Fuel cap is venting well, float bowl vents are clear.
At warm idle, plugs are on the rich side of OK. Ran the bike under what feels like lean condition for approximately 20 seconds (to the utter delight of my neighbors) and cut ignition. When I took the plugs out they were golden brown with very clean insulators, in line with TwoTired's statement regarding fuel starvation. I'm inclined to believe that's what the problem is here but I'm trying to take the approach of examining all possible causes rather than picking what I think the problem is and fixating on finding evidence to support that.
Will try to find a solution to measure fuel volume in each float bowl fairly precisely as I do not have the means to attach a clear tube to the float bowl drain other than just jamming the tube into the hole and hoping it seals. If that's generally considered a good way to do this, then maybe I'll run to Farm & Fleet for some vinyl tubing.
Unaware of what brand my points and condensers are. I replaced them shortly after I got the bike because the previous owner said that he was unable to adjust them due to having stripped every last screw under the timing cover. Extracted them all and replaced them with Allen head bolts and replaced the points and condensers with the ignition service kit from 4into1 which did not provide a brand name. It wasn't expensive, so I'm not going to be heartbroken if I have to replace it.
PO said that the bike was known to have a weak charging system, and for most of my early testing I was riding around town and spending a lot of time below 3,500RPM where the charging system begins to function. It was only recently that I started doing longer rides up around 5-6000RPM for any significant length of time. However, the issue materialized after that. In fact, it happened while I was on the expressway. The only thing I know of that can create a sudden low voltage situation under those circumstances without a relatively immediate total failure, is bad connections. I will check voltage, but I will also and clean the terminals as I expect battery voltage to be in a normal range.
The fact that the bike pulls like a rocket ship at WOT for a good few seconds before the issue occurs suggests this is probably not an ignition issue, compounded by the fact that the next time I try, the time before it occurs seems to directly correlate with how gently I treat the bike in between sprints. If I let it idle or even shut it off and let it sit for a moment, then it's good for a lot longer. If I back off, coast for a few seconds, then crack the throttle again immediately, it hits the lean condition a lot quicker. My experience working on antique Pontiacs tells me that an intermittent electrical problem would be far more random, or potentially tied to heat levels or sudden mechanical shock such as hitting a pothole.
As far as the air filter, I think that is going the opposite direction. It seems to be getting too little fuel, rather than too little air. The filter appears to be a UNI foam filter that has seen better days, as the foam seal that pinches against the airbox is falling off. I thought about just making a fresh one out of some thin closed cell foam or Neoprene and attaching it with RTV. Might end up costing more than a new filter to go that route, but if it keeps me from having to replace it again in the future, it might be worth it. The UNI filters are supposed to be lifetime filters, you just wash them and oil them at regular intervals.
I am not using external filters, just what's left of the one in the tank. I have a fresh petcock/filter on order because the filter screen on this one is in bad shape.
Regarding the emulsifier tube: I probably should have cleaned those individually too, yes, but that doesn't appear to be related to this issue. The purpose of the emulsifier tube is to pre-atomize fuel as it's coming up out of the float bowl under vacuum, and if that were failing I imagine you'd see overall poor power delivery rather than *awesome* power at WOT for a few seconds and then an immediate change.
Here's a thought.
I have the petcock with the bag style filter over the top right now. The filter is in bad shape. It's torn in a bunch of places. Is it likely that I'm just seeing poor fuel flow from that due to the filter media getting pulled over the pickup tubes? I've got a new petcock on the way anyway because I want an intact filter regardless.
