I'm not particularly familiar with ATV's or small motorsport crafts. Although, seems like the basics apply. Internal combustion engine, fuel injected, or carbed, etc.What I do not know, is the more special things related to them. So this is why I figured I should post something here as I know some of you guys are more seasoned at them.I've got a Yamaha Big bear and I think its a late 90's. Apparently had a tune up and the likes at a local dealer. My father in law begs to differ as he is having a hard time figuring out what he paid for.So I drove it a couple days ago to go looking for a deer I thought I may have killed. Took a road and got going fast. Thumb-throttle pegged all the way and in highest gear.Once it gets to speed, its like it will suddenly lose power for a moment and you can hear it. It feels like someone is hitting the brakes for a moment. It will go back and forth like this until I let off the throttle.I called the dealer and asked them, they think its the carb needing cleaned or rebuilt. I hope not as I need this for next week.I thought maybe it needs an inline fuel filter or something...Anyone have any thoughts on this?

Those fuel tanks build up a fine dust in the bottom over a long period of time. I think I just removed the tank nipple that connects the fuel line and gave it a good flushing out. The carb I tried cleaning with carb cleaner while I had it apart but, nothing worked till I got the carb sonically cleaned.I like those in line filters they sell that are clear and can tell when to change.

Pull the spark plug, what does it look like? What if you straight up pin the throttle (any gear) does it fall on it's face?

I bought my Bombadier used. It came with spare plugs... when you only have one cylinder, one plug, it must be a good spark plug. I had had it tuned & serviced in a Pro Shop... And they missed this. I couple of years later, I had it in a different shop... and they found it had the wrong spark plugs in it. But the plugs had the same brand & correct number. I couldn't believe it & insisted on the old plug back.

What I learned is there are 2 versions of the plug with the same number. W H A T ? The knobby contact must be removed & the bare threads exposed. The coil slips down inside the tube in the valve cover, and there is only a small hole for the threads to slip into the coil.

Sooo, in my case, the spark was having to jump to the coil as well as the spark plug gap.

I do not know about your machine but this is how Bombardier did it.

PS: I took the used plug, clamped in a vise & tried vise grips. The knob was NOT removable on THAT plug. I ordered more spares from a ATV shop on line & got the correct ones.

Assuming its a carb'ed unit I would pull the carb off and go back through it and double check all fuel jets and all air jets for plugging. Change the fuel filter again. Might go as far as change the tank of fuel out if it got moisture/water in it.

Any chance it's just revving higher after the work was done and now it's working against the rev limiter? If it has a rev limiter.

Does it have Gas? Low on fuel? It sounds like a fuel restriction... like it uses up the fuel in the carb bowl, and can't refill fast enough so loss of power. Could be a clogged filter, restricted strainer at tank outlet, stuck float. Possibly contaminated fuel. Draw off a sample put in a (tightly capped) glass jar to settle overnight... I used to do this often in boats... (drain water separator into container)... prevented doing work that was not needed.