1995 Polaris magnum 425 will not run with airbox hooked up

[Kurt Reiser]

Frank there is a vacuum line coming from the oil reservoir going to the airbox then the two vacuum lines that go onto the carburetor they are all clear. Which direction but I want to go with the needle up or down

Edited February 27, 2019 by Kurt Reiser

[Frank Angerano]

Ok the reason I’m asking:

1. I’m wondering if there is a problem with the vacuum where as soon as you hook the air box up the vacuum lines are drawing all the air from the air box starving the air flow into tye carburetor. Why? No clue! Just wondering if that’s what’s going on. May sound stupid but you never know   

Try to move the needle on the carburetor and see how that goes.  Make sure you remember where the clip was on the needle before you move it. 

  If that’s not the fix then disconnect all hoses going to the air box, reconnect to the carb and try to run the bike and see what happens.   

Side note. That clip and spring set up in the carburetor can be tedious. Parts can fly off and never to be found again. Work in an area that will prevent that from happening.  

[Kurt Reiser]

Sorry to ask this again should I move the needle up or down?

[Frank Angerano]

Never be sorry to ask.  Better safe then sorry.  Moving the clip to the top notch lessens the amount of fuel coming in. Moving the clip to the bottom of the needle allows more fuel to flow.  

Where is it at now ? (The clip)

[JacobSlabach]

haha yeah make sure you are where you wont lose parts.  I almost lost the c-clamp off my sportsman carb yesterday....and I was in my workshop lol

[Frank Angerano]

Any progress? 

[Kurt Reiser]

 Just got finished raise the needle did nothing

[JacobSlabach]

hmm  could you upload some pics of the bike (like the carb, air box etc?)

[Frank Angerano]

Hoses connected  to the air box like the vacuum etc ? Did you try to disconnect them lines and reconnect to the carb and see how that is ?

[Kurt Reiser]

I just got called into work I will load pictures tomorrow when I go back to my garage thanks a lot guys

[davefrombc]

Did you check the carb to see if it  had  both the  air screw  and the fuel  screw? I can't see the  air box being a problem  unless either the  main  duct to  atmospheric air  is severely  restricted or the air filter  plugged.  The amount of air a vent or vacuum line connected to the  box would affect would be absolutely minimal .

When  you  move the  clip on the  slide needle , you  should only try one  notch at a time up  or  down at a time  from the original  setting .

[Frank Angerano]

I agree with the air and vent/vac  lines having minimal impact on the air box and it’s baffling.  I’m just trying to look at everything.   @Kurt Reiser has said he’s checked for any obstruction on all openings air filter etc so how can this be? Very strange.  

[davefrombc]

That's why I suspect a  plugged passage or the carb has both an idle air screw like most  carbs , but may also  have a fuel mix screw ..  The ones I see illustrated like the one in the video  I  posted are  not the thumbscrew  type but look  more like a  regular  plug screw ( for want of a better term )

 

[Frank Angerano]

Agreed.  

[JacobSlabach]

10 hours ago, davefrombc said:

Did you check the carb to see if it  had  both the  air screw  and the fuel  screw? I can't see the  air box being a problem  unless either the  main  duct to  atmospheric air  is severely  restricted or the air filter  plugged.  The amount of air a vent or vacuum line connected to the  box would affect would be absolutely minimal .

When  you  move the  clip on the  slide needle , you  should only try one  notch at a time up  or  down at a time  from the original  setting .

I agree👍  Although....  I moved my needle on my sportsman's carb up two notches...  problem solved.  no more adjustment needed- the bike runs perfect.  Its got to be something big that we are just missing to be causing the bike to just not run....  Are you sure its starving for air and not fuel?  I mean it seems pretty obvious to me that its air...  but I wouldnt just rule out the fuel till you're sure.

