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Early 2000s Polaris Trail Boss 325 4 Stroke Fuel Issue

Brian Haley

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I wanted to ask this question before I pull this thing apart and dig into the carb. Very seldom used for a couple years but took it out the other day and it ran like crap. Blows black smoke on throttle (unburnt fuel) and sound like a multi cylinder engine with a misfire. Won't rev very high on full throttle but seems to be smooth at about 1/3 or less throttle. Choke doesn't seem to do anything and thinking perhaps it's stuck closed. If I turn off the fuel and let it run for a minute It will rev like normal under full throttle until it stalls out from no fuel but as soon as I open the fuel line again it goes right back to running rich/flooding.


Any help or ideas would be great. Trying to avoid tearing it apart and rebuilding the carb but the only thing I can think is the choke is stuck cause with the fuel off it will rev just fin seemingly as it's emptying the float bowl.

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Update and I hope someone can shed some light on this. So I cleaned the carb and no change in fact it seemed to get a little worse but I now believe that's because it needed some adjusting that I couldn't really do because of the way it was running. I still thought it was a fuel issue and it may very well be but I bought a new carb. It was cheap enough on amazon so I went that route. Installed the new carb and it starts and runs at idle better than before and smells much less rich but it would still not rev on throttle. It revved a little better with the choke on but still backfired and popped pretty badly and would not rev very high. I started thinking it could be timing so I did some searching and I found that was not likely the case but many people were saying it will run like crap without the air box installed and sealed. I had my doubts but I tried it anyway and it was the same. Without the airbox I tried covering the air intake and it flooded with fuel and stalled as would be expected. Once I put the air box on I covered the air intake again and it revved then began to stall so I covered half of the opening and bam it revs like it should.

I'm debating covering half the opening with some duct tape and just go with it but I figured I would ask if anyone had any ideas for a proper fix. I suppose the issue could be not enough fuel in the mix or too much air since it idles fine but I'm not completely sure how to adjust it. Mixture screws are usually meant to adjust at idle and my issue is off idle with both the old and brand new carb.

Does anyone have any ideas?

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  • Admin changed the title to Early 2000s Polaris Trail Boss 325 4 Stroke Fuel Issue

I don’t think the tape is a good idea. I would go back to the factory carb and set your fuel/air screw to 2-1/2 turns out from a snug tight position.  That should get you running enough to tune it. Either 1/4 turn in or out. The higher it revs while turning the better. Lower the idle if the engine revs to high. 

Now as far as not running right and air box etc.  Yes the air box and filter need to be in place. And that needs to be done prior to adjusting the fuel/air screw.  

Now after that is done you should check spark quality as well as add a new spark plug.  Make sure you have a good spark.

Is the fuel fresh ? And the valve (petcock) on the tank flowing good fuel ?



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In your initial post you described a rich fuel condition that got worse as you increased throttle. You were right too think along the line of a choke stuck closed or other obstructions in the intake. Next you removed and cleaned your carb to no avail. There is one item often overlooked when cleaning a carb. The needle jet which sets just above the main jet. It is a small (usually brass ) tube with a series of small holes in the side. If those holes are restricted or clogged your fuel mixture will be too rich. Make they are clean and be careful not to alter their size. Your new carb on the other hand has a lean condition and most likely could be corrected with a larger main jet. 

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For the time being I'm gonna close off part of my air intake of the filter box cause I need it for tomorrow and it will run decent. Unless I'm wrong and the pics I found online are also wrong it doesn't appear I can adjust the air/fuel mixture screw with the carb on the quad.

One part of my question was overlooked. Am I correct in thinking that the mixture screw only adjusts idle mixture? One more thing. On the bottom of the float bowl there is a nipple with a shut off screw next to it. The original carb had nothing connected to it and the replacement had a rubber hose on it but I have no idea where it should be connected. What is the purpose of this nipple? It appears to just be a way to drain the float bowl.

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Regarding the drain hose...it's also connected to the overflow tube inside the float bowl.  If the float valve ever sticks open, the excess fuel will run out on the ground instead of filling up the crankcase with gas.

You'll need one of these tools to adjust the idle mixture with it running.1519871349_s-l1600(2).thumb.jpg.e453529e4c76ffb4e87d4c137fd2ff38.jpg

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Starting to think I have multiple issues going on. Going to replace my very old and worn fuel lines to make sure none are collapsing off idle as that explains getting too much air and backfiring. Also I ordered a new petcock cause the original does not fully shut off the flow. (noticed that when I had the tank off the quad) It's only a drip but I still want it to shut off. Once that is done I'll have to see how it runs off idle and continue to figure out why I'm having mix issues. I appreciate all the info. 

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3 hours ago, Ahren Longo said:

may have a stuck float in the carb. That will make it flood too.

The old carb didn't idle all that well but was seriously bad off idle. The new carb idles fine and still has issues off idle which leads me to believe I had 2 problems. Old carb off idle seemed to be flooding gas based on black smoke and raw fuel smell. New carb seems to be either starving for gas or just getting too much air cause similar sound with no rev and sputters and backfires but no smell or smoke and choking the air intake partially it smooth's right out and revs fine. Going to replace all my fuel lines to make sure I don't have any collapses.

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