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Polaris Magnum 425 - No Top End


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I have a Polaris 425 Magnum. she starts fine and seems to have unlimited low end but just does not have any top end. I know she should be capable of more. Its like there is a limiter on the throttle or something. Need some advice on what to check next. I rebuilt the carb and made sure the floats and Needles are set.

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Well I have not gone that far yet but I guess I will. The machine use to run great except the carb bowl would leak. The needle seat was worn to the point that it would not shut off the fuel and it would constantly drip out the over flow. She ran like a raped ape at that point. So I decided to rebuild the carb and since that time it just seems like there is no top end. It acts like there is a govner or something and it just hits that power end and that all shes got. But I can tell you this its no were near what it use to be. This machine used to blast up hills now it just chucks along. The engine kind of seems like its at peak so I may take a look at the belt and clutch to see if its worn or crazed.

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  • 2 years later...

I think it was multiple problems. I ended up rebuilding the Carb and changing out he several components. The Throttle position switch, CDI and ignition coil out. The only thing that I could definitively say was bad was the micro switch in the ETC on the Thumb throttle. I replaced that along with the other parts and she has ran fine for some time. Although at the moment I have it tore apart I am working on the 4 Wheel drive unit. It seems to engage only one side and pulls really hard to the right when you don't want it to  

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I believe it has something to do with the electric parts since sometimes it will engage just fine, but after a little riding you almost have to fight it from pulling really hard to the right. when you disengage the 4 wheel drive by the button it will correct and stop pulling. It does not happen all the time but its a real eye opener if your not ready for it. She needs front shocks and some other work upfront and I plan on getting back into tearing it down in near future. Also the front Gear box has a wine you can hear from a long way off and I have heard this on other models of that year. I disconnected the chain to isolate it and the wine was definitely coming from the front gear box.

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I believe it has something to do with the electric parts since sometimes it will engage just fine, but after a little riding you almost have to fight it from pulling really hard to the right. when you disengage the 4 wheel drive by the button it will correct and stop pulling. It does not happen all the time but its a real eye opener if your not ready for it. She needs front shocks and some other work upfront and I plan on getting back into tearing it down in near future. Also the front Gear box has a wine you can hear from a long way off and I have heard this on other models of that year. I disconnected the chain to isolate it and the wine was definitely coming from the front gear box.

Thanks for the info and when I finally get to working on that machine again I will let you know what I find

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  • 1 year later...

Ok its been a while since I posted. I have finally got some time to work on the Magnum 425, first off Thanks for the past posts and suggestions. First I tackled the 4 Wheel drive issue and the front hubs were totally gummed up and the oil was like a thick grease and had no resemblance of oil at all. So with a few new parts like bearings and lots of cleaning I believe the 4 wheel drive is back up to par. I have encountered a few of the previous problems. The machine will start and Idle perfect. Once you give her throttle after its warmed up it stumbles and bogs down, it does not die just stumbles and will not seem to throttle up. Here is what I have Checked so far. 

  1. I Cleaned the Carb completely.
  2. I checked compression and its at the low end of the scale but within tolerances according to manual.
  3. I checked the cam mainly the exhaust lobs for wear and they seemed ok with normal wear not excessive and the lobe seems fine.
  4. It has Great spark and good plug


I had some spare parts on the shelf from a wrecked 425 so I thought I would swap them out to see if any thing changed.

  1. Sparkplug
  2. CDI unit
  3. Regulator unit
  4. Vacuum fuel pump
  5. Fuel filter

There was absolutely no effect with the above items, Starts almost immediately, idles great, once it warms up it stumbles with throttle and bogs almost like in reverse without override.

So I am at a complete loss with this one. I tried everything that I can think of. This problem seem to be starting quite a while ago and has just progressed. 

Any thoughts? What about warn intake valves. Unfortunately I am not sure how many hours this machine has since I actually bought it at a scrap yard right before it was to be crushed.  

I will appreciate any suggestions. Thanks Woody


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I'd recheck the carb..  My guess  is the  bogging is from an  over  rich  mixture.  Rich  mixture is  used when   the  engine  is  cold , but as it warms up   it  needs to  be   a  bit leaner. Have you tried setting the  main  jet needle  on  a  higher  notch  ( lower into  the  main  jet) I  searched on your  carb and  pix were the same  as the  Suzuki one . There is a video  in the forum  on how they work. The needle  may  have  worn  enough over the  years that  it is thinner  thus  passing  more  fuel  on the setting it  is  at  now .. Since  it  is  a  "rescue from the scrap yard"  ATV,  a previous  owner  may have  swapped  in a thinner needle  from  a  smaller series of  the  carb. If  you set the  main jet  needle  lower in the   bore , you  may  need  to  open  the  idle  jet  some  to  keep it from  being  too  lean  at  idle.  From  your  description  I  would  say  you've pretty  much eliminated everything  except a  possible  over rich  carb  setting. I can't think of anything else  to  blame it  on..