[Kurt Reiser]

Frank I was having problems signing in for some reason I’m very illiterate when it comes to these computers what I said was I don’t think it would be a carburetor problem both carburetors are doing the exact same thing. there is  only one screw on the bottom of the carburetor I’m not quite sure if you asked that question it seems like it’s something very simple because the bike runs great as long as you don’t hook up anything to the back of the carburetor it’s bizarre There are no obstructions in the airbox there are no obstructions in any of the vacuum lines I will check the cam and check the valves check the plug have move the needle up-and-down one groove at a time.Sorry if I sound repetitive I’m just getting so frustrated

Edited March 1, 2019 by Kurt Reiser

[JacobSlabach]

If its worth it to you, I would love to take a look at your bike if you brought it down to me-  I cant come to you because I'm under-aged and cant drive yet😣

im in christiansburg VA.  If you are interested, massage me privately and I can send you my address.

but if you want to figure this thing out on your own, I totally understand!  I'm just really curios as to what the issue turns out to be.

[Frank Angerano]

@Kurt Reiser I can’t begin to tell you how many times I’ve looked at something over and over just to find I did miss something. So being repetitive is not a bad thing.  

Edited March 1, 2019 by Frank Angerano

[Kurt Reiser]

Frank I’m not sure if it was you somebody asked for pictures of the carburetor I believe here they are

[Juliusjammer]

Ok, before someone gets mad at me for resurrecting an old thread, this diagnosis information is valuable and still very valid.

I have a 1995 Polaris Magnum 425 4X4 and I am having the EXACT same problem.

Did you ever figure out the problem and if so, what was it?

My carb has been rebuilt, new fuel pump, new fuel lines, new OEM Polaris air filter, new OEM fuel filter, new recommended spark plug, fresh ethanol free gas.

With airbox cover on it bogs down, with airbox cover off it runs way better but will not go over 40 mph, but at full throttle with the cover off I get that more powerful high rpm sound you get when you goose it.  with cover on it bogs and struggles.

It seems like something is restricting airflow but it is not.  

Could the carb just be needing to be adjusted to allow more air?  

When it is warm with airbox cover off it and in neutral when I rev it the engine quickly returns to idle like it should if it the carb is tuned.  But maybe there is room there for better tuning?

Any help would be appreciated.

Jeff

[Juliusjammer]

I did just talk to a Polaris tech and they suggested trying a smaller main jet.  He said it is getting too much fuel.  And the cover off is giving it more air.  I am going to try going down a size by 10.

[Gwbarm]

That is a good recommendation, not sure why it would need that since it is the OEM carb, you might also try the needle in a different position.

[flyinbrian365]

I've been working on a customers 2004 with the same issue and found the intake rubber boot is leaking air causing the issue. Spray contact cleaner around the boot while it's running and see if it stalls. Alot of 400 and 500 machines seem to be having the same issue all of a sudden.

[Gwbarm]

Sounds like the rubber may be breaking down after 20+ years.

[JBuckeyeJ]

If it's getting too much fuel, a leaking intake boot would help the situation, not make it worse.  I would verify it is indeed getting too much fuel as suggested by the tech.  Hook everything up and letting it stall.  Immediately pull a spark plug and see if it's wet.  

[flyinbrian365]

On 10/15/2025 at 6:49 PM, flyinbrian365 said:

I've been working on a customers 2004 with the same issue and found the intake rubber boot is leaking air causing the issue. Spray contact cleaner around the boot while it's running and see if it stalls. Alot of 400 and 500 machines seem to be having the same issue all of a sudden.

Ok update on the running issues. What's causing this problem is the needle jet gets worn oblong from wear of the needle going up and down and the forward and back pulse of the engine. I removed the needle jet and put a scratch awl up through the bottom and shined a flashlight underneath and you can see the oblong wear of the needle jet. On my customers machine the needle jet is obsolete so I shimmed the needle down with a washer half the thickness of the original and dropped the main 2 sizes and it now runs perfectly. The plastic washer is the oem and the metal washer is the new replacement. 

[mtnous1]

Try cleaning the air cleaner out