Rode yesterday, no Plug Fouling, but running rough! Help?

Hey Guys,

Well I got my 426 out yesterday after putting it up w/ couple of problems last time I rode. I took out the new UNI air filter and cleaned it and put a very small amount of oil on it. I also put in a new plug. (CR8E) Well I had done the blue wire mod but I plugged that back in. when I pulled the old plug out.(after one ride) It was very black but not fouled out. Now if I am not mistaken you either foul a plug out or it is good not really anywhere in between. Is there such a thing as a partially fouled plug??

I am just confused here, bike run great until it got hot and I started jumping and running the piss out of it. Once hot it started bogging and running rough from midrange to top end. Stock jets, BK Mod, 75 degrees. I know that I can start searching but I am at work and I have searched and read so many different things. I just wonder what my problem is. I put the stock filter back in and it ran a little better but still wasn't running right. If I was to pull to a stop and just sit there and idle then stab the throttle it would bog or cut off. I can't tell whether it is starving for fuel or getting too much. Very frustrating of course as I know many of you have found out. Right after I did the BK MOD you could stab the throttle at idle, midrange or topend w/ no hesitation. Also I installed one of those small see thru fuel filters too. I wonder if maybe that is restricting the fuel enough to make this problem happen, also when I pulled the freshly (lightly oiled) filter their was no evidence of oil in the carb boot. And what I found out in the past after overoiling the filter was immediate fouled plug. I am not getting a fouled plug though, so what the He$%? I know there have been many topics discussing this and so many different things to look at but I am at work and just wondering if anyone had very similiar problems under somewhat the same conditions maybe could help. I am going to take off the fuel filter, try another new plug, and put the UNI filter back in. I will pull the carb off and clean if U think it is necessary, any thing else, I missed a 0 problem bike, but Hey if U are gonna ride a race bike then U gonna have to work on it? Any help is GREATLY APPRECIATED!! FRANK :)

did you let the freshly oiled uni sit for a day? if not, clean your carb.

just checkin, did it have gas in it? mine runs like crap when its on its last couple ounces of fuel

Definitely start with cleaning the carb. For me personally every time it runs funny it's goo in the carb, hopefully it's the same for you.

thanks for the replies,

I am thinking it is the carb, so I guess I will pull it off and clean it. Sirthumpalot, Question, what jets are you running in FL. My elevation might be a little higher but temp's are fairly close to each other. How do you have your carb set up? Needle, BK MOD, main, pilot, air screw etc. Just curious, I will clean the carb and start w/ that. I will check jets too see what I have stock, and turn the fuel screw back in to 1 1/2 turns out to start with. Thanks, anybody else any ideas. Should I take off the fuel filter or leave it?? Frank


I went out on me 00 for the first time in a month. I too applied the Blue Wire and did some fine tuning to the BK duration.

It was fairly warm in the hills I assume about 80 - 85. The Bike Ran flawlessly.

My questions are:

since your jettigs are stock, have you played at all in the needle area ? Meaning raising lowing needle

if no I would also look at the float level on the carb. Could be a tad high Not sure just a helpful guess


Plugs do get partially fouled, or at least partially cleaned. If a plug has been fouled, but then run out, it will often act fine down low, until about 1/2 throttle or revs. Then it will cut out with crackles or backfires (not the muted pops of too lean) until it is less that 1/2 running at WOT. The first thing I would do is change the plug. It isn't likely your overall jetting changed overnight.

Overoiled filters get goo into the pilot jet without being visible in the inlet of the boot. Lots of experience here. The only place it plugged on mine (many times) was the pilot jet. It gets inside the pilot jet through the pilot air jet and then the cross drilled holes block all or part of the air from the pilot air jet. The bike then runs extremely rich on the low end, one consequence is fouled plugs. The richness does not explain your open throttle rough running (pilot is overwhelmed at large throttle openings) but the resulting fouled plug does. Again, the symptom is crackling, backfiring, and sharp lunges at wider throttle openings but running fine at lower RPM/throttle positions. Sometimes it cuts out so much it dies, giving the declining bog sound.

If this is the case, you still need to clean the pilot. On a newer ('01 and up?) carb, the bolt on the bottom of the bowl is 17mm instead of 14mm(?). If you have the 17mm bolt, you can get to the pilot by turning off the fuel, optionally draining bowl with 3mm horizontal allen, pulling 17mm bolt. Main jet is 6mm sticking down in center. Pilot is flat bladed screw fairly deeply recessed directly in front of main (still in 17mm bolt's hole). Screw it out, carb cleaner and compressed air until you can see through it and what you see is round. Then make sure you can see through all 4 sets of cross drilled holes.

Fresh plug, clean the pilot jet.

