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MOTORCYCLE CARB JETTING Tech Tip

This is one of the misunderstood areas of tuning. It is also an operation that is done wrong more times than not. Any shop that tells you that they have jetted your bike correctly is only guessing if they are not equipped with a dynamometer. While they may be very talented and able to guess closely, they are still only guessing, whether they want to admit it or not. We have set up many bikes on the dyno that have had performance work done at shops that did not have a dynamometer. Some shops were aftermarket, some were franchised, but we have NEVER, EVER, found a motorcycle set up by a shop without a dynamometer that we could not make some kind of performance improvements on. We are not saying that a shop without a dyno cannot jet your carb, we are just saying that they cannot do it right. When we dyno tune a bike, it is not unusual to get 30-40 dyno runs on a bike just to get the jetting right. To get even close to the same results without a dynamometer, you would have to put at least 80-90 miles on the bike, one or two miles at a time, on the road with at least half of them being run at over 100MPH. A dynamometer does not lie, and it is without question the most accurate way to set up carb jetting.

If you don't have a shop in your area with a dynamometer, it may be worth your while to make the trip to a place that does. If it is not possible, then you are going to have to rely on the old trial and error method. While you may find information published telling you what jet size to use on a certain bike, and it may seem to work, don't blindly believe it. Every bike can be different. The only way to tell is to buy a handful of jets and try them. Even if your bike seems to run o.k with the jet size recommended by someone with the same bike, you still want to install one size richer and one size leaner and see it makes any difference. Ideally, you want to find the smallest main jet that gives you the highest top speed, then richen it up one size, just to be on the safe side. Your bike should not foul the correct spark plug. If your bike fouls plugs or if you have to run a hotter plug than what is recommended for the bike, then SOMETHING IS WRONG. Go back over EVERYTHING again. Also make sure everything else is right before you begin. It is impossible to correctly jet an engine that is out of time, or has a tight valve, or burns excessive oil, etc. What may seem immaterial to someone who doesn't do it every day can actually have a drastic effect on a bikes performance. For example if the battery in your bike is not up to par, all of your jetting efforts may be in vain. I cannot stress enough that EVERYTHING else must be right BEFORE you make any attempt to set the jetting.

Also keep in mind that ANY time you change the air filter or exhaust pipes, you may have to re-jet the carb. While we are on the subject of exhaust pipes, I would like to emphasize that if you install drag pipes on your bike and don't re-jet the carb, you will probably go slower, foul plugs, and the pipes will probably turn blue. It's not the pipes fault, it is your fault, you were too lazy to properly jet the carb. Drag pipes can improve performance, but ONLY if the carb is set up right for them. Loud is NOT fast. Loud is only fast when the jetting is correct and the bike is tuned right. Otherwise, you are only kidding yourself. Do it right, or take it to someone who can. You will be happy you did, plus it is easier on your bike. A carb that is not jetted correctly will ABSOLUTELY, POSITIVELY, DEFINITELY take life off of your motor. Get the jetting right, your bike will run better, have less problems, and go a lot longer between rebuilds.

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