cant find air leak on polaris trailboss 250 - Polaris ATV Forum
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post #1 of 15 (permalink) Old 06-12-2019, 04:05 PM Thread Starter
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cant find air leak on polaris trailboss 250

hello just joined. ive worked on many bikes but this has me stumped. i have a 91 polaris trailboss 250 2 stroke that im assuming has an air leak since it revs out of control once the choke is off. with the choke on just a little bit it will settle down but still runs rough. ive been through the carb a few times and blew out all passages and all jets are crystal clear. this is why im assuming its an air leak. one thing i did find is when i spray carb cleaner right where the choke connects to the carb it slows the idle down?? could the choke be sucking air? ive sprayed everywhere else with no luck. ive also put 2 stroke oil around the clutch side crank seal with no difference. i guess what im wondering if any of you guys have heard of an air leak at the choke? and is it normal for it to leak a little. ive read before that they do but idk. also, can the crank seal still be bad even if they pass a fluid test?? sorry for the long first post but just wanted to add what ive done. any help or any ideas as to where it may be leaking would be greatly appreciated. thanks.
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post #2 of 15 (permalink) Old 06-12-2019, 04:57 PM
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I guess I'd check your intake boot as well, unless you've already done so checking for cracks and such in the rubber. Did you spray around the intake boot/gasket? It's possible it's a head gasket, but I'm guessing not... try spraying around the base gasket as well.
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post #3 of 15 (permalink) Old 06-12-2019, 05:06 PM
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How many turns out do you have the air screw?
Should be one (1) turn out from lightly seated.
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post #4 of 15 (permalink) Old 06-12-2019, 05:17 PM Thread Starter
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Originally Posted by Tigman186 View Post
I guess I'd check your intake boot as well, unless you've already done so checking for cracks and such in the rubber. Did you spray around the intake boot/gasket? It's possible it's a head gasket, but I'm guessing not... try spraying around the base gasket as well.
i hope im replying right. wasn't sure how to reply on this forum lol. yes i sprayed the the boot and the base and head gasket with no change. not 100 percent sure if the head gasket would change the idle but im assuming so. the only thing i found that braught the idle down or made any change was spraying the choke where it attaches to the carb but I've never seen that before. i changed the plunger thinking it might not be sealing but no change.
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post #5 of 15 (permalink) Old 06-12-2019, 05:21 PM Thread Starter
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Originally Posted by polman500 View Post
How many turns out do you have the air screw?
Should be one (1) turn out from lightly seated.
I've tried adjusting the air screw in every way possible i think. ive had it 1 turn out and went little by little all the way to two turns with no adjustment. its got me stumped idk.
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post #6 of 15 (permalink) Old 06-13-2019, 06:40 AM Thread Starter
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as far as the crank seals go will either side cause an air leak or is it mainly the drive clutch side?? ive heard its mainly the drive clutch side but not sure. any ideas are appreciated. thanks.
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post #7 of 15 (permalink) Old 06-13-2019, 09:17 AM
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Either side can leak air, but the seals are designed to hold compression in - usually if they are leaking there is a tell tale trail of splooge running down from the seal. Will it rev with the cable removed from the throttle slide?

2006 Trailboss 330
2002 Sportsman 90
2005 TRX400EX (FOR SALE)
2003 LT-Z400 (needs engine work & is for sale)
2004 CRF250X
1971 Triumph 650 Bonneville (has 5 original miles)
1972 Triumph 650 Bonneville (undergoing restoration)
1979 Honda CBX (6 cylinder)
1970 Kawasaki G3SS 90cc Bushmaster
1976 Suzuki RE5 Rotary (not running & FOR SALE)
1981 Kawasaki KZ305-A
1981 Suzuki GS450T (undergoing repair & will be for sale when done)
1982 Kawasaki KZ750-H (FOR SALE)
1989 Honda VT1100C
2007 Vectrix VCTX Electric Scooter
1965 Montgomery Ward 3 1/2 HP Tecumseh Mini Bike
1970 Triumph T25 250cc Trailblazer
1968 Triumph 250cc Flat Tracker

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post #8 of 15 (permalink) Old 06-13-2019, 09:21 AM Thread Starter
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Originally Posted by latebird View Post
Either side can leak air, but the seals are designed to hold compression in - usually if they are leaking there is a tell tale trail of splooge running down from the seal. Will it rev with the cable removed from the throttle slide?
haven't tried that. i will see when i get home today and check back. thanks for the response.
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post #9 of 15 (permalink) Old 06-13-2019, 09:28 AM
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I'm not familiar with that style carb but was noticing while looking at the diagram that it has a diaphragm of some sort on the side. If it has a leak could that cause the issue??
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post #10 of 15 (permalink) Old 06-13-2019, 11:43 AM
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Polman - what diaphragm are you referring to? I just looked at the carb and it's just a std. mikuni VM round slide - no diaphragm?

2006 Trailboss 330
2002 Sportsman 90
2005 TRX400EX (FOR SALE)
2003 LT-Z400 (needs engine work & is for sale)
2004 CRF250X
1971 Triumph 650 Bonneville (has 5 original miles)
1972 Triumph 650 Bonneville (undergoing restoration)
1979 Honda CBX (6 cylinder)
1970 Kawasaki G3SS 90cc Bushmaster
1976 Suzuki RE5 Rotary (not running & FOR SALE)
1981 Kawasaki KZ305-A
1981 Suzuki GS450T (undergoing repair & will be for sale when done)
1982 Kawasaki KZ750-H (FOR SALE)
1989 Honda VT1100C
2007 Vectrix VCTX Electric Scooter
1965 Montgomery Ward 3 1/2 HP Tecumseh Mini Bike
1970 Triumph T25 250cc Trailblazer
1968 Triumph 250cc Flat Tracker

Shop Owner and Mechanic with over 50 years experience
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