TOPIC:

It seems to me that there is a lot of basic maintenance that should be tended too. This bike is 35 years old.

Valve clearance check?

Properly clean the carbs by removing all the jets/pilot screws, soaking everything in carb dip, reassemble with new O-rings?
zeus.mtsac.edu/~cliff/storage/gs/Mikuni_...Rebuild_Tutorial.pdf

Install proper air filter

Check rubber boots on head and airbox and make sure there are no air leaks? Spraying junk on the airbox/carbs is NOT a proper test, the boots must be check physically to assure there is a proper seal all around.

Vacuum sync the carbs?

Valve clearance has not been checked -- will add that to the list.

Carb rebuild guide looks pretty good, wish someone had pointed me to that three years ago when I actually had the carbs fully disassembled. Gaskets and O-rings appeared to be recently replaced so I didn't mess with them.

Air filter... well yeah no kidding, and I WILL be installing one as soon as I can get it, but that still doesn't answer the question.

The airbox boots are brand new and are sealing up fine. The boots on the manifold side are older but still flexible and not cracked. A visual inspection will only tell you so much, they can look perfectly fine and still not be sealed --thus spraying something like ether around them will tell you if there are gaps too small to see. As mentioned I obviously don't have a good seal around #4 so that one needs to be removed and inspected closer or swapped out with another boot.

Oh by the way, regarding the leakdown test... one good sign (I think) is that even cranking the engine by hand I'm generating pressure in at least some cylinders (you can hear it escaping when I stop turning). Definitely need to check all four though and see where it stands. From what I saw inside the engine I don't expect any leaks around the valves, but most likely will hear some around the rings. Since I've always been standing on the right side of the bike while it was running I can't be certain that no air was being blown out around the head gasket -- although the gasket and rubber bushings are all new.

Another thing that has occurred to me... what about the spark? Several times while I've had it running, it just seems to suddenly die out. Makes me wonder if the coils are too weak? I did an ohm testing across the ones I have (I think it was supposed to read at 3ohms?) and all four coils I have checked out at within 0.1 ohm, but I'm sure there's more to it than that. I dunno, might be worth swapping in the other pair of coils if I don't find any other significant problems.

I'll be home tomorrow morning, hoping to get through a lot of checks, but also need to work on the starter clutch issue. Hopefully it will be a productive day.

Making some good progress but I have to stop for the day. So regarding the leakdown test... well that was a tough one. The air pressure kept moving the pistons. I tried holding a wrench on the bolt but that didn't work either. Perhaps I had too much pressure, the compressor was sitting at about 100psi. Anyway I got the outside pistons checked at TDC once I hit the exact spot, but it appears the center two sitting at BDC have a valve open so I had to spin the crank around until those two were at TDC and try to hit the right spot visually. When I could hit the exact TDC spot for each piston, there didn't seem to be any rapid leakage anywhere. I couldn't hear anything and a lot of pressure escaped when I let up on the air nozzle, so it seems like that is in pretty good shape.

The big discovery was the manifold boots. None of the bolts were really tight, and the top bolt on #4 had about 1/2 turn of play in it. Also that boot did not seem to sit properly, so I grabbed another one and that boot just kinda slipped right into a notch. I snugged down all the bolts and the engine is running noticeably better now, but still not idling. What I mean by 'better' is that I can just about get it to idle at 2500rpm now, but more noticeably is that whenever the engine starts choking itself out I can hit the throttle and actually recover it every time now. I was also able to get it to sort of idle for a few seconds at a time at lower RPMs, once even holding it briefly below 1000, so overall it is responding MUCH better now, and it even feels smoother. I can set the idle screw for about 3000rpm and release the throttle, it sits there just fine running by itself. Then I start to back off the screw and when I hit about 2800rpm it just seems to sputter and die. I'm guessing that may be about the point that the main jets get closed off and without the extra fuel from the idle circuit it just can't keep running?

At this point I think that idle circuit on the carbs should be checked. When I get back home I'll get the carbs pulled and drain the bowls so they can dry out, then see what I can get done in the next couple days. I had no idea there were separate idle jets before seeing that guide above, it'll be interesting to see what kind of crusties are hiding in there.

