Are there any resources (books, articles, etc.) on jetting techniques. My problem is, I put K&N's on a couple of bikes. I have no experience jetting, so I had the dealer (Chaparral) do the rejet. As far as I can tell, all they did was change to a larger main. I don't think they did anything to the slow jet or the needle. Both bikes rip at full throtle (so I think they got the main right), but they bog (to lean) when you accelerate hard starting from anything less than around half throtle. This is a safety issue too. I cased it off a BIG jump because the bike bogged in the middle of the takeoff ramp (to late to back out then!!).

Anyway, is there any Science to this, or is it just a matter of trying different jets and moving the needle around until you like the result? I think if I'm going to get this perfect, I'll have to do it myself.

 

What does your plug look like? You need to check it. Run it hard then hit the kill button before slowing down. This will give you a more accurate reading on the plug.

 

If you "run it hard" (full throtle?), aren't you just testing the MAIN (which kicks in from 3/4 to full throtle)? Like I said, I think the main is ok. It's at the lower (0 to 1/2, maybe 2/3 throtle) end where I need to richen things up. Does testing the plug color work down there too? Maybe I should buy K&N's Power Up kit. I think it has a different shaped needle, and must have some different slow jets too???

 

A jet kit will contain main jets and a new needle. I think the needle is shaped slightly different. It is uncommon to change any other jets. The plug thing will work but you can only really tell when using a new plug. So put a new plug in ride 1/4 throttle through the gears and kill it. Then pull the plug and see what it looks like. According to Dynojet if you are asking for power (pushing the throttle) and it bogs you are "usually" rich. Now, if you are real lean it will bog down too. Jetting is a delicate art!

 

My K&N kit came with 8 main jets and a needle valve, clip and spacer. The needle valve is shaped different for my application. I remember when I was first playing with mine and it was sluggish at low to mid throttle and I moved the clip down to raise the needle. I eventually installed a larger main jet. I would try adjusting the clip on the needle first if I was haveing the problem you are haveing. It is easier and quicker to do to see a change and it might fix your problem. What are you riding anyway? You have a Yamaha name but posted in a Honda forum. Do you ride an ATV or a motorcycle?

 

The YZ thing actually came from somewhere else. We do have a couple of YZ125's, but mostly the kids ride them. I grew up riding dirt bike, but switched to ATV a few years ago. There are 6 of us in the family, and we have 5 Hondas, 2 Yamahas, and 1 Suzuki (plus the YZ's). Our garage is a sea of bikes!!

 

I'm had the same problem I called Dynojet and they said anything from 0-3/4 throttle would be the needle and to adjust it first. Also I have a 2001 400ex it suggested to put the 170 jet in I think it may be to much I have a biggun slip K&N filter and a Dynojet jet kit if anyone can help I'm open for suggestions