Good luck,


Well thanks again, I have an 01' and the plug that is in it now is not fouled, might be partially fouled but I am out of plugs and don't want to pay 6 bucks at the dealer for a new one. I will clean the one I got. Now in regards to the main and the pilot, do I need to twist the carb or can I remove all this (pilot and main) without twisting the carb? I probably will pull the whole carb though and check my needle also. what setting (needle) would you recommend for my problems? What about that fuel filter?? thanks, frank

My carb is completely stock. I've fiddled with the fuel screw a little, but I think it's less than 1/2 turn from the stock setting. When the carb is clean it runs flawlessly. No bog at all and loads of zap. I personally think that Yamaha did a great job with the stock jetting on the '01, at least for the climate that I ride in using pump gas. :)

No BK MOD Thumpalot, Well glad to hear that it runs flawlessly. Funny so did mine before I started messing w/ it. Oh well water under the bridge now. I guess I will figure it out. It is thunderstorming and raining here so it looks like a 12pack tonight and pulling the carb off and cleaning. Thanks for the info. Still wondering though about the fuel filter leave it on or take it off? Anyone else got any ideas? Frank

I'm sure a fuel filter could not hurt. After cleaning my carb for the second time I'm thinking of adding one myself. :)

[ May 13, 2002: Message edited by: sirthumpalot ]


If you are pulling the carb, go ahead and carb cleaner and air all of it. If not, if you had a short enough screwdriver, you could get the pilot out without twisting. I twist mine far enough to get a 6" screwdriver in it without loosening the joints or throttle cable. I've kept a close eye on the cable and so far, it's fine. Let us know what you find. Maybe we should start a pool. I want $2 on clogged pilot, fouled plug.


Mark Cantrell, the limb you walked out on to place that bet looks like a bridge from here (pretty safe bet)

A bet huh? LOL Well guys did not get a chance to pull it off last night, but I think that is the best thing to do, but if I can turn the carb enough to get the bowl off and pull the main and pilot then I will. I will let you know so we can see what happens in regards to the bet!! LOL Thanks for your help guys! Frank :)

Well guys,

Got another ride in yesterday evening. Still stumped with this thing. I hate that it ran so good for months and now it is bogging and hesistating off jumps and hard on the throttle up hills. I pulled all the jets out of the carb. I cleaned each one and reinstalled. I took the fuel filter off. I also tried some bigger main jets. Before even at idle I could stab the throttle with little to no bog, now if I stab the throttle at idle it will almost cut off. The jets looked clean. Needle is set stock where it was from the factory, the main was a 162 and pilot was 42. Air screw is 1 3/4 turns out. I tried moving up on the main to a 168, seemed to run better so I went up to a 170 main and it got worse. What gives? This is getting really frustrating, People who have a stock 426 w/ no mods! Leave it alone!! IMHO. I sure wish I did cuz it ran so good before, what gets me is it ran really good thoughout the winter but now it is struggling. Whats next guys? What do you think I should do now, Do you think it is starving for fuel under the conditions I described or is it getting too much fuel. Help! Help! Thanks, Frank

Should I go smaller on the main, bigger on the pilot, or move the needle, check the float height, BK duration? Thanks again!


when is the last time you checked the valves.

I have read post prvious in regards to this, it mentions like you did

I hate that it ran so good for months and now it is bogging and hesistating

Suggestions point to a possible Valve adjustment is needed. :)

Hey Ego,

I figured I would get my first reply from you!! LOL Do you ever sleep, it is 9:30am here it's like 6:30 where u are isn't it! LOL Anyways, I checked my valves about 4 rides ago and they were all in spec. I guess it is possible they have fallen out of spec since then but I kinda doubt it but if I can't find any other reason it is acting this way then I guess I will check them again. What do you think? Frank


Only suggestion on the valve chk, persoannly I would, Keeps out of the house and out of the wifes hair :D

Ya I sleep, Me job requires me to be here for the east cost yakkers :)

I pretty much do what I have to do, to stop from being so dang bored

You said you removed and cleaned the jets, but did you shoot any cleaner into the carb passages? Be sure to shoot some cleaner and air through the carb passages as well, not just remove and clean the jets.

That said, my next best suggestion is to remove the carb when you clean it to make sure that it is spotless. Then put everything back to 100% stock settings and see how it runs. It ran good previously with stock setting so if you go back to those settings with a clean carb and it's still running bad then that pretty much eliminates the carb as the problem (unless it is damaged somehow) and we can move on from there. Let us know!

Thanks guys,

Guess that is my only option left of removing the whole carb and tearing it down and cleaning it. Just such a headache, but if you ride you are going to have to work on em' heh! Well I will try that and see what happens. I did spray carb cleaner into the bottom of carb when I had the jets out though. I still wonder if this is a lean condition or a rich condition. Plug looks rich but hard to get an accurate reading on that since I have to idle from the track to my house (1/2mi). Guess I could undo the BK mod, and put stock main jet back in. But other than that everything is stock except for the new UNI Filter. ?????????????????????Frank

Several of us had problems with plug fouling in CO, it was a nightmare to diagnose. I had people like SUnruh telling me to try race gas but I didn't. Couldn't imagine that was it. I was ready to torch my 426. BUT IT WAS THE GAS! I can go out and put 10% ethanol (oxy gas mandated here in the winter) in my bike and have it foul in 15 minutes.

If you have not yet changed gas, do it now. Look, what do you have to lose? It takes no work. Go buy race gas and see if the problem is still there. Yes, something else could be wrong but I know I would have saved many hours of aggravation if I had followed this advice.

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