Thanks for all the suggestions, but I'm generally past all of these points and down to either accelerator pump tweaks or a more subtle overall setup problem. I have parts for the pump on the way; as they take about two minutes each to swap it's pretty simple to experiment with a larger squirter and then one cam profile and another. No fix, I go back to the .031s and the pink cam and look elsewhere.
For the record:
- Stock, out of the box carb.
- Primary jets dropped two sizes (76-74); will do more tweaking later if it seems necessary.
- Pink follower cam (stock), 0.031 squirters (stock)
- Full bench prep done including butterfly adjustments and accel pump follower adjustment.
- Full baseline idle mix adjustment made using vacuum gauge.
- Idle-Eze tweaked a little, to no effect; returned to base setting.
- Ignition timing checked; 15* initial gives 38* total at about 3000 (builder mark on damper for total advance used).
Oh, yes, I did tweak the float levels to a little on the high side as well.
The engine is running beautifully - perhaps a little off optimum, but not by shade-tree standards. But it maintains a little bog off idle and my current suspect is the squirt system. I'll know in a few days.
I don't want to waste time and money on a dyno tune until spring, for a number of reasons. I'll take my personal best with no obvious flat spots for now. Once I have this bog solved, I need to tweak the vacuum advance - the engine is lower vacuum than the stock spring assumes and I need to get into the secondaries before 4000.
No one quite addressed my original question (other than to confirm that a 50cc pump is probably unnecessary)... is it likely to be squirt size or timing, or both?