I've had some (probably very temporary) positive, strange outcomes and I have very little clue why. This is going to be a detailed post with many things that are probably irrelevant because I have no clue what action caused what, and hope laying it all out here will let someone more experienced pick out some diagnoses.
Some background: One of the parts the bike was sitting for was a pair of bar-end mirrors like I had on my 450. To fit those, I had to trim the end off the throttle tube (turning it from a closed cylinder to an open one). I verified that the throttle still turned smoothly, snapped closed, and engaged all the relevant parts on the carbs, so I am (was?) pretty sure that this couldn't have effected the bike's functioning.
Today at lunch: I decided to try and drop the bowls to check out the jet situation. I got through 1 bowl and two drain plugs before I realized I needed a better way to access the inside float screws. I removed the plug on the #1 carb - fuel flowed out as expected. Dropped the bowl. Bowl looked overall clean, but there was a chunk of flat, solid green schmutz rolling around in the bowl. At first it looked like a piece of gasket, but based on the curvature it looks more like gasoline deposits that formed on the inside of the bowl and then broke free. Which means there's probably more of it somewhere in the carb itself. Oh joy. See picture #1 for this. Removed the cap on the pilot screw, sprayed carb cleaner up the pilot and main jet, waited a couple minutes, dried it off with canned air.
Removed the plug on #2. Clean fuel came out. Gave up for aforementioned access reasons.
Reattached the bowl and plugs. No change noted on start-up. I did find out that I could rev to about half-throttle with the choke halfway engaged, but at half-throttle the engine flatlined; continuing to apply throttle would cause it to then die unceremoniously at that point.
After work: Decided to explore WolfTechnic's suggestion re: boots. Both sets of boots seem soft and relatively new, but I thought I'd give it a chance anyway. Sprayed WD40 (don't worry, I wiped) over each boot when the engine was running. No change. Killed the engine.
Here's where I lose the plot a bit. Decided to play around with the idle screw for some reason. I found it very difficult to screw in, but easy to screw out, and once I'd screwed it out a given number of turns it wasn't necessarily easy or possible (or so I thought) to screw it back in the same number of turns. Played around with the idle a little while the engine was running, and eventually somehow screwed it up to the point where it didn't idle. It would start and stay running with throttle (and sometimes with choke), but would just die immediately without any throttle. I couldn't seem to get the idle screw tight enough to give it idle RPM again. I figured the idle screw wasn't contacting the throttle-turny-thingy (the thing the cable hooks into on the carb side)
Opened up the throttle on the handlebar to ensure all that worked. All that worked. Played a little with the slack adjusters on the handlebars. Closed it back up.
Found out that while the throttle *usually* fell into closed position with a nice clunk, there were occasions when I'd wind it in and out that I wouldn't get that little clunk. I couldn't really find a pattern as to why that was.
Took the fuel tank off to get a better look. While I was doing that, turned the petcock to prime to double-check we had flow - fuel spilled out as expected. Turned it back to reg. Found that the idle screw is spring-loaded as expected (though it looks very lumpy/welded/non-standard). Once I could see I wasn't in danger of stripping something out or breaking it, really wailed on it to get it to contact the throttle again. Eventually got it idling again.
Decided to try the engine one more time. To my surprise, I found it would run a little bit better than it did previously - I could get it to 5k/solidly half throttle before I hit the wall I found before, without the choke.
Packed up, ran it one more time in order to test a couple ideas I had. Running normally: got to about 4k (not 5k), same wall. Shut the bike down. Took the gas cap off - a while ago I'd suspected some kind of negative pressure situation in the tank; had ruled it out but wanted to give it another shot. No real change noticeable -
maybe the tinest fraction better. Turned the petcock from reserve to prime - no change whatsoever. Turned the engine off. Shot WD40 into what I presume are the vent holes on the gas cap - see picture 2 - it came out as expected, no dirt/grime/etc. Put the cap back on.
Started the engine one last time. No choke. Found that it jumped to 4k a bit faster - on maybe quarter or 2/5ths throttle instead of 1/2. Once it got to 4k, it flatlined like before - BUT I found that if I kept going with the throttle, instead of dying, it'd keep going after a little bit of that flatline. I saw the tach read 7k for the first time in a while. The 'flatline' seemed to exist from, let's say, 2/5 throttle to 3/5 throttle. Maybe 3.5/5 or 4/5? It was a big flatline, more than I'd likely feel comfortable twisting through on the road.
I fully expect to go down either later tonight or in the morning and find its gone back to its old ways. But this certainly feels like I've picked up some leads. Now if someone could only tell me what that lead is
You've done a great job troubleshooting the issue so far.
Assuming the ignition is really alright, I'd say do a compression check first before you spend hours and hours fixing the carbs. Do a test in cold and warm condition.
If compression tests OK then I'd say there is something wrong with your carbs assuming they get enough fuel. Could be that the filter on your petcock is missing or broken and debris is clogging your carbs. Check floatbowl levels and then check main jet for clogging.
Another candidate for issues are the intake boots from carb to cilinder head. Although they should give issues at idling too when they are faulty, they can cause weird intermediate symptoms. And on these old bikes, if they have never been replaced, I can guarantee you they have cracks and suck in "false air".
Thanks - I feel like I've mostly done a great job of finding new walls to bash my head against, but I suppose that's the same thing.
The intake boots feel pretty fresh and springy. They're also all fastened with new-looking allenhead bolts, which circumstantially leads me to think they're new. I also did do the WD40 test and found no change whatsoever.
Will have to rig up a little floatbowl gauge with some of the tygon I have lying around.
