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ATV Repair and Maintenance Having problems? Doing a rebuild? All Repair/Maintenance Discussions here.

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Old 01-09-2012, 11:38 PM
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1990 Polaris Trailboss 350 4x4 Running Issues... Carb? Cold Weather??

This machine is all OEM, ran like new this summer with no issues.
When the temps got colder, the problems started... anytime I give the quad a little bit of throttle it wants to bog down, then is hard to start back up.

It seems to run great in the garage (where its warm), I can give it all the throttle I want in neutral and it even idles great.

I thought my problem was the spark plug originally, I pulled it, it was black/shiny like the carb was running rich, then replaced it and had the same problems, then did a carb rebuild on it, everything looked great. I even replaced the air filter and the air intake filter.

I "re-tuned" the carb after the rebuild, let the machine idle for about 30 mins, everything to be running perfect, then drove it from inside the garage to outside (-20 below) and seemed to do good for about 30 yds and started to bog down again.

Im at a loss on what to do with this issue, should I adjust the air/fuel mix screw? Should I do any adjustments in the cold, where I plan on operating this machine? There should be some way of operating a 2 stroke in the cold right??? or am I just out of luck until the spring? What about adjusting the needle?? any and all suggestions would be a great help!

I fix jets for a living, a simple 2 stroke shouldnt be too complicated right?
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Old 01-10-2012, 03:13 AM
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try setting the fuel screw a 2.5 turns out and put some seafoam in the gas, maybe you have some moisture in gas
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Old 01-10-2012, 05:18 AM
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And welcome to the board!
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Old 01-10-2012, 05:28 PM
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Thanks for the Welcome! Im glad to have a forum like this as a credible tool!

Russ, Thanks for the sea foam advice, Ill head down to NAPA tomorrow and see if that doesnt fix the problem as well as turning the air/fuel mix screw, its currently 1.5 turns out...
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1990 Polaris Trailboss 350L 4x4 (Vintage)
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Old 01-10-2012, 07:45 PM
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First of all there is no such thing as a fuel screw on a round slide carb. There is a small flat head air screw that should be set at 1.25 turn. It could be possible you have water in your gas that is freezing up. If not that then you could have ice in the vent on your fuel tank. Try cracking the fuel cap when it acts up. Those would be the two simplest things. If anyone has messed with the carb they may have leaned the jets down. 350s are notorious for running too rich in the summer time so some people rejet them. Check with your dealer as to what the jet sizes should be and double check that's what's in there. Another possibility is you have a lean condition due to an air leak. Check the boot between the engine and carb as well as the one between the air box and carb. A bad crank seal could also be a culprit. When it is cold the air is more dense so the access air getting sucked in the engine can cause it to run poor. I know that is alot of answers but a lean running engine has an incredible number of possible causes! It's funny ain't it. You would think a little 2 stroke would be so simple wouldn't ya! But diagnosing one can be as complicated as a jet engine would look to me!
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Old 01-10-2012, 09:25 PM
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Red face

Thanks for the advice Cyclone, I am starting to think that I have bad fuel... maybe due to warm/cold environments causing condensation? I triple checked the rubber hoses and clamp connections to and from the carb, they're as snug as could be. The ATV is garage kept 99% of the time, unless Im cleaning it out on a nice day, I park it outside for a couple of hours but always when the temps are favorable.

Its just irritating to see the guys on their late model snowmobiles with the same carb tearing it up outside and yet I cant even get mine 30yds outside the garage , there just has to be some way I can operate it at negative temps. Here in alaska its generally -15 to -25 daily.

I'll also try playing with the air screw after getting some fresh fuel in, any advice on adjusting the idle screw? What about spark plugs? Im running NGK BR8ES, is there a better spark plug? The ones im buying come pre-gapped at about .020 which is larger than what I need .018, do they make smaller pre-gapped plugs?
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Old 01-11-2012, 04:14 AM
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That's the plug you want to use. The idle screw isn't going to affect anything other than your idle speed. It has nothing to do with the fuel mixture it just pushes on the slide to open it up more when you thread it in. I would make sure to drain every bit of fuel out of it including what's in the carb. Mays well replace the fuel filter while you are at it.
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Old 01-11-2012, 01:15 PM
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First, do you you have good fuel flow to the carb. Is the float height set at the correct height,float tangs should dead level and parallel with carb. body.
Next, with the air filter housing installed,you can start adjusting the air screw, you can start with 1 1/2 turns out. let the engine get warmed up, then start adjusting the air screw,slowly ,either out or in until you achieve the smoothest highest idle that you can, then 1/4 turn back in. then set idle speed to spec. double check air mix screw, check idle speed again.
I take it the replacement parts were excactly the same numbers and slide needle is the same and installed on middle notch?
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Old 04-05-2012, 11:57 PM
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Ok guys, I've been working on this thing off/on all winter... heres what Ive done and where I stand.. riding season is just around the corner for us up here in Alaska. I pretty much started from scratch after I pulled all my hair out ....

Cleaned gas tank, fresh gas, added seafoam
ensured I had a clean fuel filter, good fuel flow and a rebuilt carb
I did adjust the choke cable, idle screw, and air mixture screw also...

I finally got the machine back to where I was in the beginning, idling like a champ but bogging down when in gear and trying to give it half to full throttle. Almost had zero throttle response and it seemed the machine was starving for fuel...

After playing with this thing for hours on end I made a discovery today and need some advice...

I noticed if I had the choke off, the machine would idle for about 5 mins and die, or if I reved it up 3x in a row it would come back to idle and die, or if I went WOT instantly from idle it would bogg down and die.

HOWEVER, if I put the choke on "half" (the choke switch sticking straight out) the machine worked flawlessly in all tests, it may have idled all day long, who knows...

I have yet to try to ride the machine again to see if it will bog down in gear under a load, however, do you guys think the choke adjustment is slightly off causing all these problems? Any advice is welcome, could it be something else?? More to come...
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1990 Polaris Trailboss 350L 4x4 (Vintage)
2005 Polaris Sportsman 500 H.O. Olive
(50th Anniversary)
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Old 04-06-2012, 12:43 PM
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Ok, I read the whole post. All the stuff to now is fine,but you have a jet issue. If it runs good with the choke open your getting to much air,not enough fuel.So, I'd go at least two steps larger on the idle jet for starters and maybe the same on the main also. It sounds like someone has been in it before and messed it up. There's numbers on the jets,go two step than they are now higher. The dealer or a bike shop will have them. You may need to go even further to get it right,but you got to start somewhere.
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