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Sorry for the essay...

So I am a newbe to dirtbiking and like when I got started with driving I love to learn about the mechanics of the bike as I start to ride. One of the things I am pretty confused about is the carburator and the different jets. I hear things like pilot jet, main jet, etc. But I really dont know what any of that means. I did a quick search and didn't see anything. I am sure there is a sticky on the subject somewhere, but if not, can someone give me a quick run down on what are the parts that are adjusted to tune a bike.

Also any pictures would be helpful.

I am currently in the market for a XR 200/250, TTR 225, KLX 300 ranging anywhere from late 80's to present. Basically a 4stroke from the 200-300cc range.

Also what are common terms you guys refer to when describing how a bike is running so it is easier to communicate with others?

Finally, how can you tell what jets are in a specific setup? For example, my roommates TTR 125 is all stock but it has a hell of a time starting when cold. Once warmed up it has a solid idle and has good power through the whole RPM range. I dont know what it is about starting it, but after a day, it will take 20+ kicks to start. Where would be an apporopriate place to start?

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I'll give it a shot. This is for 4S. Everything is based on throttle position, not RPM

A carb in most cases has five basic tuning areas.


Pilot jet -Controls fuel at idle

Fuel screw - Fine tunes the mixture for the pilot jet


Needle - This covers the opertating range of just off of idle to about 7/8's throttle. It has basically two sections. The straight diameter (thick part) controls from just off of idle and about 1/4 throttle. The tapered part covers the rest. Needle clip position has an effect on this area. It does not affect the area of the straight diameter (because it is a straight diameter, moving it up or down does nothing whereas on the tapered area, you are moving the taper).


Main jet - This affects WOT.


Starter jet - This adds extra fuel (and a little air) when the 'choke' (actually is an enricher) is opened. Your friends bike might need a larger one, assuming all else is good with the engine. Tight valves will often make cold starting hard.


AP - On may bikes, an AP (accelerator Pump) is used to add fuel when the throttle is rapidly opened. It consists of a diaphram to pump the fuel, a timing screw to adjust when the 'squirt' happens in relation to the slide position and in some cases, a leak jet used to control the total volume of the squirt and the response sensitivity. The AP is usually only found on carb bores 36mm and larger.

Jetting is not something you can learn in a day. I have been working on bikes my entire life and I learn some more details all the time. If an engine is way rich, the exhaust note will sound wet or heavy. If it is way lean, it will sound dry or hollow. If the jetting os off by only a little, it can be very hard to tell unless you have a lot of experience or the specialized tools.

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