By David Porter, Customer and Technical Support at The Bonneville Shop
Table of Contents
Background
On May 1, 1969, Triumph published Service Bulletin Number 10-69, pertaining to the Amal Concentric carbs. The bulletin, which appears to have been influenced by the West Coast Triumph distributor, Johnson Motors, explains the changes made to the late 1968 and 1969 Triumph models to enhance engine performance in the 1968 models which were equipped with the first iteration of the Amal Concentric carburetors.
“The new carburetors offered a simpler design, with fewer parts and bottom-mounted float bowls, which effectively cured the long-standing problem of leaking float bowls that plagued the previous Monobloc carburetors.”
Service Bulletin 10-69
Late in the 1967 model year Triumph introduced the new Amal Concentrics as original equipment on some models, and for the 1968 model year all models in the range featured Concentrics. The new carburetors offered a simpler design, with fewer parts and bottom-mounted float bowls, which effectively cured the long-standing problem of leaking float bowls that plagued the previous Monobloc carburetors. The first generation 900 series Concentrics featured a removable pilot jet and angle-cut spray tube. They were specified with a short jet holder (622/080), shorter needle (928/063 identified by three annular rings), and non-cross drilled needle jet (622/079).
Amal Concentric Carbs – Parts
Other than the pilot jet, these were all two-stroke components that led to tuning difficulties and rich running conditions at lower throttle positions. These early Concentrics were essentially a two-stroke carburetor supplied for use on a four-stroke engine. The testing that ensued found that a longer jet holder (622/128), longer needle (622/124 identified by two annular rings), and a cross-drilled needle jet (622/122) all contributed to an improved four-stroke carburetor.
Amal actually had an update kit, 622/235, that was comprised of the four-stroke jet holder, needle, and needle jet, and the late production 1968 models were specified with these parts. Incidentally, the removeable pilot jet was replaced with a pressed-in pilot jet and a 622/074 flat-cut spray tube replaced the previous 622/075 angle-cut spray tube.
“If you have a 1968 Triumph twin, or any other British motorcycle from this time period, and are experiencing poor low speed running conditions that cannot be corrected using routine tuning practices, it may be due to excessive wear on a 50-year old carburetor, and the carburetor(s) may need to be replaced”
If you have a 1968 Triumph twin, or any other British motorcycle from this time period, and are experiencing poor low speed running conditions that cannot be corrected using routine tuning practices, it may be due to excessive wear on 50-year old Amal concentric carbs, and the carburetor(s) may need to be replaced. However, before you bin the carburetor(s), you may want to inspect the jet holder, needle jet, and needle to confirm you are running the proper four-stroke components.