Working on a 1998 Polaris Sportsman 500. This thing had a couple of unusual problems that baffled a few good techs. Got the starting problem, previously posted, repaired and now starts like new.
I tested this ATV with the air box removed, ran like a scalded dog at high speed, pulled some impressive wheel stands, awesome power and idled like a Cadillac. Put everything back together and it bogs badly in neutral or in gear, won't rev up.
Removed air filter, same problem. Removed air box, ran like new again.
Replaced the carb with a direct factory replacement. Float level correct, choke and throttle cables adjusted to specs. I removed the air intake tube from the air box, checked for obstructions, there were none. About 2-1/2" on the filter side and 1-1/2" on the carburetor side. I can rev the motor to 3k or 4k RPM's, slip the air intake tube over the carb air inlet and the motor practically dies. Remove the intake tube and the RPM's pick back up almost immediately. The original carburetor did the same thing and was replaced because of this.
Is it possible for jetting to be this far off on a factory replacement carburetor?
Anyone have any ideas?

 

sounds like your jetting is deffinatly off, find the pilot jet on the carb. if its a mikuni carb then its an air pilot. in that case move the pilot jet OUT 1/4 turn at a time, this should increas the amount of air mixing with fuel at idol.

i would not think the fuel/air mix skrew would be misadjusted, but the pilot pissibly. plus the port&pollish job from the factory might not be to specs, so standard settings would make it run bad.

Deffinatly try adjusting the pilot first, if you have the lid problem at all RPM's then thats my guess.

 

mine runs like crap with the airbox lid off

 

gaugedholes, that means your letting too much air in and its running lean. leave the lid on utilitys IMO

 

Is the airbox possibly rubbing on something? Like the choke cable?

 

It does this with just the air box and no filter?, Has to be something plugging the air tube.

 

Strange, but true.
I removed the air inlet from the air box, thought the rubber coupler might be collapsing and blocking the intake. I pushed the air inlet all the way up to the air inlet of carb, no chance of the rubber coupler affecting air flow. started it up and it idled like new. Attempted to rev up the motor, bogs down and practically dies. No black smoke, no backfire, just bogs down. Remove the air inlet pipe and the motor screams back to life.
Blockage, no!! I can practically stick my hand through this air intake tube. It's about 2-1/2" diameter on the filter side and about about 1-3/4" at the carburetor end. There is a couple of sweeping bends get from the air bow to the carb, but definitely no blockage.
Away for a day or two, but will try adjusting the pilot screw as previously suggested to see what happens. Jetting is the only thing that makes any sense.
Unless there are any other ideas.

 

Pilot screw should be set at 2 to 2.5 turns out.

 

I've got the EXACT same problem with my 1995 Polaris Magnum 425 4x4. Runs great disconnecting air box from carburetor but as soon as the air box is connected to the carburetor it won't rev up in neutral. Misses terribly. I should have the stock #140 main jet but didn't verify (just bought this machine a week ago).

Did you try adjusting the pilot screw yet? VERY interested in the fix for this.

 

<div class="FTQUOTE"><begin quote>Originally posted by: jbuddenberg

I've got the EXACT same problem with my 1995 Polaris Magnum 425 4x4. Runs great disconnecting air box from carburetor but as soon as the air box is connected to the carburetor it won't rev up in neutral. Misses terribly. I should have the stock #140 main jet but didn't verify (just bought this machine a week ago).



Did you try adjusting the pilot screw yet? VERY interested in the fix for this.</end quote></div>

THe air intake tube is suckking shut.Is the spring still inside the air snoot?
It is part # 19 in this diagram for a 425 magnum
http://parts.polarisind.com/Browse/Browse.asp