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dcecar

noob question: what is jetting?

9 posts in this topic

I am a total noob, just got my bike 3 days ago. I've read alot of posts about overheating, smoke etc, people mention jetting, I have no idea what it is.

Can anyone explain what it is, what it does, what adjustments make what effect etc. thanks -Dejan

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Hi,

The JET in Jetting refers to the a metering device in your carburetor that restricts the fuel rate as it is processed for combustion. This sets the ratio of fuel and air so that the proper explosion can take place and give the maximum power the engine can provide. jetting usually refers to changing or adjusting this metering rate to improve your power based on ambient conditions mostly.:thumbsup:

L@ter Bud

JD

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For an engine to run properly it needs the right ratio of fuel and air mixed together. Your carburetor does the job of allowing fuel to mix with the air as air goes through it. The jets are like small little pipes that allow the fuel to flow through before mixing with the air. Basically, the smaller the jet, the less fuel goes through it to mix with air. As temperature, humidity, and elevation change, the properties of air change and thus the amount of fuel needed to make the right ratio changes. For this reason you need to change your jets accordingly for different seasons and elevation. There is a range that the jets can cover so you don't need to change them for a one degree temperature change, but you should change them for a 15-20 degree change or you can damage your engine.

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4 strokes aren't as easily damaged by incorrect jetting as 2-strokes are. If the jetting is off enough to cause damage then it's most likely going to be running pretty bad. In general, if your stock bike is running good then the jetting is fine. These things are actually pretty flexible and if you're near sea level and don't get extreme hot or cold then it will probably run just fine year round without any jetting changes at all. For example I'm in S FL and I ride a 426 and I've ridden as hot as 95+ and as cold as 50 and it runs fine with stock jetting, maybe a little fiddling with the fuel screw. Jetting changes would surely bring improvements, but not enough to justify the effort to make changes in my personal opinion.

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Here is the stock fuel screw, viewed from under the carb:

IMG_0633.jpg?t=1210791827

Here is the location of the screw with the carb in place. (red knob is an aftermarket screw):

IMG_0632.jpg?t=1210791929

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Here is the stock fuel screw, viewed from under the carb:

IMG_0633.jpg?t=1210791827

Here is the location of the screw with the carb in place. (red knob is an aftermarket screw):

IMG_0632.jpg?t=1210791929

where does the fuel screw change the mixture at,..........at idle?.....where at in the throttle location:excuseme:

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where does the fuel screw change the mixture at,..........at idle?.....where at in the throttle location:excuseme:
Only at idle and in the first 1/8 throttle as the carb transitions to the main circuit.

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