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Do I get air miles when I re-jet?

Not likely.

To re-jet or not to re-jet? That is the question. Whether I'm at school, in the shop, or answering e-mail, the question of jetting is the question most often asked.

First of all, what are jets?

This pic shows the carb (inverted) sans float bowl.

A Jet is a type of propulsion system, predominantly used on most modern aircraft ..... Actually, Jets (as used in a motorcycle) are the parts in the carburetors that determine how much gasoline (brought up from the float bowls) is allowed to mix with the incoming air to give the correct air/fuel ratio for proper combustion.

The jets are screwed into the bottom of the carburetor body, just above the float bowl (the bowl must be removed to access them). They are made of brass with a very precise hole drilled through the middle, with the size of the jet, (based on the diameter of the drilling) stamped into the top or the side. The higher the number, the bigger the hole, and the greater the volume of fuel flow. On most carburetors there are two jets, one (called the pilot) which controls fuel flow at low engine speeds and the other (called the main) which controls fuel flow at higher engine speeds.

So when would one re-jet a set of carburetors on a motorcycle?

If the bike is completely original, without any modifications, then you wouldn't need to change jetting. It's only when changes to the intake (i.e. nonstandard air filters) or exhaust systems are made that re-jetting may be required. However, the replacement of a muffler alone with a comparable aftermarket type does not usually require re-jetting. It's only when the entire exhaust system is being substituted for a high performance one that jetting work will likely have to be done.

Why do these changes require jetting?

Twin carbs stripped of their jets.

When the bike was designed, the folks doing the work built the engine, fuel, and exhaust systems to work together in order to produce the desired performance characteristics, as well as to meet all the legal standards of many different countries.

When we take it upon ourselves to change any part of that power family we also have to adjust the rest of the system to work with our new addition. A good analogy would be that one day you decide to build up those biceps, so you begin to lift weights. By lifting the new heavy loads you begin to tear down muscle tissue. This tearing down of tissue causes the body to repair itself with new muscle tissue. In order for this to happen your body will need more and better fuel. If you continue to guzzle beer and eat a steady diet of French fries your body cannot rebuild the muscle, so the damage does not get repaired. You may think your doing this great performance-enhancing thing for your body, when in fact you're only working harder and may even be doing more harm than good.

The same holds true for your motorcycle. By adding a performance pipe or air filter and not compensating for the change you may be robbing it of performance instead of adding to it. You may also be doing some serious damage to engine.

And here with have a lurvely 4 into 1 system (circa 1972).

When you add a performance air filter it is generally less restrictive than the stock one. This allows more air/oxygen to pass through it and into the carburetors. By adding more oxygen (but not any additional fuel) you will lean out your air fuel ratio. The leaner the mixture, the hotter and more erratic the engine will run.

Changing the stock exhaust system in favour of a high performance header will also necessitate re-jetting the carburetors but for different reasons. When a manufacturer designs an exhaust system to meet several international noise standards, they usually do so at the cost of some performance. Adding a performance pipe offers less restriction to the exiting burnt gasses, usually resulting in more noise than the stock, but also a gain in peak horsepower. By fitting a well designed and less restrictive aftermarket system you not only better utilise the pressure pulses in the pipe (which help with the extraction of the exhaust gas from the cylinder) but you also exhaust the gasses more efficiently (meaning that less burnt gas remains in the cylinder to dilute the next fresh charge).

Although it may not seem linked, this does ultimately effect the intake system, usually requiring a richening of the mixture.

How can jetting solve this problem?

Removing the main jet ..

By increasing the size of the main jet, we increase the amount of fuel in the air/fuel mixture that's being burnt in the cylinders. By increasing the gasoline content you make the oxygen in the mixture lesser than the stock mixture. Less oxygen, or more fuel in this case, the cooler the engine will run. Also by increasing the quantity of mixture being burnt you get a more powerful burn in the cylinder and an increase in the performance of the machine.

To compensate for this we increase the size of the main jet. A good rule of thumb when changing the air filter OR exhaust system is to increase the jet size by 10%. So if the main jet is a 95 then 10% would add 9.5 to the jet size, giving you 104.5. Since jets generally come in increments of 2.5, you'd have to settle for a 105. If you change both the air filter and exhaust then you're probably safe to start at the 15 - 20% increase zone.

Note - It's always best to go a bit higher than too low. By opting for a 10% increase you're likely to be adding a bit too much fuel, making for a rich mixture. Slightly rich is much less harmful to an engine than a lean mix, which runs hot and can cause seizure or even melting of the piston!

So how do you know if you have it right? Because you can't actually see what's going on in your engine while it's running we have to rely on an accessible observer - the spark plug. The spark plug is the window to viewing the internal combustion characteristics of your motor.

The plug chop

Plug colours (left to right) - Rich, normal and weak (click for bigger pics).

You've just installed a new performance air filter and/or exhaust system. Presuming the bike starts, allow the bike to warm up normally, no zoom zoom, and take it for a good run - 20 to 30 km should do it. Find a straight stretch of road (a steady incline, is preferred but not essential) and kick the bike up to a high gear and open it up. As it revs out, release the throttle, pull in the clutch and kill the engine. Pull over and remove a plug.

Warning - The plug will be HOT! Use gloves.

Have a look at the business end of the plug; it's the end where the spark occurs. If all is well in the motor the end of the plug should be a tan colour. If the engine is still getting too much air then the plug end will be whitish in colour. If you're getting too much fuel, then the colour will be black. All this is assuming your engine is in good shape and not suffering from old age or oil consumption. These conditions will effect the colour of the plug and may give you inaccurate readings.

How do I change the jets?

In order to change the jets you must remove the carburetors from the bike. Once removed the float bowls must be removed to access the jets. The main jet is the located in the middle of the carburetor body and is the one that has the needle running into the tube that the jet screws into. Jets can be purchased from a local motorcycle shop and is usually done on an exchange basis. They range in price from $5.00-$15.00 each depending on the type and machine.

Because the carbs have to come out each time the jets are worked, this is why it can get very expensive in a short amount of time. Carburetors are generally a pain in the ass to get at and to remove, and jetting is not an exact science to boot. It might take several attempts before the bike runs really well.

Is this what they call a Jet Kit?

No, a jet kit is an entire kit that changes how the carburetor performs. The main jets are changed along with the needles and slide springs. Air holes are drilled out while others have restricters installed. Some kits will come with a modified performance air filter as well. These kits run upwards from about $160.00 each and you may need to have the bike run on a Dyno Machine to find out where the performance lapses, (flat spots) if any may be. That will run you a pretty penny.

I think I'll get a fender sticker that'll read;


Oh my poor ZX-7, now the gas tank is leaking...

Thanks for reading, Sonic.



Lean, too much air or not enough fuel in the mixture.

Rich, too much fuel or not enough air in the mixture.


Quick observations:

Bags of black smoke from the pipe: Too Rich

A white smoke from the pipe: Too Lean

Head pipes turning blue very quickly: Too Lean

Popping and farting from the pipe: Too Lean, possible air leak in the intake system




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