Purchased a 94 sportsman 400 2-stroke ATV about a month ago. I know the ATV was used on the farm mainly, and does not appear to have been abused. The battery on it was dead, so I had it hooked up to my battery charger running on 12V @ 75 amps to start the ATV during testing. I replaced the spark plug and while I was doing that, I performed a compression test on it. The compression reading was 93PSI. I know the manual says that anything under 100 is under spec at sea level. My location is 4800 feet above sea level so I know that my readings will be a bit lower than the manual states. The ATV turns over and fires right up. Seems to run good, so I am not sure that I need to rebuild the top end.

However, since I know it might be in the cards, I am trying to see which route to go if I do need to rebuild it. I have found some 85mm Big bore kits that come with the head, piston and all gaskets online for around the same price as the stock 83mm top end kits. If I decided to go with the big bore, is there anything else that needs to be modified other than what comes in the kit?

Are the big bore kits worth putting on, and what kind of power increase can be expected with a 2mm oversize bore? The ATV is used for trail riding, hunting, and back country rides. I don’t bury it in mud, or race it.

Are there any down sides to going with a big bore kit over the stock 83mm bore?

Thanks for any help