Topic: 1977 CB550K Rev Hang

[cr0issqnt]

I am so close to being ready to sell my summer project but I have (hopefully) one last hurdle, when I blip the throttle it just stays at that rpm it doesn't go down. I am really not well versed in carburetors but I am pretty sure its to do with them.

[denward17]

Could be a couple of things:

Check the throttle cables/return spring to make sure it is free in it's operation.

Could be the carb sync is out of whack.

Does it idle nicely before you blip the throttle?

[cr0issqnt]

Yes it Idles just fine before the throttle blip.

I adjusted the throttle cable to be looser but before that the rev would hang when I would let go of the throttle but it would go back down when I pushed the throttle all the way off. Ever since loosening it it wont go back down at all.

[kerryb]

Here are some tests to help you decide.  Wave an unlit propane torch near your carb boots to see if idle is affected.  Vacuum leaks can cause a hanging idle.  As was mentioned, synch your carbs. Make sure your ignition timing is spot-on, & if that isn’t it you could have tired old springs in your timing advancer.  That’s what finally fixed my ‘76550f.  There are several posts on here describing all the gory details, like not being able to buy new springs.

[HondaMan]

It sounds like the carbs were not correctly 'bench synced'? On the 77/78 550 I recommend using nothing (i.e., not a 1/8" drill bit), but to set the carbs all to close exactly the same with the idle screw backed off. The #2 carb on most of the PD4x series carbs does not have a slide-sync adjuster, so make sure it is closed, then adjust the other 3 to also be closed. Then they will be better synch'd by default.

The last carbs (PD46x series) are very 'touchy' about this issue.
Are the carbs the PD46A, or PD46B, or PD46C type? The "A" type uses 14.5mm float depth while the "B" and "C" use 12.5mm depth. If the carbs are, for example, "A" type and the floats are set to 12.5mm depth instead of 14.5, it will run a little too rich and will tend to 'hang' at high idle when warmed up.

Also, make sure of the idle-mix screw setting: it should be between 1.25 and 1.75 turns, tending toward 1.5. Hopefully you have the OEM Keihin brass inside the carbs: with aftermarket brass (like from 4into1.com and other non-Honda sellers) will not mix correctly and will cause you to have to figure out your own jetting because theirs is calibrated differently: the size numbers do not match up with Keihin's numbers.

[cr0issqnt]

Tomorrow I will keep investigating the simple stuff with the throttle cables but if nothing succeeds I will go and check the carb syncs because that is something I have not fiddled with before.
I am not sure on the type of carbs I have but the only experience I have with this problem is just short stationary startups so it could very well be the problem with the type A carbs.

[Deltarider]

You didn't specify your model, but what you describe is typical for a K3, which model had carbs designed with a lean mixture. In spite of all the posts about sloppy spring, I never had them on my advancers (both TEC). That 'heat annealing' I never quite understood. Temperatures are too low there, is my estimation.  What I did have, was some hardened grease inside the advancer that hampered the return movement. Cleaned it, reassembled with only very little machine oil and no grease at all. Still good after decades.

[cr0issqnt]

I do have a K3 so that very well could be.

I also have a related question I am checking my ignition timing and the continuity on my multimeter for cylinder 1-4 goes off when the rear timing plate is at the right position for cylinder 2-3 could that be the problem or was something put on upside down.

[HondaMan]

I do have a K3 so that very well could be.

I also have a related question I am checking my ignition timing and the continuity on my multimeter for cylinder 1-4 goes off when the rear timing plate is at the right position for cylinder 2-3 could that be the problem or was something put on upside down.

I don't quite understand your explanation, but if it is that the 1-4 points are closing when the 2-3 points are opening, that is correct, and normal. Each set of points gets 180 degrees of closed dwell to charge up it related coil.

[cr0issqnt]

You're right I didn't explain it very clearly looking back. What I meant was I was using the equivalent of a test light and when the timing marks lined up for the 2-3 cylinders the 1-4 cylinder gap was closed.

[HondaMan]

You're right I didn't explain it very clearly looking back. What I meant was I was using the equivalent of a test light and when the timing marks lined up for the 2-3 cylinders the 1-4 cylinder gap was closed.

That's normal: only one set of points is open (or closed) at a time on these bikes. That way, it uses a steady current from the alternator despite running 2 sets of points.

[M 750K6]

The high voltage and spark at the spark plugs occurs the instant the points open. With your test light you're checking when that happens. Should be on the 'F' mark.

The manual is helpful. The order I do things:
- plugs out, 3rd or 4th gear engaged and rotating the back wheel to rotate the crank
- check points faces are clean and undamaged
- at widest point opening gap, adjust the points gap to the manual spec. Obviously, do this for both in turn.
- on 1 and 2 first check timing is on 'F', adjust the main plate until it is then lock it down. Double check by going a further rotation
- on 3 and 4 do the same and lock it down.

I don't like to leave the ignition live for long periods, to stop the coil overheating,  so I turn it off at the ignition until I'm ready for each timing check. Don't know if that's necessary, but makes me comfortable.

You can then run the bike in a dark garage, to check no serious arcing at either set of points. Just a crude check of your condenser.