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Old 12-02-2008, 11:38 PM   #1 (permalink)
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Backfiring 1999 Polaris XC 700

I have a 1999 Polaris XC 700 that bogs down and backfires on hard accel from idle. It seems to idle fine (1500 RPM) and runs great on slow/light acceleration. Just when you hammer the throttle from idle it's like it falls on its face (bogs down) and backfires loudly. Even has shut the engine down on backfire a couple times. I have cleaned the carbs thoroughly, soaking and blowing through with an air gun. Also removed pilot jets and cleared with a wire & carb cleaner. I have replaced the plugs also and sync'd the carbs. I just bought this sled from a guy who said it had been sitting in storage for quite some time. I don't know if the gas is bad or if this could possibly be a bad fuel filter (seems to have good fuel flow from fuel lines to carbs). Any ideas? Any help would be greatly appreciated.
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Old 12-03-2008, 08:33 AM   #2 (permalink)
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First thing I would do is get that gas used up and get some new stuff. Run high octane. If the problem persists, you might have to change your jetting (overfueling). Did you ask the guy what altitude he rode at? he may have set it up for mountain riding or something...
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Old 12-03-2008, 11:51 AM   #3 (permalink)
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He said he only rode it in northern Wisconsin. The main jets are 185's so I think those are stock main jets.
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Old 12-03-2008, 04:23 PM   #4 (permalink)
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make sure youre getting good spark as well, check the ohms on the stator coil.
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Old 12-03-2008, 05:29 PM   #5 (permalink)
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man, my 600 does the same thing and i did all the things you did and my jets are all stock. goin to the dealer as its under warranty...for a month
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Old 12-03-2008, 06:51 PM   #6 (permalink)
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Does anyone know how to disable the throttle safety switch? Someone had mentioned the switch may be cutting out.
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Old 12-03-2008, 08:01 PM   #7 (permalink)
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Ok... I disconnected the throttle safety switch, and that didn't fix it. I checked spark with my inline spark tester, and it's nice and bright both cylinders. Anyone ever heard of a fuel filter doing this? Thats about the only thing I haven't done.
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Old 12-03-2008, 08:02 PM   #8 (permalink)
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In theory, it'll backfire if it's starving for fuel when you hammer it. Have you rebuilt the fuel pump yet?
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Old 12-03-2008, 08:33 PM   #9 (permalink)
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No, I haven't. How do you go about doing that?
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Old 12-03-2008, 09:18 PM   #10 (permalink)
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2001 xc 600 7000 mi. same thing bog - backfire ???????????????????????????
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Old 12-05-2008, 11:32 AM   #11 (permalink)
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Adjust

sometimes if the air screw is out too much in colder weather it may backfire.turn your air screws in 1/8 of a turn and go for a blast.make sure it is warmed up really good.
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Old 12-05-2008, 11:52 AM   #12 (permalink)
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Where is the air screw? Are you talking about the idle screws on the carbs?
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Old 12-05-2008, 12:19 PM   #13 (permalink)
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screws

the screws are a small slotted screw on the side of the carbs.not the one with the plastic knob that is for ideal
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Old 12-05-2008, 08:14 PM   #14 (permalink)
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can you let me no what happens at the dealer please
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Old 12-05-2008, 10:31 PM   #15 (permalink)
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I'm not taking mine to the dealer.
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Old 12-06-2008, 06:26 AM   #16 (permalink)
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Follow the fuel lines back from the carbs, they'll go to the fuel pump(s). You can get rebuild kits for them.
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Old 12-06-2008, 08:30 AM   #17 (permalink)
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seg are you suggesting the fuel pump doesnt pump enough at higher rpm? starving the carbs.
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Old 12-06-2008, 02:39 PM   #18 (permalink)
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I am thinking that if the fuel pump cannot keep up with the fuel demand of the engine the problem would be apparrant at wide open throttle more than or even as well as off the line. Are you experiencing high speed running issues as well or just holeshot? If the fuel pump is not up to the task or even if the filter is partially clogged from sitting the carb bowls will be full of fuel for the holeshot but will run low when the fuel demand is higher at high speed. Tell us more about your symptoms. Also check the resistance of the spark plug caps because they could be robbing spark intensity if the resistance is too high, this would be most noticable during hard accelleration. Also make sure you are running fresh fuel as mentioned above.
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Old 12-06-2008, 04:37 PM   #19 (permalink)
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The tank was just about empty and I filled it up with fresh 93 octane - no change so far. I have let it warm up at idle and it still does it. I haven't ridden it more than just around my yard so I don't know about higher speed accel. When I hammer the throttle from idle it falls on it's face and backfires. Seems to idle just fine (1500 rpm's) and if I slowly take the rpm's up to 3k or so and hit it, it seems to respond fine. I was thinking the same thing that there should be enough fuel in the carb bowls to supply fuel to the engine immediately off idle, thats why i'm not sure it could be the fuel pump or filter, but I'm not too knowledgeable about 2 cycle engines either. I would rebuild the fuel pump just to eliminate it but no Polaris dealers around me have a rebuild kit for it. Do you think I should take it out and drive it for a while, or will this potentially cause engine damage?

Thanks for all your help and suggestions, I really appreciate it!
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Old 12-06-2008, 08:13 PM   #20 (permalink)
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Quote:
Originally Posted by curd28 View Post
The tank was just about empty and I filled it up with fresh 93 octane - no change so far. I have let it warm up at idle and it still does it. I haven't ridden it more than just around my yard so I don't know about higher speed accel. When I hammer the throttle from idle it falls on it's face and backfires. Seems to idle just fine (1500 rpm's) and if I slowly take the rpm's up to 3k or so and hit it, it seems to respond fine. I was thinking the same thing that there should be enough fuel in the carb bowls to supply fuel to the engine immediately off idle, thats why i'm not sure it could be the fuel pump or filter, but I'm not too knowledgeable about 2 cycle engines either. I would rebuild the fuel pump just to eliminate it but no Polaris dealers around me have a rebuild kit for it. Do you think I should take it out and drive it for a while, or will this potentially cause engine damage?

Thanks for all your help and suggestions, I really appreciate it!


if it is backfiring through the intake it could indicate a lean misfire look for air leaks and anything that would cause a lack of fuel . Another thing to look for would be a damaged reed valve or piston skirt causing the engine to breathe incorrectly

If it is backfiring through the exhaust that normally indicates raw fuel in the exhaust being subsequently ignited which generally points to weak spark or possibly low compression
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