® The Needle Jet

When a dealer tech I had to learn by experience this interesting fact: many carburetors' rich symptoms are caused by worn needle jets. A classic situation is when the bike works fine but stalls at traffic stops. Especially if the stall is immediately preceeded by a gradual decline in engine rpm. In the absence of an improperly adjusted pilot screw, this is almost surely a needle jet issue. The engine basically floods out when the throttle is shut completely. A less common indication is when all the engine adjustments are right, bike actually runs very well, air filter and intake are squared away, carbs clean and perfectly set up, ignition properly tended to, all of that, and yet the bike soots its plugs. In both cases, the needle jet often is what needs to be replaced.

The needle jet is the longish brass tube that sticks up into the carb bore. The jet needle is the spikey thing attached to the slide that strokes into the needle jet. Almost the same terms, needle jet and jet needle. I used to tell my students the last word in each term is the description, the first is the location. In other words, the needle jet partners with the jet needle, but is itself a jet. Conversely, the jet needle partners with the jet but is itself a needle. Make sense? Honda used to sell these together as one part number, not because they are a matched set as many suppose, but because Honda's contract with Keihin prescribed that level of spare parts availability. Just like the situation with vacuum tops and slides. Economics, in other words, not technology.

There are two kinds of bikes that exhibit the most wear of the needle jet: smaller machines, and machines whose carburetors were designed and manufactured prior to 1978 for the U.S. market. Smaller bikes' carburetors, because of the machines' low power, get more throttle use, thus see the most slide movement up and down, the jet needle "sawing" away at its needle jet all the while. Bikes whose U.S.-spec carbs were made before 1978 have needle jets and jet needles made of brass, materials not as resistant to wear as the berryllium and stainless steel that replaced them after the U.S. emissions era began in 1978.

The jet needle may contain the most careful engineering of any part in the carburetor. There is a reason a given model carburetor will contain a different needle in almost every year of its production. They are that highly developed. Consequently, the aftermarket, specifically carb rebuild kit suppliers, cannot hope to match this level of sophistication. The result is they make do. And not very adequately at that. The very poorest quality part in any rebuild kit, the one that will do the most to ruin your carburetor, is the jet needle. I have been comparing them to OEM for decades. They have never measured up. I have said this before, but I view replacing the OEM jet needle with a kit jet needle as carburetor "abuse" of the worst kind. And guess what carburetor part has long been discontinued by Honda. Unavailable.

In the old days we used to ascribe to the needle jet and jet needle pretty specific ranges of operation, i.e. 1/4 to 3/4 throttle. With the advent of CV carbs throttle opening is less relevant and traditional mapping became less definite. However, in any carb having a needle jet and jet needle they still control the bulk of the carburetor's fuel delivery.

The needle jet and jet needle. The heart of the carburetor and the ultimate challenge for the rebuilder.

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