I have a 2001 YZ426F. I have the Thunder Alley pipe ( stock header ) . I live at about 1500 ft and its around 85-90 degrees here. I installed a 168 main and a 45 pilot. I installed the Factory R&D accelerator pump mod to my carb and changed the needle to one that the R&D guy told me to. I believe he said its only used in Canada ?? anyway . I put the clip at #3 position, like he told me to and 2 out on the pilot screw. It has a very bad miss at the very bottom of the throttle at all speeds ( steady throttle speeds ) . I cannot seems to cure it from this adjusting the pilot screw. So the be sure I checked the plug to see if maybe I had a bad plug and it was very black . Too rich , so I changed the pilot and the needle clip leaner and the miss is still there. So I thought too many changes at once , I put everything back the way it was stock except the main jet. I was gonna do one change at a time . It still misses at a steady pace at the very bottom of the throttle. It cant be the main jet becouse I am not running that hard. I pulled my carb apart about 10 times yesterday and I know its not dirty . I have hardly ridden this thing since Feb. when I bought it. Maybe 1 or 2 times a week and I dont race or ride hard. I'm a novice rider. I havent a clue of what this can be anybody out there have any Ideas? is there a jetting guru out there? I am too nervous to do the BK Mod as I dont quite get the Timing of the pump squirt. Thats why I went to the P-38 lighning mod. Supposed to do about the same. PLEASE BAIL ME OUT HERE!!!! I know someone here on the best damn website period can help me . :):D:D:D:D


You need to describe in detail what you mean by your miss.

1. Is it a stumble, like an absence in breathing, when you go to accelerate?

2. Is it a pop and miss in the exhaust note when holding the throttle steady?

3. Does it only do it under a load? Or does it not do it while sitting on the stand?

4. Or you getting a backfire at any time though the carb?

5. What happens when you’re coasting, in gear?

I'm asking these questions because I'm not sure if you have provided us enough information to know if this is an electrical or fuel delivery issue.


Mine does the same thing. I went leaner, it cleaned up a little, but its a little ratty while ya rev it just slightly. I think its the ingition map. Youd think you could get it sweet, but I think the camshaft is radical enough to make the slow speed carburation difficult. I played a bit with the TPS, and I plan on riding it in an hour or so, so Ill report back what I found.

Rvan, at 1500 feet and 95 plus degrees, you should probably be at 40 on the pilot, possibly down to postion 2 on the needle and the stock main. If your pipe is black in the end, your too rich. You should be a nice soft shade of grey in the pipe.

Do you know what needle you put in? Part no. or something? I'm at the same elevation and this is what I'm running- main 165 - pilot 42 - EKP needle #5 - 1 turn out on the fuel screw - BK mod - pump gas - Powercore 4 silencer(?) I'd put the stock needle back in and maybe play with the clip postion. Also go down one on the main and pilot. Good luck!

These guys have given you good advice. Try putting in the leaner pilot and changing the clip position.

[ July 26, 2002: Message edited by: skthom2320 ]


Let start with the easy stuff and just put a new plug in.

Save the old one and tell us what it looks like.

Also, let me know if you have ever cleaned the plug in the past, and if so, how.

This may or may not solve it, but at least it eliminates a simple possibility.


Just another thought. Also try some different fuel. I have now gotten bad fuel twice since 2001, and the bike would barely run. Even got it from some decent Gas Stations, but you never know! Maniac

i have had my share of troubles myself jetting my bike..and ive been a mechanic for 16 yrs..i finally bought a o2 sensor, the 2 wire type..hooked it up to a millivolt meter and went from there..shooting for 1000 milivolts..spent 2 months trying to get it perfect..took 15 minutes with an o2 sensor...jetting the top end was was the bottom and half throttle that was weird..the jetting was, lean, then rich, then lean again..i went to a 45 pilot..clip on #5, with a 175 main jet running sunoco race gas,with bk mod..the bk mod leaned the whole deal out alot..i know my jetting is as weird as it gets but the o2 sensor dont lie and the plug is nice and tan..cranks first or cold..and runs like a raped might go to a Napa parts store..get a 2 wire o2 sensor, hook it up to a millivolt meter and go from there..worked for me..hope this helps

JR :)

[ July 26, 2002: Message edited by: txthumper ]

TX thumper has a bitchen idea. I wonder if that would work on a two stroke too? Not that im going that direction, but I see a way to make some cash.

Tx what kind of sensor did you buy? I got the two wire thing, but I dont know enough about the O2 sensors to pick one. Do they all measure about the same?

Second question, how in the heck did you read the meter while you were riding?

Its more of a pop and miss at steady throttle . I had my buddy ride it away yesterday and it just had a bad miss at steady throttle . only at the bottom end of the throttle. I could rev it and it would clean right up .Moving or sitting on the stand. Also I had no backfires or popping on deceleration, just steady throttle. I am totally stock now except for the main . I know it cant be the main becouse it runs like a mother when I am on the pipe. Maybe I do need to go leaner on the needle and pilot. I dont remember my bike doing this when I first got it . Since the Thunder Alley and the P-38 and different needle I began to notice it . So I went back all stock Except the Thunder Alley pipe and the Main is richer . I also run 91 octane pump gas . I am gonna Freak out here . This jetting crap sucks. I love my bike but Its really hard to dial in right now. :)

i just went to a napa part store and asked for a 2 wire o2 sensor..they all read the same just some hook up differant..i cut me a little bracket and used a hose clamp to fasten the sensor in the end of pipe..needs to be inside as much as possible to be accurate..then i connected the volt meter and taped the meter to my handlebars until i got the jetting right...some cities have shops with dynos..they can do the same thing for ya but can be expensive..and yes you can do a 2 smoker this way..i know a guy with a ktm 300, he put his on a dyno in dallas with a o2 hooked up...after the jetting was done they picked up 7 horsepower..just think how much you would have to spend on motor work to gain 7 cost him a total of 125.00 and 2 for me, i think i paid around 30 bucks for my sensor and alredy had a meter..hope this help

JR :)

Yes I have cleaned the plug in the past - I used a sand blast method that we have in our shop. Specially made for cleaning plugs. Right now I have the stock needle in at stock setting and the stock pilot in . Which way should I go on the needle and what pilot should I run?.

What an awesome idea ! I have my 426 setup with the BK mod and thru trial and error came up with the same jetting specs you are running except for the main. I have a 168 main installed. I am going to run out and get an O2 sensor tough and see just where I am at. Awesome Idea.

txthumper...FYI...I was told by my dyno guy not to use sunoco high octane. Last year I had my bike jetted on a dyno using an exhaust 02 analyzer. The dyno guy told me to stay away from sunoco octane fuels becuase they are a petroleum / methenol / ?? blend. He said they are OK for cars/trucks not good for higher reving motorcycles.

His explanation was highly technical. It had something to do with the fuels speed of ignition, combustion and expansion characteristics, the peak rpm and compression ratio of the bike, heat and detonation, and cylinder lubrication. He said a lower octane, pure petroleum fuel is better for high revving bikes.

This guy (Jeff Bloor @ Cycle Max Services 905-791-9677) is a highly respected tuner that builds and dials in race bikes for many CDN superbike racers, I trust his judgement.

That has to be one of the best ideas I have ever seen on this forum. I'm going to run out and buy a sensor and a good multi-meter and start having some fun. Wooo-Hooooo!!!

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