I hooked up the vacuum gauge this morning and made a couple of tweaks to the sync. I think I moved the #1 and #4 screws about 1/8 turn each, so it was REALLY close off the bench. Then I put about 50 miles on the bike. Overall it ran well. Throttle was smooth and responsive and it pulled decently up to about 7k RPM (I didn't push it any further than that yet). The only thing I noticed is that once the engine was good and hot, it idles at 2k rpm when coming to a stoplight. If I fiddle with the idle screw, it will drop back down to 1100, but it will want to to die or it will die when I come to the next stop. I just left it alone eventually and let the bike idle at 2k when stopped.
Also, somehow my choke is no longer working. It worked last night (bike would fire up and run at 4k rpm for 10-20 seconds). However this morning (before doing the sync) the choke had minimal if any difference on rpm. I can see the linkage is connected, and the lever moves up and down, but its not having any impact!
Also my tach gets stuck at 4k rpms and take a good thump to unstick and start working again. I suppose I need to get that taken apart and cleaned!
The 1978 550K has real "touchy" carbs, and they run lean at idle (for emissions reasons). This, combined with the too-soft springs in the spark advancer, tend to make this particular 550K want to idle high, or not at all, when it is warmed up. Part of this today is caused by modern gasolines, which burn much slower than 1970 fuels, and when some unburned gas wets the intake tubes the heat of the engine vaporizes it and causes the engine's idle to tend to wander about 1500 RPM from your desired point - either up, or down (as, in, stops).
The first "cure" I've always had to apply to this particular 550K3 is the same one I use with the 750K8, as it suffers similar issues: slow down the spark advancer by tightening the springs. This normally involves clipping off at least 1/2 turn form each spring as a start: alternately 1 full turn off 1 spring, which yields the same result. The general idea is to slow down the spark advancer: it is rushing to full advance now around 1500-1600 RPM, which, since it is close to idle openings of the slides, makes the advancer stay there when the idle won't slow down. I can't think of a single 550 that I have seen in the past 15 years that DIDN'T need this fix in order to idle at all here in Colorado, where the high altitude makes this a really big problem for the bike. I watched a nearly new (4000 miles) 550K3 sell for $250 on Craigslist here because the owner noted in the listing: idles at high RPM, have not been able to fix it". That was 5 years ago, I think, and was one of the reasons I started on the 500/550 book.
It's the same story here for the 750K8 and most of the 750K7 bikes I've worked on, too. Even the earlier ones benefit, especially here, from slowing down the advancer to more closely match modern fuel burn rates. My 750 returned to its normal happy low-speed self after I made it reach full advance at 3300 RPM instead of the OEM 2500, which by 2005 had become 1800 RPM with the original springs becoming heat-softened. Now I do it to every SOHC4 that shows up here, and every rider likes it afterward.
The characteristic symptom of too-soft springs is: when cold, it will idle. When hot, if the engine is coaxed down to idle speed, it will idle. If the throttle is blipped open when hot, most of the time it will not settle to idle, but hang around 1500-1800 RPM, depending on which type of gas, carbs, and pipes it has. Real soft springs will make it hang even higher, like 2500 RPM.
