Topic: CB550 suddenly poor idle, low speed bog?

[JLeather]

Rode my 550 to work a couple times this week, which is ~20 miles each way.  Did fine for the first trip, but on the second day the trip out went great, but on the way home about 10 minutes into the ride it suddenly developed a terrible low-RPM bog and poor idle.  Above ~3,500 rpm the bike runs fine.  In fact when it transitions from the low-speed bog to 3,500 rpms it's like someone flips a switch and the bike just takes off.  At first I thought maybe I had the carbs jetted a little rich for the winter, but the idle screws have no effect on this.  I also replaced the condensers because one looked like it may have been leaking, also no change.  I have double checked the timing to make sure it is in-spec and that the advancer is working properly (with a dynamic timing light).  All seems fine there.  Not quite sure where to go next.  It happened so suddenly I feel it must be electrical?  Also it doesn't seem to be throttle position as much as purely RPM related.  At low RPM's the bog persists up to 3,500 no matter how much throttle I give it, and above 3,500 the bike runs great whether I'm flogging it or puttering along It's still noticeable even when the bike is cold, although it does start and idle a bit better cold/choke than hot.  I don't want to just start shotgunning parts at it, but I'm considering new coils/plug caps as they're original to the bike. Anyone ever been through something similar?  A couple of weeks ago I gave the bike a full tune-up; carb synch, valve lash, points gap and timing, etc.  Bike has been running great since then up til now.

Just to add, the bike is bone-stock except for an oil uni air-filter.  Original exhaust, airbox, etc.  Carbs are stock jetting, except the idle screws are 3-turns out (I guess the uni-filter is a bit less restrictive).  It was running perfectly all the way through the rpm range until this issue suddenly popped up.

[seanbarney41]

Check the battery voltage.

[seanbarney41]

Also check voltage drop at the coils...turn on ignition, check battery voltage, check voltage at coils, the difference is the voltage drop.  Be careful to not take too long doing this test, as leaving the ignition on without the motor running can be bad for ignition components.  Also do this same check with the bike running.

[Beau’s Bar and Garage]

Sounds like dirty has to me. Plugged jet ?

[Pappy]

I would go with debris of some sort in one or more idle circuits as well. Since you are transitioning off the idle circuit into a main circuit as RPM and load go up the engine smooths out as it depends less and less on the idle circuit.

[RickThomas]

+1 on checking the battery voltage, but also check the regulator by verifying the proper voltages at different rpms.

Had the exact same problem several months ago on my '74 CB550. I knew timing was perfect and engine/carbs were tuned properly. The bike would run great, but it seemed like it would really struggle at idle after about a half hour of riding. I could stop and after it cooled down, it would be fine until after about 30 minutes again. I initially thought it was overheating since it continued to happen most at idle, but eventually I checked the battery voltage at different RPMs and realized that the regulator was bad. It was not providing the proper voltages. It would give just enough voltage to run properly at cruising rpms, but would slowly drain the battery at idle (the normal voltage drain at idle is significantly less). The battery voltage was able to slightly recover while I let it "cool". This is simple to check and worth avoiding more difficult headaches.

[Deltarider]

... but would slowly drain the battery at idle (the normal voltage drain at idle is significantly less).

But... that's normal. See Fig.284 at the bottom of p.94. http://manuals.sohc4.net/cb500/service_manual/HSM500550_4.pdf
Before any work/modification whatsoever on the electric system, verify you have a good battery. That means: charge it to a 100%.

[JLeather]

It turns out I'm an idiot (or at least occasionally forgetful).  I pulled the tank off last night to start checking coil voltages and found that my #1 and #2 vacuum port screws had fallen out.  #3 and #4 were present, but loose.  I have this trick Proto-tools screwdriver that pinches the screw slots that I use to thread these back in when I sync the carbs, but it isn't used for tightening them.  I guess I threaded them all in but didn't go back and make them tight, and they've been one-by-one vibrating out over the last couple hours of riding.  It's raining so I haven't been able to test ride it, but I'm 99.5% sure this is the cause of all my sudden idle issue!

[RickThomas]

... but would slowly drain the battery at idle (the normal voltage drain at idle is significantly less).

But... that's normal. See Fig.284 at the bottom of p.94. http://manuals.sohc4.net/cb500/service_manual/HSM500550_4.pdf
Before any work/modification whatsoever on the electric system, verify you have a good battery. That means: charge it to a 100%.

Exactly, what I meant is that I could it idle for about three minutes (one light cycle) before I'd have issues. Under the normal drain at idle (especially with LEDs), it should last significantly longer.