I've been developing this strange bimodal idle on my SU's, predominantly after having the engine at speed for a reasonable length of time ( >2K rpm). When I let off the gas and take it out of gear, the engine does one of two things. The idle either settles down to some high value, like 1250 rpm, or it falls quickly to near 0 rpm and barely hangs on, or it completely dies. If it does die, it starts right up.

The SU's are freshly rebuilt, and I've got them tuned pretty well, about 12 flats. There is no hesitation when pressing on the gas, and all indications are I've got my fuel mixture dialed in. If you let the car idle for an extended period, it will return to a normal idle of about 800 rpm, which is where I've set it with my idle screws.

My only thought is that after being open for a while, the butterflies either close to far, thus killing the engine, or they stick open for a while and then settle closed. After playing with the throttle linkage and movement, everything looks free and clear, however.

Any ideas?