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Worn needle jet orifice?
Titanium, Stainless Steel or Aluminum

OK - now that a manufacturer has actually produced a service bulletin that says that jet kits cause needle jet wear, there needs to be some clarification about needle material and needle jet wear -

  1. All needle jets wear while being used, at varying rates.

  2. Needle material and surface finish  DO affect the needle jet wear rate.

  3. Needle Jet material and surface coating affect the wear rate.

    1. Unplated brass wears quicker than nickel plated brass.

    2. Stainless Steel needle jets KILL aluminum and titanium needles (yes, we tried it!)

    3. Sadly, titanium is a quirky material material to use for needle jets, as you have to be careful about what needle material you  - or - somebody else, down the road, installs. A stainless needle in a titanium needle jet will hollow out the ti orifice in a few 100 miles.

  4. Riding habits affect wear rate.

  5. Different engines, used in different ways, have higher and lower rates of needle jet wear.

     Large bore singles and twins, used at idle and low rpms, will wear out needle jets quicker than the same engines used on highways at higher rpms.

    Small bore engines, used at higher, smoother, more even intake pressure rpms, will perhaps never have enough wear to cause a problem.

    Engines with large intake tract pressure fluctuations (like big bores at in town cruise operation) rattle the slides back and forth and bang the needle back and forth in the needle jet - causing accelerated front to rear direction needle jet wear - Almost as if someone was using a little ball peen hammer on the needle jet.

    1. Examples:

      1. Some "Eurosports" with 38mm downdraft carbs (unplated brass nj) wear quickly, with or without a jet kit - in as little as 5000 miles of low rpm, in-town type of operation. The same models, operated at higher operating rpm's, commonly go 2 to 4 times as long before nj replacement is required. Yamaha TDM/TRX 850's use the same type carb and, the same holds true with them.
        That's common internet knowledge - and we make replacement needle jets for the 38 downdrafts because of the wear issues. Actually, when rejetting for the first time, most people purchase new "tubes". Click here to buy

      2. Some 1200 - 1500 twins have very large cylinders and very high pressure fluctuations at idle - rattling the slides back and forth at low rpm. Evidence? Check out the wear on the "slide guide" in the carb when the slide is inserted fully down in the idle and low rpm cruise - if the slide is full seated, there is a lot of for and aft play in the slide / slide guide area. Some people have even resorted to "fixing" the slide guide to put the slide back to a "new amount" of rattle.
        You can look at the wear - it's "for and aft" - exactly in the same direction as the slide rattles....

      3. Quads - especially those use in farming and ranching applications, have a large "appetite" for needle jets (emulsion tubes), too.
        They are commonly left idling or just about idle while being used in spraying and other utility operations. Most "savvy" mechanics know that replacing the "emulsion tubes" will fix plug fouling problems resulting from overrich low rpm conditions.

      4. UK sportbikes with 36mm carbs (along with the plethora of 600, 750, 1100 and 1200cc Japanese, German and other bikes that use the same basic 36mm carb with brass needle jets) have always had issues with needle jet wear -

      5. A roughly machined needle IS bad, though!! And we CAN rebuild UK ZX9 carbs that had somebody else's super rough SS needles in it....... Call

So..... If you see a service bulletin that states that jet kits "cause" the needle jets to wear out, take it with the qualification that it is "only a matter or time"....... And it would have happened one way or the other.


Needles and needle jets that are STOCK and WORN -

Hey Marc,
 I've got the new emulsion tubes in and am finalizing the tweaking.  
Wow, what a huge difference this has made so far!  Anyway, I scanned my 
old emulsion tubes and superimposed a circle on them to show the wear 
John Bolhuis
Worn needle jets from 38mm downdraft Mikuni carbs
99 Monster 750


Sent in by:

John Bolhuis
November 15, 2004

all stock needle
TDM 850

37,000 miles
60,000 km

all stock needle
Suzuki gsf750 Katana

28,000 miles
45,000 km

about .3mm wear