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Thanks Dave,

  I agree this thing has got me stumped. I just ordered a cheepy carb off Ebay. We will see how that pans out but for the cost I figured it was worth a shot. I was thinking the same thing that it is acting like its dumping fuel when given throttle. The only problem is the plug is never black its always clean and light gray brownish. Pretty much what you want. I also picked up a spark tester so I can see the spark while its running. So here is my plan for this weekend.

  1. I want to open up the inspection plates for the cam timing area and check to ensure my timing marks line up correctly.
  2. Open up the flywheel cover and check that those marks correspond with the timing gear on cam. I just cant see it through that little hole. (getting old)
  3. I want to meter out the trigger coil, stator and ignition coils for proper ohms to see if they are good.

Of course that is if the carb does not change the problem.

Yeah I rescued this machine from the scrap yard back in 2010. When I got it the flywheel was loosely on and the covers where missing. I checked it out at that time, got the parts and put it back together and she ran great for several years. Then it started to loose power at top end and it progressively got worse over time until  I got tired of fiddling with it and parked in the back of the shop for a couple of years. I just decided to get her going again and see if I can tackle this problem. 

The reason I am thinking valves is because I had a Suzuki that ran bad once hot and turned out to be valves overly warn. When cold she had decent compression but as it heated up the compression would drop off and it would become hard to start until cool.

Anyway this problem is a bit different since it idles great and starts every time. It just does not want to throttle up. When I first got this machine up and running it was great fast and responsive. Then over time it seemed to loose top end power. I decided to figure it out and one thing led to another and she sat for a couple years and here we go again. I may end up tearing the top end off eventually if I cant figure this out. I read somewhere that the ETC can back feed through the reverse light to the limiter. I will need to check that out as well.

Thanks for the suggestions I appreciate it



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Quick update:

   02/15/18 I had sometime last night so I opened the covers to check the timing marks. The cam chain does not seem to be overly warn and the tensioner keeps it nice and taunt. The timing marks and top dead center all line up. So I also checked the Stator and trigger coil for resistance readings and they all fall within the attached Chart.

So here we are again I cant seem to figure this thing out. What is left to check?



I am at a total loss with this machine. I am usually pretty good at figuring these things out but this one has me completely stumped. So any suggestions will be appreciated.

Thanks Woody.

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I was able to use a known good carb. A good friend has a good running 425 and was gracious enough to allow me to use his carb for quick swap to eliminate carb. Well my carb ran great on his machine and his carb made no change on my machine. So I am still getting kick around by this machine.

So without anymore suggestions I think its time to do one of two things. Tie the throttle open and put in gear and let it roll across the fields and get a few of my old friends and have some target practice or tear the engine down and see what is going on inside. The first option sounds like more fun but I cant give up and I refuse to give in to a mechanical machine. I have a sickness when it comes to machines and I hate to be beat by a mechanical thing. There is always a reason why a mechanical item does not work I just have not figured it out YET.

I have to put this on the back burner for a couple of weeks due to other obligations. I was really hoping I could get this going again.

If anyone has any other suggestions please let me know.


        The only thing I can think of is Valves. Here are the symptoms again

  1. Starts and Idles perfectly. Uses choke to start, warms up and off choke. idles without a miss.
  2. Once you give it throttle, meaning you go past idle maybe 1/4 of throttle it bogs down and will stall if you keep there, it will stumble for a bit before it completely stalls out.
  3. Plug is grayish brown and not fouled out.

Please read previous threads to see what has been tested and changed (there is a long list)


I will continue to update once I get back on it. Thanks to everyone

P.S> I did not have time last night to check the compression once it was hot since I had to put my buds machine back together and get it back over to his house. Once I am able to get back on it I will fire it up and try that before I tear it down.


Again thanks to all and hope to find a solution soon 




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  • 4 weeks later...


  Well I finally got a chance to let it warm up and test the stater and other winding's to make sure they are not breaking down when hot. The readings were within tolerances so I moved on to checking the valves. I decided that this engine has some miles on it so I changed out the valves, springs, and cam. Started right up and again at 1/3rd throttle she would start to sputter and die. 

