Carburetor Adjustment for Stock EX500s

The following are suggestions for improving the EX500’s carburetor response without purchasing an aftermarket jet kit. These modifications are not street legal and may cause your motorcycle to fail certain U.S. states’ emissions standards. These modifications are not approved by the EPA for use on street legal motorcycles.

General Suggestions:

For stock motorcycles with little or no modifications, idle jet adjustment and jet needle adjustment are very useful in improving the EX500’s performance. Mainjet replacement is not usually necessary with such a stock set-up.

Idle Jet Adjustment:

The stock EX500 CVK34 carburetors have small aluminum caps that cover the idle jet adjustment screw. These screws are set and sealed at the factory at 1¾ turns to pass tough U.S. emissions standards. For most of the world, the stock setting is 2 turns out and are not sealed. If you have a U.S.-spec EX500, you can improve off-idle response by removing these caps and adjusting the screws to richen the mixture.

To remove these aluminum caps, the carburetors must be removed from the motorcycle. Drill a small pilot hole into the center of the caps, making sure that you stop as soon as you breach the cap. If the drill bit goes too far, there is a possibility of damaging the adjustment screws. A pick or other suitable device can then be inserted into this pilot hole and the cap can then be pried off. Turn the adjusting screw clockwise until it lightly seats, then unscrew them counter-clockwise 2 or 2 1/2 turns.

Jet Needle Adjustment:

The jet needles on this constant-vacuum carburetor do not have removable jet needle clips with which to alter the position of the needle. In the past, CV carbs used to have such a feature, but EPA standards have mandated that they come from the factory with a fixed spindle. To richen your mid-range response, shim up these needles by placing small washers underneath the jet needle spindle. The more washers that you use, the more you will enrichen your mid-range. Remember, this adjustment is dependant on the state of tune of your motorcycle and the altitude at which you operate it. After installing a K&N stock-type filter in my airbox, a flat spot developed at 7,000 rpms and I found it necessary to further shim up my jet needles.

Main Jet Replacement:

Since emissions standards are measured more strictly on idle and midrange areas, motorcycles usually come from the factory with sufficiently rich mainjets. Therefore, if you have a stock airbox, filter and exhaust, you may not need to replace your mainjets. If, however, you have a free flowing exhaust and/or filter, larger main jets can be useful. The EX500 comes stock with Keihin 130 main jets and Kawasaki has on its parts books both 132.5 and 135 main jets. These, unfortunately, are relatively expensive and buying several can cost as much as an aftermarket jet kit. If you cannot source inexpensive mainjets, it is probably best to buy a jet kit as they usually include several sets of larger mainjets. Mikuni jets of the same number designation are NOT interchangeable with Keihin jets.