Thanks, I'll give one of them a try tomorrow or friday.

Incidently, I removed both HIFs from my motor and took them into my well-lit basement shop. I discovered a few things of interest.

#1 The SU rebuild kit throttle shaft seals are not very good. One was not sealing well at all and the other had a split down one side. Probabaly not helping the whole problem. I put some tight-fitting neoprene O rings on in place, but a couple stacked rings puts extra tension on the throttle shafts...

#2 the throttle plates are very particular about how they are installed. A tiny fraction to one side prevents a good seal and no doubt contributed to my frustrating week.

#3 a jury-rigged return spring is not sufficient to overcome the pressure of the intake and therefore #1 plate never gets close to near-closing so I couldn't possibly get my idle and mix adjusted on the front carb.

I'm hoping one of those two fellows also sells good throttleshaft seals too...