Sportsman 90 won't start after sitting on it's side for a while
 

I got a Sportsman 90 that had gear issues (2014 four stroke). When I got it, it started up and ran fine, but wouldn't shift into gear. I opened the gear box, replaced the problem bearings and got everything back in order - didnt have to touch the engine at all (other than draining the oil while working on it). I reassembled it all and when it came time to start, it wouldn't stay running. If it sits for a couple hours it will turn over and run for a second or less, but then die. And if I ever touched the throttle, that would kill it immediately. And after it ran for a second or less, it wouldn't do it again until it sat again for another couple hours (it will just crank then)

I put fresh oil in it, so I know that is fine
I put a new spark plug in it, so I know that is fine
I drained all the gas and put fresh gas in it, so I know that is fine
I put a new fuel filter in it, so I know that is fine
I tried it with the gas dial on both ON and RES (in case one was blocked) and both were fine
I pulled the carb and cleaned it (bowl, jet, ...), so that should be fine
I know the starter is good and I know the battery is good

I am at a loss now for what else to try. I am not an expert with engines but I don't know if maybe it is something with the top end or cylinder or something? What is odd is it ran fine when I got it. And even immediately after I got everything put back together I DID have it running for 30 seconds or so - revving it and all. So I am not sure why it is giving me such issues right now.
It is worth mentioning that while I got time to fix the gear problems, it sat on it's side for a month or two (with fluids in it). Not sure if that might have something to do with it.

Any ideas on what I should check next? I am hoping it is not anything with the engine since I am not as familiar or comfortable working on that.

I can upload a video showing what it does when I try to start it if that helps, but it is pretty much just the starter cranking

UPDATE:
I did a compression test and it was just under 110 psi - so I think we are good there? The manual I have shows 115 - 155 psi
If I remove the air filter and spray starting fluid in there, it will fire up and run for a second or so (until it burns through the starter fluid)
I am far from an expert when it comes to this stuff, but the fact that it fires up with starter fluid - that tells me the compression is good and it must not be getting fuel? Does this mean the valves are fine?
I unplugged the fuel line where it connects to the carb and gas is flowing freely out of it - so the carb is getting gas fine
I removed the drain plug in the carb and gas comes out of that - so that tells me gas is flowing freely though the carb

Any other thoughts on what to do next? I am at a loss. Do people think it still might be a valve problem?

I'd look into valve adjustment.

Thanks for the suggestion. I updated the original post saying I sprayed starter fluid in the air filter and it fires up right away and runs for a second while it burns through the starter fluid. So does this mean the valves are ok? I did a compression test and it is right around 110 psi which I think is good (?). The manual states 115 - 155

I confirmed the carb is getting gas.

Any other thoughts on what to do next? I am at a loss. Do I still need to look into a valve adjustment?

It was the valves! I did a valve adjustment as described in the service manual and that did it. Note that it didnt fire up right away after. I used some starter fluid in the air box to get it going and then it kept on running

Thanks for all the help