about 600 hours on it. It would start and run but it definitely didn't run very smooth and was sluggish with trouble idling. After getting it home it wouldn't start to even get it off the trailer. I did some research on this forum and decided to pull the heads and found both intake valves had stuck open which isn't surprising as the gas was at least two years old. I decided to put a carb kit on as well since I had to pull everything off to get the heads cleaned up. I followed the service manual for all specs and got it all put back together. It started right up but only lasted about 20 seconds before it started knocking so I immediately killed it. I pulled the spark plug on both cylinders and found coolant in the rear cylinder. What is the likelihood that the gasket didn't seat properly? Or could I have an issue with the head itself? I'm at a loss as it ran right up and prior to this when I first loaded it up to come home. Any ideas on where to go from here?