1981 Suzuki GS650G - dies under load when throttled past ~1/3

[seinwave]

Redid the valve clearances (took me much longer than I want to admit) and reseated the airbox boots a tiny bit better, along with a couple other small things I'm probably forgetting. That work yielded... a refusal to rev above quarter-to-fifth-throttle. Which I'm actually counting as progress vs the prior state, because now I've got something concrete to look in the carbs for. Proper cleaning will still be a couple weeks off as I wait for parts to arrive but I'll probably drop the bowls and see what the pilot and main jets look like from underneath (as well as the floats).

[seinwave]

Good news: nothing wrong with the coils, and I'm getting consistent readings on the coils. The installed caps were 10k ohm and at least one of them seemed to be dead - couldn't read any resistance/continuity over it. I installed 5k caps and am now getting a good 23k. I do think there *was* some sort of ignition issue as does runs a little smoother now. I double-checked the coils and they're reading good on the primary; no visible damage.

Bad news: near-zero change in the main issue: still can't put it under any load without it dying. Spent hours and hours today with nothing to show for it apart from finding the crankcase breather hose was disconnected (no change).

Stock airbox and air filter is installed (and filter is clean and lightly oiled) but very close inspection makes it seem like the airbox is twisted, such that #3 and #4 carbs are a little loose or off-kilter in the airbox->carb connectors. It's actually a pretty significant amount. I'm about to try and go down and reseat them and maybe that'll help.

Changed the plugs: the ones in there were dry-fouled black in less than 200 miles, which bodes poorly for my hopes of it being an airleak. #1 and #2 were not just black but it seemed like there was oil on the first few threads of the plug.

There seemed to be a tiny bit of... something leaking or seeped from #1 and #2 carbs, just around the drain bolt. #2 a little more than #1. I've heard that a bad petcock could foul cylinder #2 - could that be the cause? I almost want to say there was oil in it; it wiped off much 'orange' and thicker than I'd expect gas. I recently did an oil change and overfilled a little - but oil shouldn't make it into the carbs with overfilling [right?] and I might be paranoid. There's no big "I was riding it and suddenly it made a bad noise and slowed down" event that could point to serious engine damage - this all happened after sitting for 2 weeks while I got another part sorted. If it persists I'll get a compression tester but I'd be shocked if that were the case. (edit: after further reading seems like this and the oil on the plugs is likely to be from the blowby gas recycling via the breather hose I just reconnected, rather than some horrible internal engine damage - if it is not in fact just old gas turning yellow on/in the carb). I'm going to check the valve clearances again just to be sure. I have put off the carb cleaning but this seems more severe than some rough running and I'd expect carb issues to manifest in neutral to some degree as well as in gear.

A little more background: Prior state was: difficult to start, ran fine. I say difficult instead of hard because it was like you just had to mess with a) the petcock setting [vacuum issue?] b) the tank cap [maybe a clogged passage there?] c) the choke lever for a given amount of time - there wasn't *a* thing you had to do, just play with it - and then it'd come to life without much issue the next time you pressed the starter. I figured this was a fueling issue.

Now: easy to start (first stab of the starter), idles alright... no power.

Further details and thoughts as I find them:
- Removing air filter has essentially no effect on idle/running etc. That makes me think there's a leak upstream of the filter - but that doesn't seem to jive with the seemingly-rich running I'm dealing with otherwise. Haven't been able to identify the source of that leak yet, will dig in further.
- #1 and #2 having a tiny leak makes my mind goes to a stuck float -- but I've had that happen to me before (on the 450) and it was nothing like this. Seeped from the bowl-body interface (not the drain screw), took months to go from a seep to a slow drip after I turned the bike off to full-on overflow, etc.

Questions
- Am I wrong in thinking that most carb issues would show up somewhat in neutral (no load) as well as under load?
- Currently thinking it's a partially clogged petcock (internally or in the filter) that lets enough fuel in for idle revving but not enough for proper power; got a repro in the mail. Does that seem likely?

[seinwave]

I’d get new plug caps …..remove old ones and snip off about 3/8 inch off plug wires - this gets you fresh copper for the new caps to bite into. With 5k ngk caps the overall resistance cap to cap will drop to about 23kohm.

this is a good time to check your mechanical advance on your signal generator setup….

edit: ignition coils are very durable, but look for obvious breakage at wire/coil joint

The mechanical advance was the first thing I checked when it was having trouble at 6k - it was in perfect order, which was unfortunate because I was SURE it'd be seized and I'd have found the root cause.

I don't think the stock caps or recommended plugs are resistor - are you suggesting I change to resistor caps and plugs? scratch that, looks as if the OEM caps were resistor, not the plugs.

EDIT: the more I think about it the more your diagnosis of it being the caps makes sense. To get the 33k reading in the first place takes a lot of what can best be described as "*@%^$*@%^$*@%^$*@%^$ing around" with the multimeter and caps; sticking it and out and moving it around inside the cap. I do the same on the 2-3 coil for a while and yet get nothing. I figured this'd be due to something with the coil themselves but seems simpler and likelier to be a bad/intermittent connection between cap and wire. I'll investigate the coil connections themselves next time I'm down there.

[tom203]

I’d get new plug caps …..remove old ones and snip off about 3/8 inch off plug wires - this gets you fresh copper for the new caps to bite into. With 5k ngk caps the overall resistance cap to cap will drop to about 23kohm.

this is a good time to check your mechanical advance on your signal generator setup….

edit: ignition coils are very durable, but look for obvious breakage at wire/coil joint

[Nessism]

Is the airbox installed?

[seinwave]

My bike had trouble pulling above 6k - which was seemingly fixed for a while with new plugs - and now sputters and dies under throttle with nearly any load (it can rev to the moon in neutral, or with the clutch pulled in). Mercifully the aftermarket reg/rec and stator from the previous owner seem to be functional, but as I chase down the issue I'm increasingly suspicious of at least one of the ignition coils. the 1-4 coil I read 33k ohms over the plugs, but for the life of me I can't get any reading whatsoever from the 2-3 coil.

In shopping for replacement coils, am I right that I need to match the following:
- dual-plug/single fire
- 3-5 ohms over the primary coil
or do I need to also worry about the resistance of the secondary coil? Most coils I see for sale don't list this. I'll be sticking with the current ignition system until and unless I absolutely positively have to change.