New Thunder Alley and jetting problems

I have a 01 426 that has a slight stumble when the throttle is barely cracked. I have tried jetting with and without the bk mod and have not been able to cure it. It was a very slight stumble and I could live with it. I installed my new TA pipe and it made this problem more pronounced, eventhough the bike runs better everywhere else in the powerband. I think with the louder tone of the TA, you can hear the stumble better.

I can whack the throttle open from an idle and the bike will almost never die. It pulls hard and runs great. The problem is when the throttle is barely open. (maybe a 1/4" of movement)

I notice this when creeping very slow in 1st gear or when I am breaking into a turn and slighlty blipping the throttle. You can hear the stumble and it sounds as if it is going to die at times. But as I exit a turn and roll on the throttle, it runs great.

I guess my question is what should I look to? Is this a pilot jet problem because the problem is only during very slight throttle openings? Has anyone else dealt with this??

I do give the TA pipe a thumbs up. It makes the bike rip, and has an awesome sound.

You're most likely off on the pilot/fuel screw. I'll assume that you are adjusting the fuel screw, if you are more than 2.5 out then get a bigger pilot. Usually, if you're lean on the pilot circuit you'll get decel popping. Also, if you haven't taken the carb apart and cleaned it real good be sure to do so. The pumper tends to get dirty and grimy which affects the performance noticably on these bikes. Check the squirt timing when you get a chance i.e. make sure it's not hitting the slide.

I've had my TA pipe for about 2 weeks and haven't had to rejet at all. I will say that when I rode on Sunday it was around 40-50 degress and I could tell I was borderline lean on the main (warrenting me switching to the 168) but I normally don't ride when it's that cold so I'm not going to switch it out. For what it's worth my bike will cough (and sometimes stall) if I wack the throttle while standing still but I never notice a hesitation while riding so I ain't touching it. Elevation here is 2500, I run a 50/50 mix on race/pump gas, jetting stock except 1 leaner on the clip position.

The bike is running absolutely flawless (except for the standing still wacking of the throttle) from bottom to top with this pipe! I can chung into a corner in 3rd and roll the throttle on like nobody's business without touching the clutch at all (thanks to the TA).

Good Luck, and remember to include stocks in tires & chain companies in your portfolio!

I think you may be right about pump timing. I have been thinking about it, and I think that could be it. It seems that it would affect it at even the slightest throttle opening.

Right now I am stock main and pilot with the next leaner needle in the middle position. I run 100% phillips b32.


By the next leaner needle, do you mean an EJQ?

The stock needle EJP or a bigger pilot jet may be what you're looking for IMHO.

Hope you get it sorted out.


Thanks for the advice. I'll pop the stock needle back in to see how it acts with the TA pipe.

i would get the EKN or EMN needle if you can. i recognise the symptoms you talk of and i cured it with a move to EMM needle.

i could have got the EMN or EKN. look at the last letter, this is the fatness of the needle straight. the larger the letter the leaner you run.

so drop that Q, forget that P and get an 'N'!!



With my Thunder Alley, I did the BK mod and run this jetting: 168 main, 45 pilot, my needle is the stock one, but Its up a notch from stock, in other words its richer 1 notch. Plain and simple, my bike rips and doesnt stumble whatso ever. Not off idle to wide open, not rolling it on, never ever. Usually I run race gas, VP c-12 or Trick 108. My bike starts first kick almost everytime, hot or cold. And I dont even have to kick it that hard. I can start it wearing flipflops. Cleaning your carb really good will help you sort things for sure. I read in MXA that if you have to use the hotstart button, you need to clean the carb if everything started out right. Jets are cheap. Go get yourself a 45 pilot, (get the honda jet, its 4 bucks cheaper) and put the stock needle back in it a notch down from stock. You'll be stoked. Start out with the screw at 1.75 turns with the 45 pilot. Im sure if you get the pump timing squared away, these settings will make you very happy.

James Dean - If he was too lean on the pilot wouldn't he have popping on decel or hanging idle? I've ridden Wyatt's bike and that "stumble" feels like when I have the fuel screw a tad too rich on mine. It just seems odd since we both have '01 bikes and ride the same areas. I recall him saying that the stumble wouldn't go away with fuel screw adjustment. Maybe I should spring for your jetting spreadsheet!!

Johnnie (Wyatt) - I have a 45 pilot if you need it and want to try JD's suggestion.

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