The needle jet or the main jet?

It seems to me that the needle jet would be most important since it controls the larger throttle range and it's the range you're most likely to spend the most amount of time in (provided you are a trail rider)....

So why all the fuss about the main jet? Simply because most people don't know any better or they are only concerned with top speed or it's the best tuning jet? Or is it because it's generally easier to change than anything else?

And when doing plug checks, do you use new plugs for each throttle position? Is it essential that you run it under load? Do you start with the bottom end of the throttle checks and work your way up or is it better to work your way down?

I'm not doing any changes yet but I am trying to get more info and trying to clear the air about jetting.....thanks for any info!

In doing some searching I found this great article: Carb Tuning

Here's another I've had for a while: Carb101

Another:Carb

Even after reading all this I still don't really know and don't have any real world experience......thus my question here...

 

Needle jet or Main Jet?
You mean Needle versus Main Jet.
Unless you are just cruising around at half throttle all the time, the main jet is the most important. Get that wrong and you either fowl plugs or blow your engine. The main jet influences 1/3 to full throttle. The needle only influences 1/4 throttle to 3/4 throttle, so if you feel you really need to fine tune that range, then try different needles or adjust the c clip. Most people would change needles and never know the difference, and the most common position on the needle for the C clip is mid way. Really, I would say, only change something if you are having problems, or if you have a dyno to see the small results frm changing needles.
I don't do much trail riding, so I am usually in the last 1/4 of the throttle, but I have never messed with the needle or clip. I know how to, and understand the effects, but I just havn't seen the need to.

This is according to a dirt wheels magazine article I have and self analyzing

 

Thanks for the info but again I see conflict....where I was getting some info says that the main jet is only 3/4 to full throttle and the needle is from 1/8th to 3/4 and from 3/4 back down to 1/2 throttle (from White Bros tech info) White Bros Tech It's a PDF file so you need Adobe

And trail riding I spend more time in the middle of the throttle range than anywhere......according to White Bros the only reason to change the main jet is if you're having a problem 3/4 to full throttle....

See why I'm confused? I appreciate the input though and real world advice is far, far better than "theory".....and I'm probably not a good enough rider to notice much of a difference by changing either......now I can tell a difference between my old stock tires and my new ones! [img]i/expressions/face-icon-small-wink.gif[/img]

 

The main jet effects the middle throttle position because the needle and jet are working together. Basically the needle is blocking the jet, but since it is tapered, it blocks it less as it goes up(and completely opens the jet at full throttle). At any given positon (After about 1/3) the needle to jet relationship is important. In exagerated terms, imagine if the jet was so small that it hugged the needle and no gas got by, but air would rush in, creating an "Extreme" lean situation, vice versa, imagine the needle in a ten gallon drum, where it has no real effect and fuel just pours in. The engineers of carbs figured out that if the main jet is correct for full throttle, then it will be suitable(maybe not perfect) for the mid positons. I assume you can just run around in the throttle position that you use most and check the plug. If it is really dark, jet down, if it is really white, jet up. Otherwise you are running in the correct range, then you can fine tune by moving the needle. With the same jet, if you move the needle up(putting the c clip down a notch) you ritchen the mixture, and vice versa. So if the plug is a little too gray or a little to brown, adjust the needle up or down.
True the main jet has the biggest effect on 3/4 and up but it isn't dicounted below that
Hope this helps

 

Excellent info!!!!! Thanks for taking the time for an explanation....it's starting to make much more sense now.

 

Cool, Glad I could help.
I dont mind explaining how jetting works, but I hate it when people ask which jet to run. Like we know what there machine is doing.

 

Nice explaination. As you are running mostly in the "middle" if the fuel circcut, I would try playing with some diffrent needle profiles vs adjusting the "E" clip (if you can). As they both have input in the middle