I put a new set of throttle shafts into my SU HS-6 carbs this weekend. The old ones were a little bit worn but had always passed the "spray test" (probably because I had nylon washers on the throttle shafts - brass washers were not available).

I got some new shafts and throttle-plate screw from Moss Motors this last week and while I was installing the new shafts, I also installed some new jets from an official SU kit listed for a B18B P1800 (same motor and the same carbs).

The initial result appeared good, but when I went to get everything into proper adjustment ans sync, the mud hit the fan.

The carbs are now nearly un-tunable. With throttle screws completely backed away from the throttle cam, I can barely get the idle down to under 1200 rpms. Leaning the mixture past 9 flats drops the idle to <1000 rpms, but the car runs VERY badly.

I omitted the nylon washers when I installed the new shafts. The carbs now fail the spray test - which seems pretty damning for the throttle shaft bushings.


What bothers me, is that when I removed the "original" jets I found that the original jets had a single groove on the brass jet, while the new SU jet lacks a groove at all. The holes looked to be the same size, but you never know unless you measure.

I know the wrong jets will muck things up, but this appears to be a fairly clear throttle-shaft bushing problem.

Any thoughts?