I have a new 250 AC 2wd 2000 that backfires loudly when i release the trottle from wide open .The spark plug is greyish-white.Tried hi-test gas on my third tank of gas;same problem.Could i neeb bigger high speed jet?It idle normal.

 

you need to check valve clearance same prob on mine

 

Somebody please correct me if I'm wrong here, but the greyish white plug is too lean is it not??? My 'Cat would backfire when running full throttle, then letting off immediately. It was too rich, so I dropped a few sizes on my main jet, everything has been perfect ever since. Judging from your plug color, I think you're too lean, so I'd suggest going up maybe two sizes to see how that does. One at a time might be a better option, but two should be about right I would think. You'll need to change your main jet, these are very easy to swap, and very cheap too-I think around $3.00 each??? I don't know how your relationship is with your dealer, but mine let me try out a few different jets to find which one I liked the best, didn't charge me for any of them except the one I kept-pretty nice of him really to try out all the others, I think I went through 3-4 before I found the right one.

Anyway, Best of Luck to ya,

Mike

 

I don't know if this helps:
During my first ride on my 250 4x4 it back fired when the throttle was released from 3/4 to 1/4. This only occurred for maybe 2 minutes, in the middle of a 10-mile ride. Since then I haven't had anymore trouble.[img]i/expressions/face-icon-small-wink.gif[/img]

 

My AC 250 2wd backfires when you let off the trottle for the first 10 minutes of the ride, then it goes away, usually.

 

Corndog-
When the machine is cold it sounds like-when it does backfire, is it from about full throttle, then letting off immediately, or a gradual let off? The main jet operates from a little over half throttle, to wide open. Again, on my machine it would backfire when running wot (wide open throttle), then letting off immediately. My plug was very black with deposits, so I changed my main jet from a 150 down to a 145 to lean it out a bit, as black is a condition of too rich. Actually I dropped to a 140 at first, but found that to be too lean, and moved up to the 142, then the 145. The 145 works great, haven't had a problem since. Jetting is directly related to the elevation you ride, AC jets each machine according to the altitude of the dealer you buy it from. My dealer is located around 400' elevation, and I ride from 3,000' on up to around 6,500-7,000. So, naturally it was too rich, as it was set for the 400' elevation where my dealer is located. This could very well be your problem. Check your plug first, see what color it is-to do this, you have to run wot, then trip the kill switch and not let off the gas till the engine has died. Coast to a stop, then soon as it cools down enough to touch, pull it and see what it looks like. Black is too rich, drop down on main jet size. White or greyish colored is too lean, go up a size or too on your main jet.

If it's at idle or low rpm, that is your pilot and needle settings, which could be slightly out of adjustment. I think on your carb you have an adustable needle-on mine I don't. You can move it up to lean it out, or down a notch to richen it a little if need be-that might help with idling trouble.

Also-the first 10 minutes or so sounds like the machine is cold, hasn't warmed up properly yet-do you run the choke to start it? How does it start up then? Also how long does it need the choke? You can try priming it with the priming button on the carb too if need be.

Hopefully that will help with some of your backfiring problems-if not, like Bossarcher said, check your valve clearance. I still have not checked mine, but have heard of them being slightly off from the factory, which could lead to other symptoms as well.

Best of Luck, and hope that is at least a start in the right direction,

Mike

 

I always use the choke, cause if you don't it won't start. I just pull down the choke, start it, and rev it for a minute or two. Then I put the trotttle to about 1/4 of the way and push the choke off. Usually it is ready, if it isn't I just do it some more. My AC only backfires when downshifting, but I am going to check the plug tommorrow.

 

I ordered a 135 main jet.I now have a 130 main and a 45 pilot.I also found that i had an exhaust leak at the muffler clamp which made it lot worse to backfire.Two dealers told me that it was normal to backfire if you let go the trottle when going wide open.I will let you know in two weeks after hunting.By the way the valve clearance on the 250 is 0.03-0.08 int and 0.08-0.13mm exh,metric.Talk to ya later and ty all.

 

I have a '99 250 & it has backfired since day 1. It seems to do it mostly on long downhills with the throttle closed. If I have the throttle opened, even slightly, it doesn't backfire. Also, if I pull the choke on slightly it doesn't backfire. I haven't tried rejetting like the folks in the previous posts, but that may be something to try on a slow weekend.

Tracy

 

Lil250cat mine does exactly the same even after rejetting from 130 to 135.Maybe cause these machines have so much motor braking power compared to others which build the back pressure to backfire.We'll have to stay on the trottle a bit when slowing down from wide open.Thanks again everybody.