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Hello, I have this issue and it is probably two found. Last season my rhino ran pretty well, It had a problem back firing through the carb at idle. I richened open-end up the mixture screw and that seemed to help, didn't get rid of it totally, but better. The rhino was built prior to me owning it and it has run well, has pipe, open airbag lid, I think 686 kit, and a cam. I was out last weekend, (sea-level) and it ran terrible, ie, back firing on the decelerations, almost stalling, and the backfiring through the carb is back. I cleaned it thoroughly, so I think, I have monkeyed with the air mixture, to no avail. I had the carb off 5 times while on my last trip.... can't figure this out. I feel like it is lean, I have a 55 and 175main jet in it. I looked for exhaust leaks, none that I can find. Runs great until full throttle, then when I let off it backfires, and wants to die. The plug didn't look so bad, I changed it, nothing. I am hoping someone can help. I have combed the forum, and would appreciate any help.
 

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Assuming the cam timing is correct, backfiring through the carb usually indicates retarded ignition timing. Since the cdi unit controls the timing, I don't think that's the issue here. I would suggest checking the cam timing and making sure it's spot on.

Exhaust backfiring is often caused by exhaust leaks. I cracked my exhaust in two last weekend. I had checked before for leaks but didn't find any. It cracked right in front of the muffler halfway through the 90 degree bend. I was having an occasional backfire, but nothing consistent. Might want to check again for leaks.

I ride mainly at 6000+ feet. So my jetting experience is different than sea level, but a 175 main seems pretty rich. I am actually running a 142.5 main at altitude. I have opened up the airbox and added a 2" exhaust tip to the stock muffler. I have researched this a bit, and it seems backwards, but if you have a 686 cylinder, you actually need to go leaner on the jetting. This assumes you are using the same carb as before the 686 build. The reason is with the bigger bore, there is a stronger suction signal to the carb from the bigger cylinder. To counteract that, you need to go leaner with the jetting. This has proven true on my 686. I would suggest going to a 165 or a stock size main jet and see if it runs better. I know from my own experience a very rich mixture can cause a lot of backfiring when letting off the throttle. A lot of people end up going too rich when rejetting, thinking more fuel is better. Unfortunately, it's not that simple.
 

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Hey Racer Dave. I ended up putting in a 60 pilot jet.... from a 40. The back firing at idle and deceleration is gone.... I am heading out this next weekend and will really see how it runs. Thanks for the reply.
 

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I know you got your issue fixed, but I just wanted to put this out there for the future. My 2004 660 was doing close to the same thing. I bought it a year and a half ago. 2019-2020 hunting season it did it a few times, this year it started doing it a lot and would quit running. I would have to wait a 5-10 minutes and it would run good for a while then backfire and quit. After diagnosing a few things, it turned out that it had the original fuel filter on it. It was also running lean according to the plug. I replaced the fuel filter and it is running great and no longer lean. While I was replacing the filter I switched the lines so that the short line runs from the tank to the filter and the long line runs from the filter to the fuel pump. This puts the filter under the seat where it is easy to get to. Also i leave a new filter in my toolbox so when it happens again I can change it and not have to try to make it back to camp a few miles at a time.
 
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