HELP! Backfiring Problems

Guys, I'm new to this board but would really appreciate any of your insights. I have a 2002 WR426 with grey wire mod, std. timing, full throttle mod and lid removed. I experience severe backfiring and plug fouling since making the gray wire mod. I've made no carb adjustments. The bike runs better with the stock (no baffles) pipe than with my IDS2 - I've tried as few as four and as many as twelve rings in that pipe. The plug is BLACK and sooty even after only a short ride. The timing has never been played with, but it bikefires so badly I was thinking that could be the cause. I'm no kind of mechanic, but is all of this a indication of too lean or too rich a mixture. By the way, even with the backfiring, this bike is still bad FAST. I will really be thankfull for any insight. :)

Forgot to say. I ride in South Carolina = at or near sea level most all of the time. I have never messed with the stock carb at all. Thanks

was the bike ok before,if so reconnect the wire and run it then look for grey wire threads with some detail info you may have by mistake cut the wrong wire,lotsa luck.

Bike has always fouled more plugs than it should. Plug was only good for two or three rides before. Now, have to change almost every ride. I know backfiring can cause major damage...... i just dont know where to start remedying my problem. My Yamaha dealer actually did the throttle and gray wire modification for me. One thing I noticed was that they apparently just cut the wire and electrical taped each end. Shoot, i could have done that. What I asked them to do was to install a switch, so that i could switch it back and forth. Honestly, the bike was more rideable in the woods before the gray wire mod..... or maybe I'm just a wimp :)!!!! Thanks for any ideas

Tcreek, what part of the world are you riding, what temps now as apposed to earlier in the year. Black plug is to rich. This is common when the temp drops. Cold weather needs leaner jetting.

Cold weather needs leaner jetting.

Sorry but that is not right. Cold weather will cause a bike to run leaner, not richer.

My suggestion? Move the clip either one position up or one position down and see if it either improves or worsens. Then you will know whether you need lean or rich. It's a 15 minute trackside job.

Thanks, I'll try your clip movement suggestion....

Skthom, I am not a jetting expert but a wet black plug is a rich condition. Taffy, James Dean and Yamakaze all jet leaner as temp. drops. As far as jetting these are the three most experienced jetters I know of so I chose to follow there lead. I know it doesn't make sence but they all go leaner.


Be sure to check the AP squirt. Mine was about 4-5 seconds every time I opened the throttle. Just drowns the plug. It should be around .5 sec. You will need to do the BK mod to adjust it.

Also the disconecting the blue wire might also help.


Black wet, rich condition.........

Black dry sooty, lean condition.........

The colder the air gets, the leaner the bike runs, you need to jet a little richer........

Popping, or a kind of back fire usually denotes a lean condition, requiring richer jetting..........

Dodger :):D

Just a thought!,do any of you guys think that part of the reason some folk`s are jetting leaner in the cold weather is that it may be raising the operaiting temp of the motor just enough to stop plug fouling,i`m not sure if the colder weather may be making the motor run that little bit colder,thus affecting the plugs ability too run hot enough!.I also beleive that a motor does get leaner in colder weather,or perhaps maybe some fuel`s are better than others in cold weather,i`m not sure why it works for some and not others!,but i do know that when a plug gets close to failure from carbon build up u find that the motor starts to give probs with back fires and poor running,so always take a look at the plug first when this starts,no point in trying to sort fuel probs if the plug insulator is all black and fould up,if it is,lean it off put in new plug and try again,when you see that the plug is running hot enough to keep it`s insulator clean you know you are starting to move in the right direction,just a little trace of soot up inside the plug where insulator meets plug body is what you want,now the real heart-ache and head scratchin and heavy-drinking starts trying to get it to play the game!

That's a keen threory Captain, I'd be curious to see if that works. I'd be much more inclined to try differnet things if the gad damn plugs weren't so fricking hard to get to. At least they could have made the tank a quick release.............

Dodger :D:)

Captain, interesting theory. Wouldn't it be nice to just go to a hotter plug and not have to mess with the jetting. If is wasn't winter here I would try it. This lein or rich issue in cold weather certainly creates opinions totally opposite of each other. I think we should change the subject to maybe, "Whats the best oil"


How cold is it out now where you're at, and what's your elevation? You do have to jet richer for colder, denser air...You need to make sure your engine is running at a normal operating temperature by covering part of rads when riding in temps of about 50*F or lower, as there is no thermostat to control cooling. It's obvious when you touch the engine and it feels nothing close to summertime, when you can actually hold your hand on the clutch cover without burning yourself. Running it cold seems to be the elusive jetting problem and it can cause cold seizure damage and oil sludge formation.

Have any of you guys tried the n/denso plug!,a guy in a bike shop in town told me that they can cope better with different riding conditions,i have stuck with the ngk because i got a better feel for it while i try different things,i also found that i could tell better when the plug was almost fouled up and adding too fuel probs,i found that it is really important that the plug is in good condition when trying to clean up the jetting!.As for the oil i have noticed that when i`m trying different carb settings that it does get more prone too drip sludge from the breather pipe,i dunno if the testing with carb settings is maybe part of the cause due to fuel contamination in the oil due to running the motor that little bit harder while testing it on high speed road run`s,i did note that when i was just running it under normal off road conditions it was less prone to the sludge,and also climate change is a factor in this area too,as other engines do this in winter.i dunno what engine oil you guy`s prefer to use,or find to work best in different climate`s,i change the oil in my wr at 300mls,or even before this if i have been trying different carb settings and think the oil has had a hard time!,so for any one who is doing a lot of testing with carb set up,do not forget the oil,the motor is under more strain while testing!.A neghbour of mine comes past with his kid`s and they come into the garage and say too me"what are you doing to your bike now!",i just laugh an take another swig o the happy juice!

An easy alternative to the BK mod is to change the leak jet. If it is backfiring when coasting go up on the pilot jet. :)

Colder temps require richer jetting.

Warmer temps require leaner jetting.

Thin air requires leaner jetting.

Thick air requires richer jetting.

That is the way that it is...........without a doubt.

Cold air is denser thus the engine needs richer not leaner jetting.


pv=mrt, where P= pressure, v = velosity, m=mass, r= constant for medea, & t= temp. use r as constant foe this use.

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