carbs inner workings?

i work on cars mainly and therefore have little experience with carbs since newer vehicles all have EFI so i was wondering how many different needles/jets there are in a carb that can be adjusted and what their effect on the running of the bike is. i am planning on rejetting my bike soon but first i have to understand how the carb works in order to do it efficiently. my bike is a 2007 YZ450F if it helps. thanks for ur help

It would be best to search on this subject. The explanation is lengthy. How to make the adjustments is lengthier. How to best going about doing all of this comes with experience.

The primary adjustments are:

Pilot Jet


Main Jet

Leak Jet

Accelerator pump linkage.

You shouldn't need to worry about the air circuits for the YZ carb to rejet it well. Here's a link to the pilot circuit:

Welcome to TT and good luck! There are plenty of folks here willing to help if you get stuck. :thumbsup:

are there any good instructional books specifically for jetting bikes?

Please don't take this the wrong way. Tuning bikes mostly seems to be an experience thing - more hands-on than book work.

This book may be what you're looking for:

When you're done wasting your time reading it, come back to TT, search for how to tune each circuit listed above, open your tool box, and give it a go. If you want to know how to tune your '07 FCR carb and address the quirks that are specific to your '07 YZ250F and why or why not you should use aftermarket parts to solve FCR design issues, I'm not aware of a book that does all of that. But there are plenty of posts on how the carb works and why certain things should or shouldn't be done.

Some people have taken the time to post goldmines of info here on the subject you're looking for.

Please don't take this the wrong way. Tuning bikes mostly seems to be an experience thing - more hands-on than book work.

You are right on saying this if all you want is to know how to fix it/tune it.

Do a search, find a fix, apply it to your bike and probably it'll work. After all, it's been done successfully before, so it should work, right?

But if you're looking not only to fix the thing, but to develop an understanding of how it works, then search&fix isn't enough. That's where the book comes in handy...IMHO... I've seen many posts, and there's a lot of information on this website, but I have yet to see a post that really explains how a carburetor works. It isn't rocket science, but there's enough science behind it.

The more you understand how things work, the better. If you're out there, in the woods, there's no thumpertalk to search on, but your knowledge of how the bike works might help with tracing the cause of the problem and fixing it...

there is plenty of "how a carb works" on the internet.

i really do want to understand how a carb works, so i can understand why a certain thing is happening and how to go about fixing it. or when to rejet for temp and why. ive read that u should rejet ur bike for every 15 degree variance. what are the jetting ranges for elevation?

I suppose the point I was trying to make was in the time it takes to read through a book on this stuff, one could be tuning their own bike and going through the "aha!" moments where theory becomes clarity and have an interactive dialogue with others along the way. I had a feeling that post was gonna go south when I wrote it.

hope this helps...


is pilot jet and fuel screw the same thing?

No. That carb schematic has an airscrew which changes the volume of air mixed with a constant* volume of fuel being metered by the fixed pilot jet.

In the FCR carb, the fuelscrew varies the amount of fuel being metered through the pilot jet while being mixed with a fixed amount of air through a fixed pilot air jet.

A carb will have a fuelscrew or an airscrew. In practical application, the fuelscrew and airscrew adjust the pilot circuit similarly.

*as venturi velocity increases, more fuel is pulled through the pilot jet, but so is more air pulled through the air jet, so the resulting emulsion has a relatively consistant air/fuel ratio throughout the low rpm range that the pilot jet primarily contributes to.

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