I have a 1999 Polaris 300 Xpress. If it has not been started for a few days I have to prime it a few times to start it. Carburetor was rebuilt a couple of years ago. It runs like a scalded ape once started and warmed up. I thought about just putting a primer bulb on it but there is not room for one. I have reason to suspect that the oil pump might be djmping oil into the carburetor when I shut it down but oil consumption seems normal. I am going to remove carburetor rebuild it and soak it in cleaner for awhile. Also thinking about replacing the carburetor adapter at the engine. It is rubber with a clamp and tight. I run only ethanol free gas with a little Sea Foam.

https://offroadingpro.com/polaris-xpress-300/
Some ideas. Mikunis are pretty good carbs. If fuel leaks from carb bowl, takes a while to get mixture right. Same as if bowl needle sticks a bit.

Familiar with fuel leaking out of the bowl. When that happens I know the needle needs to be replaced.

I joined the Polaris forum and a mechanic on there said check compression. He said having to prime is an indication of possible low compression specifically less then 100. I tested and it is 114. Spec for motor is 115. I did not open the throttle when testing. Not sure if the spec is throttle open or not. The Polaris mechanic said to retest with throttle open. I suspect it will be lower. I mentioned that I don't like to use starting fluid on a 2 stroke. He said it is no harder on a 2 stroke then a 4.

Most compression readings are with the throttle wide open

I’d check the reeds when you pull the carb off since the compression seems to be in spec however 115 seems really low to me. If their busted or leaking that will definitely make a 2 stroke hard to start and ever so slightly boggy on the bottom end but will still run like a scalded ape in the mid range and top end after you get in running.

The compression should be checked with the throttle wide open and if you have a camera snake you can look though the plug hole and check for a scarred cylinder and when you remove the reeds you can get a decent look at the back of the piston as well. From what you’ve described it sounds like it’s low on compression and or the reeds are shot.

Checked compression again with throttle open. It is 119. Replaced the coil and put in a clean plug. I only replaced the coil because I broke the spark plug cap on old one. The OEM coils are no longer available and with the aftermarket ones you have to remove the aluminum cap on plug. They fit real tight and they are plastic. It cranked right up. Let it sit for a day and then 2 days and starting fine. I will check specs again on compression but I read 115 to 120.

Your compression is fine. If it bleeds down SLOW you are good.

Looks like clean plug and coil fixed it. After not starting for a week it fired right up. It has always been cold natured so waiting to see what happens when temps drop.

I had to prime it to start last week. I decided to put a good charge on the battery. With a strong battery it fires right up every time. I had a problem last year with hard starts on lawn tractor when cold. I had to replace the battery and went with a 330 CCA Interstate from tractor supply. The higher CCA battery makes a big difference versus the crap that the big box stores sell.

Hard to start again after sitting. Put a good charge on battery and cranked up. I have not checked CCAs on current battery but thinking low 100s. I am looking at batteries and see several AGMs with CCAs I want. Some are AGMs and wondering if anyone is running AGMs in older ATVs. I know some charging systems don't fully charge AGMs.

I use a battery tender to keep the battery topped off. Sure helped my hard start situation.

Likewise and I have had this battery on my ATV for over 4 years. I do think some fuel injector cleaner helps too once or twice a year too. One little spec of dirt in the injectors will shut an ATV down.

This sure helped us for sure this year. We started seeing some issues last year when the weather was cold and then they were also popping up this past summer which we have never had issues then. We started using the Lucas injector cleaner and the problem went away. We put the small bottle in every other fill up which might be overkill but it works for us.

I don’t use my ATV near as much as I did 3 years ago. If it’s working for you keep doing it. Carb injector clean out is above my pay grade and not cheap.

I only run ethanol free in my ATV and add some SeaFoam. I had some issues with the lawn tractor running rough when the blades were not engaged. A tank of gas with SeaFoam added and it runs fine. Before I buy a better battery I am going to test it again with a good charge. I would like to get something with 300 CCAs but limited on size by the battery holder.

if that model has a compression release on the cam, id almost guarantee thats your issue.
theres a detent ball on the cam itself and it gets debris behind it and doesnt open the valve like it should, it will also ride in the rocker arm and groove it out, not opening the valve