05 Sportsman 400 spits and sputters, doesn't stall
 

Hello everyone, I have come here for some help. Lets start with a little history. I have 2005 Sportsman 400, that I bought back in 2012. Ran good up until about 2015. It all started when I had gravel hauled in for my driveway. It was heavy crushed garbage thats used for underlayment on major highways(it was free). I was using the ATV to drag the driveway, and was working it pretty hard making sure I was always in low gear. It had gotten to a point where the muffler got so hot, it melted the plastic side shield. At that point it had started to run a bit rough. When easing into the throttle it did okay, but try to go half throttle to wide open, it would begin to bog, and sputter. Since that day, it will always do that once the engine is warmed up. Some days it would take 20 minutes of riding to start acting up, some days it would take an hour. As of late, it's begun to do it after as little as 5-10 minutes of riding. It also, wouldn't do it in low in the beginning. It now does it in low after just a few minutes. I use this ATV to pull a mower around the yard. It acts and sounds like the rev limiter is kicking in. Will not stall, will just sputter. If i ease into the throttle it might do okay. Also, there has been a few times, it would act up, and in the middle of it acting up, it would clear itself and run fine for about 20 minutes. When it is acting up, I can hold the throttle all the way down, and will spit and sputter until I let off, but does not stall. As of late, it has started blowing a little blue smoke when acting up. I have noticed as well, in the past, it does okay for the most part in winter. If I take it ice fishing, it does great on the lake, no issues. Things I have done: Rebuilt carb(was doing this before I rebuilt, hence the reason for rebuilding), wrapped the exhaust, and shielded the gas tank(thinking maybe vapor lock), replaced muffler(thinking it was maybe plugged). Next to replace, will be spark plug(drawing a blank, if I've replaced it or not), plug wire and coil and possibly a whole new carb.

This ATV was used and abused before I purchased it, but the motor seemed to run good. I'm coming here hopefully to save myself a trip to the dealer, as I don't want to put a bunch of un-needed labor costs into it. I know the exhaust lobes were an issue on the older models, and am hoping this isn't my issue. But it sure is acting like it could be. It's just weird, because it will come out of it every once in a while. Any ideas?

Check your valve clearance.

Also, take a look at the intake boot/tube from the air box to the carb. Mine was running like yours this weekend on a ride, and I found a big hole in mine, right where it meets the carb.
Take a look at the carb boot as well.

Okay guys, its been a little while since I updated. I like the suggestion of checking valves, I just don't have a lot of knowledge on these things, so I need extra time to work on it, but don't have the time at the moment as I use it to pull my mower to mow a few acres 2 times a week. Was hoping to dig into it this winter. But I was mowing with it yesterday, mowing at about 4 mph, in low, It was spitting and sputtering, then clear up and run good for a little bit. When I got done mowing, I decided to run it a little in neutral, and noticed that it was blowing a little black smoke when I revved it. I had just changed the spark plug by the way, so was wondering if maybe it was burning a little carbon(sounded like a good theory). But then I heard what sounded like the belt let loose and grind a little, and saw a poof of smoke come from the housing. It smelled like burning belt. After that I thought I would ride it, and see if the belt was going to slip. It never did, but it was weird, cuz it started running really good in high gear. Only a couple times did it bog down on me at WOT. But I put it in low, and it wont go over 4 mph, it spits and sputters, but never dies. Could this have something to do with the clutches, or belt? I'm not 100% convinced its carb, cuz it idles great, starts great. Also, I have checked the intake boot, and all is good there.