Despair sets in.
 

Okay, so the consensus at the local shop (actually a bike junkyard, but the service manager has a Superhawk himself) is that it is in fact carburation. Replacing the black boxes gave me no change.

I have no idea what else to do. I have been through these carbs with a fine tooth comb and I can not find anything wrong.

I can get a set of stock carbs for $175. Thoughts on trying that?

Any other suggestions before I just try to find someone who'll take this depressingly useless bike off my hands for whatever I can manage to get out of it, and write the whole thing off as a several thousand dollar lesson in why I shouldn't try to get back into having a bike again?

New carbs. (My spouse is wonderful.)

Infinitely better than what I started with, at least once I got 'em warm and ran a little gas through 'em.

But still occasionally stumbles (not bad, and only once or twice on a several-mile test ride), and still doesn't like WOT. Both the stumble and the no-WOT still act like fuel starvation.

Petcock diaphragm?

Aha!
 

So, clamping off the vacum line to the petcock while the bike is idling (and thus has good vacuum) and then taking it for a spin that way removes the stumble/WOT problems entirely.

So the petcock is snapping shut any time the vacuum drops down a bit.

Any ideas how to fix that one?

Hmmmm, strange one. I've never heard of this. Have you disassembled the petcock, cleaned, and reassembled? If so, did everything go back together correctly? You have most vacuum at idle, so it sounds like there's a pin hole leak in the diaphragm or the spring is too stiff (if there is a spring). How about a vacuum leak. Bad hose at the petcock maybe?

Just for S&Gs I think I'll take mine apart this afternoon. My tank and plastics are done at the painters but not delivered yet and it's sitting on the workbench. I'm curious....

Part of why I clamped it was to see which direction it's going. It can't be leaking, or clamping it would've meant the vacuum leaked out (well, air leaked in, but you know what I mean) and it would've closed instead of staying open. So instead, I'm thinking something like 'the spring is too stiff'...which is really odd. See any holes in that logic?

However, given (for example) the abuse that had been done to the stock needles in the carbs (I should post a picture, it's quite profound, now that I've seen what they should look like) I wouldn't put it past the previous owner to have replaced the spring with something random they had lying around... *sigh* Especially since I know it was assembled wrong when I bought it, because the first time I took it apart I put it back together exactly the way I found it, and you helped me track down that it was sticking (among the many problems).

I was considering pulling the petcock apart (for the...hrm...third time?) tomorrow and seeing whether I could sort out anything else about it...

Okay, here's what mine looks like. It's never been apart until now...

http://home.nycap.rr.com/ghlbo/Petcock 2.jpg

and the spring....

http://home.nycap.rr.com/ghlbo/Petcock 1.jpg

So I got the bike back from the shop yesterday. The final verdict?

My original carb bodies were screwed up, so the carbs did in fact need replacing.

The fuel petcock was so screwy it was just replaced. He couldn't get it to both not leak and actually flow fuel.

The rear ignition coil was bad and was replaced.

And my bike is much, much, much happier. And he only charged me a little over $200 for the diagnosis and the new petcock and coil, so I was pleased with that as well.

Went for a short ride last night as it was getting dark, and oh does she purr. And pull like a demon. I am so, so, so happy to have this bafflement sorted out. (And a little bit satisfied that it was at least multiple problems that were masking each other, so I don't feel quite so stupid for being unable to resolve it.)

I love a happy ending...

Just like in the Fairy Tales.

The abused and misued damsel is rescued by a caring and patient hero.

Good job :-)

my bike has the exact same problem, but happened right after i ran out of gas, the bike shp did a 1/2 assed cleaning,, what should i do next???

mine does the exact same thing, even down to the clearing up when I pull the choke. I think I will take a gander at the diaphram next.