Jetting for Dummies?

Hi I was wondering if any of you Guru's out there could explain jetting for me. If I knew better what was going on in the carb I think I would be able to tune my bike better. I have an XR650R with the baja kit stroker exhaust and vented airbox cover. Right now I am running a 65 Pilot jet, "airflow?" and a 200 Main Jet "fuel flow" I didn't check what the clip position is. It runs good now but as the weather gets colder and elevation higher it might backfire and pop. "to lean?"

Other mods. Pro-Taper GPS steering stabilizer. 3.4 IMS w/dry break excel gold wheels CR500 shock, precision concepts fork. Very smooth and very fast. :)


Here a pretty good article on the basics of carburetion:

Keihin Carb overview You might take a look at the thread started by moderndayxr regarding jetting, Qadsan gave a pretty good overview of jetting your beast.

Here are several articles you can read to help you gain some knowledge about carburetor jetting. The same carburetor theory applies to all kinds of carburetors whether it's on a motorcycle, car, airplane, etc. While I don't agree with everything that's said in some of these articles, they are never the less worth reading to learn many of the basics. There's more to jetting than what's listed in many of these articles, but it's a very good start toward understanding some theory behind this.

As the weather gets warmer, your mixture gets richer due to less air density. When the weather becomes more humid, your mixture also becomes richer due to increased moisture displacing oxygen. The following is a quick summary on how weather affects your mixture.

Cold weather causes your mixture to become leaner (increased air density).

Warm weather causes your mixture to become richer (decreased air density).

Higher altitude causes your mixture to become richer (less oxygen in the air).

Higher humidity causes your mixture to become richer (increased moisture displacing oxygen).

If you have an OEM service manual, look at the carburetor section and it will have one or more correction factor charts. If you're happy with your current jetting, then simply use the correction factor charts to determine your new jetting based on changes to your riding environment. After digging into this stuff, you'll soon notice that humidity has much less of an affect on jetting than temperature and altitude. Four strokes are not normally as sensitive to jetting changes as two strokes are, especially when tuning for optimal performance. Running way too lean of mixture can cause serious damage to an engine. There's a very fine line between having the optimum air/fuel mixture and running slightly lean, so many tuners will setup their bikes to run slightly rich, especially during WOT. Running a bit rich is safer than running too lean. When tuning a bike from scratch, it often takes hours of persistent work even from a knowledgeable tuner to get a specific bike setup optimally for a particular riding environment, but once that setup is nailed down, it doesn't take too long to make the necessary changes to keep it in perfect tune.

A good tuner who’s very familiar with various bikes & their setups can often get you close to being optimally tuned based on their experiences. A person can also take their bike to a shop that has a dynamometer and have their bike dyno tuned for optimal performance. While the bike is on the dyno, the dyno operator can change out various jets in addition to other tuning changes between dyno runs to determine which combination makes the best overall power for your specific bike. They can also plot the air/fuel mixture throughout the entire engine RPM range for your bike to determine if it’s running too lean or too rich at certain places, but all this costs money and the results can vary based on the experience of the person making the changes to your bike along with the equipment being used, etc. You can achieve excellent results from tuning the bike yourself, but it can be very frustrating and take a lot of time to get it optimally tuned. To get it tuned so it’s running good doesn’t take so long, but squeezing out that extra inch of power from your bike so its optimally tuned is what takes the time & patience.

Here's an excellent article from the legendary Gordon Jennings about reading spark plugs. It's the most accurate article I've read on the internet in regards to reading spark plugs and well worth reading a few times to completely understand it.

Hey thanks a lot for the info on carbs. I looked up a couple of the articles you suggested. Perfect. Do you have any ideas where I can get similar information on setting up my suspension it is a similar mystery to me. :)

Suspension huh, well here's some articles you might like :)

If you're just starting out, the most cost effective things you can do is to get some new springs to match your riding weight, then setup the static & dynamic sag. Also make sure your fork/shock oil isn't broken down if your bike has been ridden a good bit. From there you'll setup the clickers which help out with all the low speed stuff. Setting up the high speed stuff or significantly changing the mid stroke performance requires partial disassembly of your forks/shock and internal valving which can be very time consuming and intimidating to some people that have never done this before. Getting the knowledge or assistance from someone else like a good suspension tuner who has the experience and knowledge regarding the valve stacks for a particular bike & application for your specific riding needs can be very helpful in nailing things down quickly, especially if you're looking for the ultimate in performance, but a lot a performance can be gained by doing the simple inexpensive stuff yourself (springs, sag, clickers, fluid level & viscosity choice). If you happen to weigh around 165 to 175, then you're probably in good shape with the stock springs, but if you weigh much more or much less, then you may not be able to get your sag properly setup, which is important in the overall handling of your bike if you're looking to get the most from it.

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