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Kurt Reiser

1995 Polaris magnum 425 will not run with airbox hooked up

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 My quad will not run with the airbox hooked up  Will not go past quarter throttle as soon as you unhook the tube at the back of the carburetor it runs great  does anybody have any ideas what it could be I have a bad fuel pump one on the way can that cause the quad to run like that

Edited by Kurt Reiser
Dictated wrong

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There are air intake openings on the air box. Have you checked that they are all clear? 

Also the filter may look clean but may be clogged.   

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Also how is the bike running if the fuel pump is bad ? If it’s still running and the pump is not delivering the proper amount of fuel then maybe that’s the cause but I’ve never really seen a partially broken fuel pump.  

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 Sorry I have it gravity fed right now. I was reading on here earlier there were things as simple as airbox leaking to Cambria albums to carburetor problems the carburetor was rebuilt by Polaris I bought a new one also tried that did not work Cam is fine Valves are adjusted correctly I just don’t know where to go

Edited by Kurt Reiser
Wanted to add more

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Frank there are no obstructions even before I gravity fed it I believe the fuel pump wasn’t working the way it was supposed to because it was still doing the same thing you give it a third a quarter throttle and it would bog  out you disconnect the airbox runs fine

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 I’ve been playing with cars mobile life no nothing really about bikes very minimum so I decided to take it to the Polaris dealer he rebuilt the carburetor I asked him to check the fuel pump he told me it was fine I think it may of ran OK for a very short time started doing this with continually run the minute when you went over quarter throttle it would bog out I disconnected the fuel pump at The carburetor and gravity fed that’s when I noticed if you take off the airbox hose it runs great you put it on it will do the same thing quarter throttle bogs out. The fuel pump should be here today so I will try it laterThe fuel pump should be here today so I will try it later

Edited by Kurt Reiser
Not enough information

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The only thing that comes to mind is there is possibly a constant pressure that the fuel pump delivers keeping the carburetor properly fueled.   

I would do exactly what you said and start with putting the new pump in and go from there.   See if that corrects the problem.  Is not you will have to go a few steps further and maybe make some adjustments or check the carburetor for an obstruction. 

Keep in mind yiu have to prime that pump before you hook it up to the carburetor.  

Keep us posted.  Good luck.

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 Frank I’ve been looking in my clymer manual it  doesn’t give a procedure to prime the fuel pump any chance you could explain it to me on here 

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It’s just a matter of taking the fuel line that goes to the carb off and putting it in a cup and crank the engine until you see fuel come from the line line in your cup.  Once that’s done you should be good to go.  I like to take the spark plug wire off and tuck it away safe so the bike does not start if you have any fuel left in the carburetor. 

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 No luck did not work still doing the same thing the carburetor was rebuilt by Polaris I bought a new one off eBay neither one works Not quite sure what to do now other than run it without a airbox

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Ok so let’s go back to the carb. It’s obviously starving for air with the filter box on. Juts because Polaris rebuilt it does not mean they didn’t miss something. Maybe it’s set wrong ?  Any backfires? 

I would try to adjust the air/fuel mixture screw and see what happens.  You should remember where it is before you do anything. Screw it into a snug position but count the rotation of turns to that position and write it down so you know.  This way you can always go back to that setting. Then try setting it at a different point.  For instance if it’s 1-3/4 turns out from snug then go 2-1/2 turns out. Test the bike and keep at it. You will get it.   

 

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I do understand what you’re telling me but I don’t see any screws on this original Polaris carburetor I also put a second carburetor on this that I bought off of eBay it is doing the same thing yes I do have a backfire only when the air boxes on I do not have a backfire when I disconnect the tube that goes to the airbox

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It’s on the bottom of the carb.  There may be a small metal plug covering up the screw but it’s there someplace.  All carburetors have a mixture screw.   

Also have you checked the manifold that the carburetor mounts to on the engine side for any cracks ?

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Yes I found the screw it was 3/4 of it turn out I want to 1 1/2 and 2 1/2 still doing the same thing I did check the manifold

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Was there another other change in the way the engine ran while making adjustments? The engine should have changed the way it ran worse or rom change ? 

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Frank I did not have a screwdriver small enough I had to take it off each time and turn it out I was going to make a small screwdriver but it got late I’ll try that today

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I'll be following along too.  I think since I bypassed my pump, My carb is overflowing and flooding the engine.  If I have the af off, the engine turns on and races, If the af is on, it floods out and wont start.  Anyway, thats my problem for my thread, I would not have a clue whats wrong with yours.  What elevation are you at?