Slowly but surely, progress is being made...

Unless the carb holders are new they won't seal for SHT even with sealent.
Steve

@SWest -- are you talking about the rubber boots on the manifold or something else?

Had more time than expected laster this afternoon so I pulled the bowls off the carbs and checked all the jet. Hoo boy, I sure screwed up there! Pulled the rubber stoppers over the idle jets and found grit from the sandblaster. Obviously not a good start. Then I pulled all four jets, and every one of them was plugged. I used the air hose to blow out the carbs with both sets of jets removed, then cleaned up the idle jets until I could see the hole clearly in each. Got everything back together and tried it again...

Much much better now, but still not quite there. Once I got the engine back up to temp it idles below 1000rpm now, mostly. I was able to dial down the idle speed screw to just under the 1000 mark and let it run there for several minutes. Then an odd thing happened -- the speed started climbing. I flipped the throttle but it wouldn't slow down, and as it was approaching the 5000 mark I decided to kill the ignition switch. Started it back up and it ran nice and slow for awhile, then did the same thing again. I tried this a few times. There doesn't seem to be any pattern to how long before the speed starts climbing. Sometimes it's a few minutes, sometimes it's within the first minute of restarting it. Initially I was thinking maybe it was leaning out, except I would expect it to just die from fuel starvation by the time the speed got up that fast. A few times I re-adjusted the idle screw while the speed was climbing -- that dropped the speed back down but it wasn't stable and would die soon after. So maybe more grit has come loose in the carbs and is blocking a float valve, causing it to flood out? I guess I could try rocking the bike next time so see if that affects the speed climb. I'm still trying to figure out how the sand got in around the idle jets, obviously I missed taping off something when I was prepping the bodies for paint.

All right, one other item that is probably related... Even when the engine is behaving and sitting nicely at an idle, the right side of the engine and exhaust are quite noticeably hotter than the left side. The exhaust on both sides has a regular rhythm to it so it feels like all four cylinders are firing. If I remember right, excessive heat indicates a lean condition?

Since I'm making good progress towards a stable idle I got the air filter ordered today. Maybe this weekend I can at least stick the vacuum gauge on it and see if the four carbs are anywhere close to similar. I would imagine if they're pretty far out, even a course adjustment to close the gap will be helpful.

I'm talking about the carb holders. The smallest leak can cause idle problems. Carbs out of sync too. One or two can cause the symptoms you are describing.
Steve

OK I had to check google... yep we're talking about the same thing. Carb holders? I never would have thought they would be called that. Ah well...

So yeah, they're definitely not new, but they also seem too flexible to be the original equipment. Sounds like there's not much else I can do with them until I can afford to get some new ones. However as I mentioned, I CAN at least start checking the vacuum between carbs this weekend and try to spot any significant differences and try to adjust against that. Unfortunately that may be about all the free time I have, the wife wants some help getting things together for a garage sale the following weekend. Apparently I've been ignoring her too much lately while working on my other girl...

Have you checked out my Mikuni thread? I used Permatex to seal them against the head until I could get new ones.
Steve

Finally had a chance to read through the carb rebuild doc, and I realized my pilot jets still have the factory caps on them (I believe it's in the risers next to the vacuum lines?). Since I have gone back to the factory airbox should I worry that one of these could be significantly off enough to be causing my idle problems, or should I just leave them sealed? I imagine once everything else is running better I may want to fine-tune them, I'm just wondering if the factory setting might have wandered enough to be considered as a source of the problems?

Also based on that document, it sounds like my runaway RPMs is likely from a lean condition, and I seem to recall that running lean will also cause excessive heat. Can't wait to get the vacuum gauge on the carbs this weekend. I'm thinking if any of them are radically different it could also mean I still have grit in a passage that didn't get cleared out. Considering what I found around the idle jets, if things don't start to clear up after installing the air filter and syncing the vacuum, then I'll do a full tear-down again and re-check all the passages.

I'd do it now.
Steve