I have two nearly identical 2001 Trail Bosses. One is a pre 12/11/00 the other is a post 12/11/00. The post machine will start to bog down at about half throttle. I have swapped almost everything between them, ( carb, fuel pump, lines, tank, etc). Nothing seemed to work until I swapped air boxes. Now the post machine runs fine. The pre machine will run well if I take the cover off the air box. Anyone have any ideas. I would hate to drop a hundred bucks on another pre 12/11/00 air box and filter if I don't have to.
Thanks
Mike

 

Like you mentioned the major difference between pre and post 12/11/00 were the air boxes and also the updated oil lines(hopefully someone replaced yours) that had screw type fittings at the engine versus the first models that just used hose clamps that would blow off.(That may have been on the early 2000 models,hard to remember sometimes) Funny though that you have better performance with the old style air box with the long air intake snorkel versus the short stubby one.We had to end up cutting these back a little because they would rub against tanks blocking off air supply and fall on their face.. Just my two cents is that the problem is in the carb itself especially if you have to remove the air box lid for it to run right. Usually that means the carb is getting too much fuel,removing the air box lid leans it out. I'd rebuild it,plus on BOTH models I'd drill out the block off plug under the carb and check to see where the pilot screw(fuel screw) is set at. Most 325 engines were so cold blooded that on some you had to leave the choke on for them to run right until they warmed up. I found several set at 1/2 turn out and one I found was completely closed from from the factory(Mikuni) Both need to be set at around 2 turns out from a lightly seated position and this should help on cold starts. Back to the offending atv,get a rebuild kit and then see how it does.Sounds like the needle and seat are worn allowing too much fuel. This is the best rebuild kit around.New Shindy Carb Repair Kit Fits Polaris Magnum 325 Trail Boss 325 03 423 | eBay

 

You can be sure that the pilot screw is misadjusted, I messed with it myself (hate to admit that). I now have a tach and will go through the pilot screw adjustment procedure in the service manual. Hoping I don't have to rebuild the carb. I had one of them done (swapped so many times I don't remember which one anymore ). I will work on it this weekend and let you know how it goes.