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I have had to make a few different types of screwdrivers to get In there while the bike is running.  Each bike is didfent of course.  Even small mirrors to see what I’m doing.   You cannot fully adjust a bike without it running.   I like to hear and feel a change in the engine as you adjust.   And you should clearly see the engine change it’s attitude as you adjust. If not then the air fuel screw needs replacing as it’s worn or damaged.   

  • Thanks 1

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 Frank I really appreciate all the help I will try that when I get back home I bought a carburetor off eBay brand new aftermarket how are they? I put it on several times it kept doing the same thing should I go back to that carburetor and try to adjust it? I was reading on this forum it could be the Cam it could be valves I’ve checked all that stuff though  i’m assuming I just dissed just like I would a car is that correct 

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No problem on the help I don’t think I’m really making any progress with my suggestions.  It’s just the way I would go as far as troubleshooting for now. Since the cams etc are much more involved.  I like to go the simple stupid way at first and get more involved if needed.   But most of the time it’s a surface problem not so involved   Maybe a fuel delivery problem or a clogged oraface in the carburetor etc hopefully.  If that’s not working then you have to go deeper. I can’t see the bike running great without the air box on and running bad with it installed as anything other then an air fuel thing.  Maybe im wrong but I just don’t see how it can’t be a localized carburetor or fuel problem.    

I am not a fan of any aftermarket carbs except for the super high end manufacturers which would cost you the same price as the oem carb. 

Aftermarket carbs work and start the bike up with no problem.  As soon as you hit the gas the problems start.  Flat spots bogging etc.  

oem carbs can be rebuilt and are the way to go with replacement.  Even if it’s a used oem carb from eBay.

I would download the manual and start to go through the carburetor settings and even go as far as pulling off and cleaning it yourself.   Making sure that it’s properly dork and the right fuel/air screw was used in the carburetor when it was rebuilt. 

Go back and double check the air box. Hook it back up and take the cover off of the air box and see what that does.     

 

 

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 Everyone I talk to says the same thing as you and I it doesn’t make sense it runs without the air box hooked up but that’s definitely the symptom I know nothing about bikes been rebuilding cars my whole life this is definitely something that is different I read a blog from a guy that said he jumped his because he couldn’t figure it out  it will drive me crazy if I don’t figure it out not saying you’re wrong but I have a hard time believing it’s  A carburetor problem I would think the aftermarket one would at least not have the same symptoms I’m going to try to adjust it again Frank appreciate everything you’ve done for me  I’ll let you know what I find if I ever do

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Double check  your  carb..  Some carbs  have the  idle air needle , but also  have a  fuel   needle.  Here is a video on a carb either the same or very similar to the  constant  velocity carb you  have.  If there is a fuel  adjust valve , that is most likely  the one you  need to  adjust.  I didn't think  of it because my  carbs  have only  had the air bleed screws.

 

Edited by davefrombc
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12 hours ago, Frank Angerano said:

No problem on the help I don’t think I’m really making any progress with my suggestions.  It’s just the way I would go as far as troubleshooting for now. Since the cams etc are much more involved.  I like to go the simple stupid way at first and get more involved if needed.   But most of the time it’s a surface problem not so involved   Maybe a fuel delivery problem or a clogged oraface in the carburetor etc hopefully.  If that’s not working then you have to go deeper. I can’t see the bike running great without the air box on and running bad with it installed as anything other then an air fuel thing.  Maybe im wrong but I just don’t see how it can’t be a localized carburetor or fuel problem.    

I am not a fan of any aftermarket carbs except for the super high end manufacturers which would cost you the same price as the oem carb. 

Aftermarket carbs work and start the bike up with no problem.  As soon as you hit the gas the problems start.  Flat spots bogging etc.  

oem carbs can be rebuilt and are the way to go with replacement.  Even if it’s a used oem carb from eBay.

I would download the manual and start to go through the carburetor settings and even go as far as pulling off and cleaning it yourself.   Making sure that it’s properly dork and the right fuel/air screw was used in the carburetor when it was rebuilt. 

Go back and double check the air box. Hook it back up and take the cover off of the air box and see what that does.     

 

 

I agree that carbs can have an affect on how a bike runs.  However, I am running a cheap chinese carb on my bayou that is taking me up to 50+ mph with 0 issues.  Starts in any weather (except under 10- then the battery goes dead haha).  I found that my aftermarket carb needed the air screw out further than what was called for in the manual.

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