Thunder Alley jetting

8 replies to this topic
  • XR250_RIDER_14

Posted October 29, 2006 - 08:45 AM


What jetting should I go up on with my TA. Bob said just go up 2 on the main but I have problems with my pilot screw on the bottom of the carb. I gotta back it way out alot more than 2 turns. Should I go up one or two? Should I mess with the needle jet?

The pipe seems really really flat with stock jetting. It wont pull worth a crap.:mad:

  • XR250_RIDER_14

Posted October 30, 2006 - 03:51 PM


I guess nobody on this forum used Thunder Alley pipes or rejets huh?

  • Fastest1

Posted October 30, 2006 - 06:05 PM


Here goes, the pilot jet is the wrong size. Go up in the next increment til the fuel screw is effective. Flat where? Top? Go up on the main generally but not always. I personally would go with the JD Jetting Kit and follow the instructions. You have to experiment. Go buy main jets the next 1-2 sizes up and try them all. Lift the needle if the mid range is weak. It always helps just to try it one adjustment at a time until you understand the circuits and their overlaps. It takes patience. The fuel screw turning out past 2 turns and no results tells me the pilot is too small. Also buy an adjustable/extended fuel screw. BTW going up in size on the pilot will have a small effect on WFO though not much.

  • Wyatt

Posted October 30, 2006 - 10:32 PM


Here's an idea....

Try doing what Bob says.

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  • XR250_RIDER_14

Posted November 01, 2006 - 03:26 PM


All he said was 2 sizes on the main and my pilot is messed up. Its stock too! I took it apart. I dont know &%$#@! to do the bike is stock and the fuel screw isnt right. By the way it is an a/m fuel screw red and knurled.

  • Fastest1

Posted November 02, 2006 - 05:21 AM


Go buy the next two sizes of mains and pilots and install them. It will be better to have both sizes on hand for tuning.

  • grayracer513

Posted November 02, 2006 - 08:34 AM


Its stock too!

A bike with an aftermarket exhaust is not stock. It's entirely possible that that one change has caused the jetting to be off by as much as two sizes in either direction. My 250F needs a smaller pilot with the WB R4 on it than with the stock pipe.

If you think you have a jetting issue, the only thing you can do is to change something, one thing at a time, and see whether it gets better or worse. You already know that you have to back your fuel screw out farther than normal. Check your hot start to be sure it closes, check for air leaks, and then increase the pilot jet size. If your low throttle performance improves, try a bigger main jet for the flat full throttle problem. Don't be surprised if it gets worse instead of better, either. If it does, go a size smaller than you started with.

There's no magic, or quick trick solution to it, just patient and orderly trial and error.

  • XR250_RIDER_14

Posted November 02, 2006 - 07:16 PM


Well I got a two size bigger main jet and the guy from the honda shop put in a size bigger pilot. He said compared to the last time he rode it when it had the stock exhaust on (I still had to back the fuel screw all the way out) It was a totally different bike. He thinks the jetting is on. He said it has instant throttle responce and you can charge things alot quicker by just twisting the throttle really fast:eek: . I think he knows what hes (hopefully) doing since hes been a mechanic for probably 10 years and he races. Guess I got lucky.

He said the fuel screw is adjusted right now.

And by the way grayracer he fiixed my clutch too. He said that some spring or something was backwards?? It creeped really bad. And hes lubing the cable for me. So hopefully ill be thumpin around on my new TA!:mad:

  • grayracer513

Posted November 02, 2006 - 08:31 PM


He said that some spring or something was backwards?? It creeped really bad.

Probably the "boss spring", a Bellville spring that goes in at the back of the stack. It can cause trouble if it's backwards.

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