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Old 06-13-2013, 08:53 AM   #1
Keithert OP
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Need help with carb issue on ATV

On my Polaris Trailboss 250 2 stoke ATV it starts and idles find but I have to be very slow and gentle with the throttle to get it going. Once moving it accelerates just find until I let it idle down again. Please see the video for a demonstration. What circuit in the carb is likely to cause this?

Demonstration
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Old 06-13-2013, 03:50 PM   #2
Range Motorsport
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Probably the circuit that blew your piston.

Do a compression check, you will thank me later.
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Old 06-13-2013, 03:55 PM   #3
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Heres how a carb works, basicall, idle is off the idle circuit, which consists of the idle valve, and pilot jet, once you crack the throttle open, the needle and part of the main jet come into play, and it stays that way till the throttle is WFO then the main only is working. The description is general but fairly accurate. I don't thing the problem is entirely in your carb unless the power is fine and like normal above where it is bogging off idle.
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Old 06-13-2013, 05:32 PM   #4
Keithert OP
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The power is fine once past the point where it bogs. I had it up to 40 mph today.
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Old 06-13-2013, 06:02 PM   #5
BillsR100
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I would look for air leaks anywhere in the intake such as loose clamps on the intake between the cylinder and the carb.
just something I would look at given the symptoms.
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Old 06-14-2013, 10:20 AM   #6
Suzuki Phil
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I'm at work so I can't watch your video but here are some tips. Have you recently cleaned the carb? Did you take out all the jets to do this?

Carbs have three systems, idle, main metering and WOT. Idle jet controls idle mixture, pilot jet, jet needle and main jet control main metering, jet needle and main jet control WOT. Pilot jet is the transistion jet between off idle and main metering (cruise). If you're sputtering off idle I would start there first.

If you took the carb apart to clean did you put all the jets back in? If you took the carb apart you need to inspect all the jets to see if they are not plugged. You need to check the air jets also. They control the emulsion of air/fuel. I usually use welding tip cleaners to make sure nothing is plugged.

Check for air leaks at carb intake boot as stated above. I've learned that carbs give me the most issues on my bikes. There simple as heck but can be quite frustrating.

Keep us informed,

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Old 06-14-2013, 11:12 AM   #7
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I am also leaning (no pun intended) toward an air leak. Use a spray bottle of water to spray round the carb area and see if it idles up at anytime you are spraying it. If so, you have a leak.
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Old 06-14-2013, 01:46 PM   #8
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Check for a scored piston or broken skirt. Very common on the 250's.
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Old 06-14-2013, 05:27 PM   #9
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Probably the wrong year and make, and probably not your problem (it's never this easy), but my brother recently told me about a 4-wheeler he worked on for a friend. He had the thing rebuilt, back together correctly and everything, but it had similar symptoms.. although he said it backfired more than yours once it was up and running at speed.

He said he took the carb off and rebuilt it 4 times one day trying to diagnose the issue. In an obscure section of the manual, there was a reference to a parking brake switch that would limit the functionality of the 4-wheeler.

Sorry I'm being vague, I can't remember his wording exactly.

He checked and noticed the front brake lever was pulled in maybe 1/8" and engaging the switch. He let it out, and the thing ran beautifully from then on.
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Old 06-14-2013, 09:37 PM   #10
Keithert OP
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Thanks for the replies. I ordered a carb rebuild kit.
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Old 06-14-2013, 11:16 PM   #11
tkitna
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Quote:
Originally Posted by Range Motorsport View Post
Probably the circuit that blew your piston.

Do a compression check, you will thank me later.
I'm with Range. Probably has a shot top end in which hampers the idling and low end. Probably hard to start too, but OP never mentioned it.
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Old 06-16-2013, 05:12 PM   #12
Keithert OP
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I'm with Range. Probably has a shot top end in which hampers the idling and low end. Probably hard to start too, but OP never mentioned it.
Quiet you doomsayers. The compression is 115. It starts very easily. I'm pretty sure it's a carb issue. I'll rebuild it soon.
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Old 06-16-2013, 07:30 PM   #13
tkitna
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Quote:
Originally Posted by Keithert View Post
Quiet you doomsayers. The compression is 115. It starts very easily. I'm pretty sure it's a carb issue. I'll rebuild it soon.
Sorry. I believe it is just a carb issue now.
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Old 06-18-2013, 02:02 PM   #14
Range Motorsport
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Quote:
Originally Posted by Keithert View Post
Quiet you doomsayers. The compression is 115. It starts very easily. I'm pretty sure it's a carb issue. I'll rebuild it soon.
As a Polaris Mechanic for 9 years I know where to start looking for issues. Once those are ruled out then narrow it down to a carb or fuel issue. It's very rarely a dirty carb in my experience.

Even odd things like loose spark plug caps, bad plug wires, and carb boot cracks are more common.
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