Hi Guys! I’m back to jacking with my 2005 SP500 (Carbureted) and have some new information (the choke stops the backfire). It is still back-fireing on deceleration. It pops when going down a hill in our neighborhood when I drive it down the street. If I come off of full throttle it will give a big pop. When I coast from about a speed of 25 MPH and the engine brake is doing it’s thing is will pop… pop… pop… pop. I can eliminate it when I pull the choke and push it in when I get close to the stop sign at the bottom of the hill so it doesn’t stall. It hasn’t done this before except on rare occasion over the last few years. It was common during our Wisc. Atv trip back in June. Only thing that changed from last year to this year was valve adjustment, fuel filter, spark plug, new air filter (& blew it out after Wisc. trip). Doesn’t appear that there is an exhaust leak that I can find. Would cracking the seal on the fuel mixture screw and playing with that work? Can anyone out there help or offer any advice with what might be happening? Thanks guys!!!!!!

 

You are running lean. The valves may be too tight, not allowing all exhaust gasses to escape the cylinder, which causes premature detonation of the intake charge, OR you need to richen up the fuel mixture.

I would check valve clearance first.

 

As far as the valves when I did the valve adjustment as stated in the service manual and there was only one valve that was out of adjustment. All others checked out. I kind-a hate to do it again because it sucks taking all the plastic off, draining the fuel, removing tank, etc., but it may be what I need to do. Would the spark plug cause this? I replaced the old plug with the same number new plug. The old plug was tan/brown and looked good. To richen up the fuel would you suggest I move the clip on the needle or jack with the fuel mixture screw that is sealed on the bottom of the carb? Keep the ideas coming. Thanks!!!

 

Move the needle in the carb up one notch, you will move the e-clip down, you should also remove that plug for the fuel screw and set that at 2.5 turns out, to remove it drill a small hole through the center of the brass cap, its only about 1/4 of an inch thick, dont drill into the screw under it, insert a sheet metal screw a little larger than the hole, pull the plug out with vice grips and thow it away. If you remove the air box you can do all this from that location, you will have to cut the tie wraps on throttle and choke cables to get enough slack.

 

You don't have to remove the plastic to do the valve adjustment. All you really need to remove is the seat and the side pannels. I have done 3 sportsmans this way. You have plenty of room.

 

I like the idea of turning out the fuel mixture screw 2.5 turns. (I had to do the same thing on our SP400 when I put in a new needle & jet kit.) Since this happens when going down hill I'm hoping this will kick in a bit more fuel when the throttle is in the closed position & the EBS is doing it's thing. I'll look at the needle adjustment if the fuel mixture screw doesn't improve anything. Let me know if you have any other ideas. Thanks!!

 

I don't think it's the valves.
I think you have an intake leak or exhaust leak. Check the intake boot between the carb and head. Then check for an exhaust leak like the doughnut gasket.
Check these things first before making any carb adjustments.

 

It could be as simple as a throttle cable adjustment. If the ETC thinks the throttle is stuck it will cut the ignition. Try holding the throttle off idle a little bit and see what happens. Doc

 

I thought about the throttle cable to but if giving it more fuel by pulling the choke during decel makes it stop backfiring then I would still focus on intake or exhaust leaks.
But without looking at it and hearing it all we can do is give suggestions.

 

OK... Here is what I did so far. ETC is good. Checked it when it first started to happen and again today. I took the plastic & tank off to have a better look and from what I can see I don't have any intake or exhaust leaks. It all seemed to look good. I then took the carb apart and cleaned it with carb cleaner and put it back together again. I drilled out the plug on the fuel mixture screw and turned it back in and then set it at 2.5 -2.75 turns out. Also, replaced the spark plug with a new one. Put it all back together again and took it for a ride. I went down the long and kind-a steep residential hill at about 25 MPH and let off the gas, it was doing great and I thought it was fixed, then almost to the stop sign and POP... another loud backfire. (back to the drawing board!) I may just replace the seals in the exhaust, but they are about $20 each from Polaris. (Asked that when I picked up a new plug from the Polaris dealer.) What do I need to look for regarding an exhaust leak... black soot coming out? You guys have any more ideas??? (And thanks again for all your help so far!!!!)