Motorcycle Carb Identification


  • I just bought a Yamaha with down draft type carburetors on it. Is this better than older carbs? Do I need to do anything special for downdraft carbs? {Dan, CO, late 80's Yamaha VMX1200 VMax}


Unless your bike has problems, there isn't anything special you need to do on a bike with down draft carburetors.

If you are experiencing a really rough idle or fouling plugs and adjusting the air mixture screws does not help or makes it worse, you may need to have your carbs re-synchronized at a shop. Especially if you are repeatedly fouling spark plugs on just one or two cylinders.

For those readers who are wondering what a downdraft carb is, this is a good place to cover carb identification.

There are several different carburetor types used on motorcycles and all terrain vehicles. The three most common are mechanical, constant velocity or CV and down draft.

There are also butterfly, updraft and side draft styles. We aren't discussing those last three styles here.

Mechanical Carburetor

The first carb type is mechanical. It is sometimes called cable operated.

Carb identification of a mechanical carburetor is simple. It has a cable hooked to a linkage that pulls the slide barrel up when the throttle is pulled. Generally, mechanical carbs have a heavy spring to snap them shut.

They will be found on 1960 through early 1980s motorcycles, snowmobiles and ATVs. Some will have an additional accelerator pump on one carb.

Constant Velocity Carburetors

The constant velocity or CV carb design uses engine vacuum instead of a linkage. Vacuum from the engine side of the throttle plate is used to lift the slide barrel. There will be a rubber diaphragm on top of the carb.

CV carbs are easily identified by pushing the slide up with your finger. You will hear the air exchange out of the air chamber.

Down Draft Carburetors

Down draft carbs are a style of constant velocity carburetor but are mounted on a downward angle. This type is used commonly on sportbikes.

Carburetor Manufacturers

Most carburetor bodies or housings are stamped with an identification number. The carb brand or manufacturer is usually cast into the metal.

Some of the more popular OEM manufacturers are Hitachi, Mikuni, Zenith and Keihin. There are also many smaller, aftermarket carb manufacturers.

Mikuni has tons of great information for DIY mechanics from hi-res images and detailed descriptions of their carburetor models to tuning manuals. Visit their site.

Search by Vehicle

It is not common, but occasionally a motorcycle manufacturer will change carbs partway through a model year. Service and repair manuals will list one carb for VINs xxxx001 through xxx998 and a second carburetor for xxx999 through xx3000. In this case, carb identification becomes very important.

Some possible reasons for this could be problems with a supplier or problems with the carb design itself, that did not become apparent until production had already begun.

In addition to your bike's year, make and model, you should always bring the VIN with you when ordering parts.

Also, if you know you need jets or a carb rebuid kit, get the number off the carburetor housing too.

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