[wilf]

Ah - you got to play whilst I was still at work! Great.

I suspect you may still be rich in both modes - going to the stock step up springs has delayed the transition to a lower vacuum level - i.e. wider throttle opening.

The stumble on opening the throttle - if it is the accelerator pump giving too much fuel, it should be a very transitory event, so going to the smallest shot should give you an instant feel for whether this is down to the accelerator pump, or an issue with the overall mixture. Are you talking about going to full throttle suddenly or just rolling the throttle open to about 1/2 to 2/3rds fully open?

I would also repeat my advice to get a vacuum guage hooked up - without that you just can't troubleshoot an Eddy carb properly, since it tells you when the move to power mode takes place (at the inches WG of the step up spring rating). You will be surprised at how much throtle it takes to do this, more so with any revs on. You should also see about 1" WG at WOT and max revs - any less and the carb is too big (could contribute to your current stumble as well) any more and it is too small. If any combination of throttle opening and revs drops the vacuum below 1"WG, no amount of fiddling with the mixtures will help - basicallly the carb cannot meter properly in that condition - but will resume working as the revs (and vacuum) build up.

I believe you should get WOT mixture set up on a rolling road before you start in on the cruise mixture adjustments. That is the most critical (from an engine safety perspective) setting to get right first. Once that is done you can just keep on leaning out the cruise mixture untill you get a flat spot, then back a stage and it's done!

Lastly - if you disconnect your vac advance, does that affect your flat spot?