I just put a carb from a polaris magnum on my 98 arctic cat 300 4x4. I plugged the top vacuum port but it wont idle unles choke is on and it revs up pretty high. Carb is clean but it has a 40 pilot and the AC uses a 45 so I ordered one. Even so, shouldnt it still idle with a 40? It get very hot so much that the gas started boiling. Is the mixture screw for air for air or fuel? Also, how do you check float height?

 

Mixture screw is for fuel not air. Theoretically the Polaris Mikuni carb should work since it's similar to the cv carb that was on it,at least by the parts list on the AC I looked at. Haven't come across something like this before on swapping carbs from two different machines,but since your system didn't use a fuel pump,just a direct flow fuel valve could have something to do with it. May not have enough engine vacuum to lift the slide? Plus was the original oem carb a Mikuni or Keihin?Jetting is not the same on these two carbs. Doesn't show on any used ones I've found,but did find a new Keihin that is supposed to fit an AC400?? Keihin Carburetor Arctic Cat ATV Brand New Artic Suzuki | eBay

 

They had mikunis from 98-99. Then switched to keihn

 

How could using a pulse fuel pump create more vacuum for the slide? Dont vacuum come from the engine?

 

The vacuum from your engine may not be enough if the slide doesn't lift under throttle,butterfly open.That's what I'd check first. If compression is good and valve clearance is ok it should be able to do this.

 

But would that affect idle?

 

Nope, different circuit on just the idle. Could be all kinds of reasons why on the idle:intake manifold leak? The carb may not be as clean as you mentioned,trash,etc may have been sucked in the carb,plus did you just clean it as many do and not order a carb kit or a needle and seat? Float height? Sets at 13mm(0.51). Plus I know jetting will be different as you mentioned from your first post,but shouldn't be enough to stop it from at least idling if everything else is right. Too many variables as both the AC carb and the Magnum are close,but not the same carb. Only other thing I can think of is the fuel mixture screw. Some early Magnum 325 and 330s also were bad about cold starts and idling. Screw is under the block off plug at the front of the carb. Check how many turns to a lightly seated position it is. Some were only open one turn,found one only 1/2 turn out. Set at 2 turns and see if it idles.

 

Well the carb was supposedly rebuilt when I got it. Ive already adjusted the fuel screw out 2 1/2 turns and it idles really high with the choke off. If I block off some of the intake with my hand it idles down.

 

Don't ever trust what any one says unless it's your mother. I'd still check the float height along with any cable or slide sticking,air leaks if it idles too high with the choke off and if the idle adjuster doesn't have any effect on it.

 

So float height set to 13mm? That seems low. Ive read around 16mm is that not right?