• SU H6 Carburetor Will Not Decelerate

    I have a 1962 Triumph TR3B with SU H6 carbs. After a recent disassembly, cleaning and reassembly of the carbs the RPM would not return to idle after a load. It continued at an accelerated rate whether under load or at a stop - racing. Only by "goosing" the throttle with the accelerator did the RPM drop, but still not to the proper idle - 600 - 900 RPM (or anywhere close to it).

    I checked that each throttle plate was centered in the barrel, that they closed easily and no "daylight" around the edges of the plate. The carb pistons went up and down easily, dash pots properly filled with the right oil, jet needles centered in the jets and not binding. No vacuum leaks at gaskets.

    After much investigation I discovered that I had two problems: (1) the throttle return springs were not set in high enough torsion and (2) the coupling assemblies on the connecting rod were slipping under load. I could not detect slippage at curb idle by manually manipulating the throttle linkage but under load it seems the connection to #2 carb slipped. This allowed at least #2 carb to remain with an open throttle plate vs. #1 carb. The coupling was just tight enough to respond to "goosing" the accelerator but having slipped it was now out of balance. Also, the shafts are made of brass and over time will score from the connectors, also contributing to possible slippage.

    By torsioning the springs another half turn and tightening the connecting couplings I now have proper carburation. (Note: I also have a slight vacuum leak on #1 at the throttle shaft due to wear. This affects balance and fuel consumption but was not the cause of the "racing" problem)

    I hope this helps someone. I couldn't find a similar discussion anywhere.

    I have attached the Moss plate showing the connectors (66) and the springs (77):

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