Motorcycle Carb Rejetting: When Is It Necessary?

Cars and motorcycles both emerged in late-19th century Germany. Karl Benz designed and built the Patent Motorwagen around 1885, which was when Gottlieb Daimler and William Maybach created the first gas-powered motorcycle, the Daimler Reitwagen. Both had carbureted engines, employing a mechanical device to mix fuel and air to be fed into combustion chambers. That fuel delivery technology remained the standard for motorcycle manufacturers until Kawasaki introduced fuel injection in 1980 as a $500 option on the Z1000. The 1969 Munch Mammoth was actually the first fuel-injected production motorcycle, but that bike was expensive and made in small quantities. Fuel injection systems employ a computer to measure fuel flow and air intake to meter the mixture of air and fuel, making them better performers than carburetors when temperature and altitude change. Harley-Davidson used fuel injection for the first time in 1995 on the Electra-Glide, which was one of several models to be reimagined later.

The same factors that trigger calculations by fuel injection system computers make adjusting your carburetor necessary in certain circumstances. Often, this can be accomplished by turning a mixture screw or changing the position of your carburetor's float, but sometimes these tweaks still leave your bike sluggish and sputtering. In that case, it's going to be necessary to re-jet your carburetor. You might also need new jets if you make changes to your air intake or exhaust systems, move to a different climate, or if you've neglected routine maintenance like oil and spark plug changes.

Carburetor jets are nozzles that spray fuel into a chamber, where it mixes with air and is sent to the cylinders of your engine through the intake valves. Carburetor jets are sized by number. Larger jets provide more fuel and a richer mixture; smaller jets feed less fuel and make your engine run leaner.

Signs you might need to re-jet aside from poor performance are a bike that runs better as the engine warms up, when the choke is engaged, or when the air cleaner is removed. All of these conditions change the air/fuel mixture, indicating that your engine is running too rich (too much fuel in the mixture) or too lean (too much air). Your engine might also backfire, run hot, or ping (try to burn the fuel before it's ready). 

To check for a wonky mixture, pull the spark plug or plugs. If your engine is running too lean, the plugs will have a light gray powdery coating on the head and electrode. If you're running rich, you'll see black, sooty deposits on the plugs. 

[Featured image by Scheinwerfermann via Wikimedia Commons|Cropped and scaled|Public Domain]

Before you change out your jets for larger or smaller ones, make sure you have fresh spark plugs and a new air filter, and try adjusting your carburetor's float, idle, and mixture screws. If you still can't get the right fuel/air mixture, check your owner's manual for the stock jet size before shopping for new jets. If you can't find this information there, call your dealership's service department and ask for help. Another good source for this information is the factory service manual, which can be found on eBay or through the American Motorcyclist Association.

Motorcycle Shippers has an online calculator to help you figure out what replacement size jets are appropriate, considering your stock jet size and expected environmental conditions. Replacing the jets involves draining and removing or disassembling the carburetor. The aforementioned factory service manual will provide you with step-by-step instructions for your particular motorcycle, eliminating guesswork and keeping you on the right path. Online forums can also be of help, but be cautious when using any technical information that doesn't come from a professional.