  So I am really frustrated with this machine. Everything tells me its electrical. So I needed to walk away. 

So this morning I went out and worked on it and tried to approach it like it was the first time it messed up. i started the machine and checked voltage at the battery. 12.2 static and when started 14.2. No problem there. So I thought I never checked that voltage with the machine above idle. So i got some clips and left the meter attached. At about 1/3 rd throttle I am pushing 16 volts if i push it or rev it I can get the voltage to go clear up to 19, 20 volts at the battery

  So the rectifier is bad because the voltage at the battery should never get about 14.5, which in turn most likely f,d the CDI. I am going to do some more checks and will post.  Hopefully this is it. This machine will have almost all new parts. I probably should have pulled the jug and got new rings and piston while I was at it. 

 Thanks for the inputs and if anyone has any other suggestions please let me know. Thanks and I will try to ensure I post once i get the new CDI, Voltage Regulator and rev limiter. To day I am going over all the wiring and checking and cleaning all the terminals to ensure there are no other electrical issues    

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  • 2 weeks later...

Hey Rwoody,

I have been through all of the frustrations you have mentioned.  My 98 425 magnum went bad on me just over a year ago.  While hauling a very heavy load of wood out and through the field it seemed as though there just wasn't the power there should have been.  Usually it would practically bury my trailer with thrown mud but this time I made it out without much spinning, matted all the way home.  Over the next few weeks it continued to decline till it got to the point to be nearly unusable, no power, backfiring, blown engine oil smell etc.  I'll try and keep this short but... Anyhow I replaced the carb with a $40 special cuz the needle & seat were shot/leaking & the rebuild kit cost nearly the same as the carb.  No help.  Replaced diaphragm pump cuz it was easy & cheap. No help.  Replaced coil with a $10 Ebay deal.  Nothing changed.  Metered all test points OK.  I was not yet retired during that time and just did not want to take on replacing the cam which much of my research led me to.  So I parked it and took the advice of the Mrs and bought a new 570 Sportsman Utility Edition.  Problem solved temporarily.  Within the last couple of months I got the bug to start poking away at it again and through some friends I ended up put in touch with a retired good factory trained Polaris technician that gave me a reasonable price and a 4 or 5 day turnaround to replace the cam so I loaded it up  and 5 days later had the machine back with the new cam.  I did run much better and would rev if I accelerated slowly but it seemed to flatten out at around 60% power.  Also if I punched it off just above idle it would bog, spit, sputter and just basically tell me it ain't happening.  So now what?  With all the thing I had done I thought I had introduced another problem.  I decided to first focus on the carb, checked the float level & raised the needle from the middle notch to the bottom.  Seemed to help a little but not much.  Got & installed a new air cleaner (without the foam outer cover), replaced the broken air cleaner hold down bracket & installed a new spark plug.  No change.  Are we having fun yet?  SO getting tired of taking the tank and all associated parts off to get to what I was wrenching on I decided to temporarily mount the tank to the front rack with bungee cords and tie down straps.  This allowed me to tinker without having to do so much BS to get to the goodies.  Well with no seat, except for a boat cushion and the tank mounted to the front rack I started making test rides and forcing it into failure mode.  Riding slowly, punching it and trying to reason out what was wrong.  Apply the choke did not seem to make any difference.  On one test ride after punching it and getting the bogg & slight blowback through the air cleaner inlet tube I put my left hand over the tube to try and prove whether it was a fuel/air issue or electrical and BAM it takes off like a raped ape.  Take my hand off the inlet and it would nearly die, back on & we're off to the races.  Deciding that I didn't want to ride with my left hand on the inlet, the same inlet that had not had the prefilter on for years, I made up a pre filter out of some filter material I had and installed it over the intake.  This somewhat solved the problem but it would blubber from an obviously rich mixture because the prefilter I had made was being to restrictive.  My next attempt at a permanent repair was to place the oil bath outer filter back on the air filter and to make a less restrictive prefilter over the intake with a spacer in the intake made of a coat hanger (much like the OEM part) to keep the prefilter from being sucked into the intake and my machine runs like it should.  So my last problem was definitely fuel/air.  I had seen one post somewhere in the last year about the need for the prefilter having to be on the intake but kind of discarded it as I had run mine for years without the prefilter on.  It seems to me like there needs to be a certain amount of restriction on the intake for the carb to work properly.    

Hope this helps you and anyone else that has had these problems.  

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Thank you very much for the detailed message i will see if i can get to it this weekend 

What is funny I mounted a small gas tank to the front because I was tired of removing all that stuff as well

I will respond as soon as I can thank you

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