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Backfiring '09 850 XP - How To Fix What I Found?

27K views 61 replies 18 participants last post by  bhard 
#1 ·
Hey Guys,

Sorry to beat this dead horse, as I did use the search feature and found many users with a similar backfiring problem on an 850 xp and adjusting the throttle cable seemed to fix. I did that procedure and mine is still doing this. My quad is a 2009 850 XP, very well maintained machine. 131 hours of use. After a few months of sitting in the garage I went to fire it up for hunting season yesterday and took it for a spin.

At idle it's missing, making a slight "popping" or "burping" sound occasionally as it burps or misses at idle and it's backfiring very loudly consistently on decel. Can't do much hunting with gunshots following me around, lol. It also just seems like a loss of power too.

After adjusting the cable, the problem still occurs. So i added a zip tie in between the contacts. Same problem. I then took off the plastics and found this:



Here is a further back photo so you can see where it is...



Looks like a god damned mouse chewed the wiring!!!! I think this might be the source of the rough running/backfiring. How should I go about fixing? Just electrical tape and call it good? It doesn't look like they made it all the way through any of the wires.

Do you agree this could be the issue?
 
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#2 ·
Something like that could definitely cause all kinds of gremlins I would think. You could try and tape it. Check each wire individually and tape each wire over any damage, then bundle the wires back together and tape the harness back up good. Won’t hurt to try.
 
#3 ·
I would be inclined to believe that's the source of your problem.

To fix those wires I would pull back the loom and separate the wires. Fan them out nicely and clean the damaged ones. Then put liquid tape to the bare spots. I would apply two coats allowing enough time to dry between coats. Then I would tape those spots as a final step.
 
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#9 ·
Hey SteveC25,

Yes, it had some gas that was a few months old (maybe 1/4 tank) and then I filled it right up with fresh mid-grade gas (89 octane). I took it for a 5 min burn up to the end of the road and back thinking it might have cleared it through the system but nope it's still doing it.

I just checked the plugs

#1. (closest to front) was really white and smelled of un-burned fuel.
#2. (closest to back) was normal black and smelled of un-burned fuel.

Not sure what to do. Dealer is a few hours away and I can just imagine what they'd charge for this, a there is no check engine light or anything. I just want to go hunting, lol.
 
#10 · (Edited)
white plug is not good, it can be antifreeze in cylinder or a lean air fuel mixture, usually lean air/fuel is grayish color on plugs and that's usually really not good. Did you check if you have a good spark on plug that was white and smelled fuel in cylinder. ?
 
#11 ·
On the earlier XP's a lot of guys switched to NGK iridium spark plugs with success in curing the mis and backfire.
 
#12 ·
So I went out and tinkered with it this morning a little more...

#1. Yes, both plugs are getting a good spark.

#2. Not sure it's a head gasket leak, as the coolant mark is just below Min. and I've never had to add Coolant.

#3. I continued removing plastic and found a tiny rub on a wire under the steering plastic. I traced it back to the thumb throttle. While the wire was rubbed and a tiny portion exposed (like a pinhead), it wasn't touching any metal or anything else (rubbed on plastic). Fixed with liquid electrical tape but I am not sure this is the issue.

I have included a photo of the plugs. Might be time to replace them as they haven't been.
 

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#19 ·
It appears to be getting fuel in both cylinders... I had both spark plugs out and was turning over the engine and a good atomized spray was coming out of each cylinder. I am not sure what the issue is. If it was not getting fuel, do you think it would idle within spec? Confusing!! Hopefully new plugs might fix it, I'll know in a few hours. Not sure how to test or troubleshoot the injectors, that might be beyond my skill level.
 
#20 ·
Ok it's getting fuel.

I'm confusing you. Lean is getting fuel, just not enough of it. That's what the white plug is indicating. Not getting enough fuel with the same amount of air, each stroke, makes it burn hotter.

If it wasn't getting fuel plug would probably be black. You want a brown/tan plug

Confusing I know. Less fuel = hotter combustion.

Can you tell if they are getting the same amount of fuel. May not be noticeable but maybe it is.

For a test you could switch the injectors and see if the white plug follows the injector.

Or just take them out and gave them cleaned/ flow tested.
 
#21 ·
For a test you could switch the injectors and see if the white plug follows the injector.

Or just take them out and gave them cleaned/ flow tested.
Okay, I gotcha. After the plug change I could try this. Are the injectors fairly easy to remove and swap like that? Nothing under high pressure or anything? I couldn't find much for a tutorial with a simple search. Heading out now to get some plugs. With fresh plugs it should be easy to see if the problem follows injector.
 
#22 ·
Well, threw in some new NGK spark plugs and the issue is the same... It seems to run and idle OK on startup for about 30-60 seconds and then it starts doing it. It does sort of sound like an injector miss and that "popping" sort of noise is coming from the throttle body, or at least it sounds like it. Would an injector problem be constant or intermittent like this? It backfires really loudly on decel, usually right before you come to a complete stop.
 
#25 ·
Usually backfiring on deceleration when throttle is closed indicates a lean condition.
See it often on carbed bikes. Not on EFI bikes from what I have seen.
I've tried adjusting the throttle cable and even put a zip tie in there and still no go.... Last time I touched the bike was a few months ago, I put in a new starter and changed all the fluids, took it for a good rip and parked it till now. Injector is what I am thinking.
 
#27 ·
Just a quick comment. Check your battery connections and make sure they are clean and battery is fully charged. EFI does not like a bad connection. Also check inside the battery box for mouse nest. The 850 also likes to have a clean air filter. A mouse made a nest in my buddies 850 air box with cat food and fried the motor.
Good luck
 
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