Topic: cb550k running problem - where to start?

[cb550k-boy]

Hi guys.  I have a 1977 cb550k that has a problem and I'm not sure where to start looking. I've had this bike for a couple of years and it's my first bike.  I have no technical/mechanical experience, but am not afraid to tackle something new.  I planned on taking it to the local honda shop but they told me today that it's too old tor them to work on. Bummers! There goes the easy way!  Last summer I had been riding the bike to work daily (about 25 miles of mostly expressway riding) and it seemed to be running fine.  Then, on my way to work traveling 65mph, it slowed down to 55mph and wouldn't go any faster.  Five miles later the top speed was about 45 and when I got to work it wouldn't go faster than 35.  After work, the top speed was again 55 so I decided to stay out of rush hour expressway traffic and head through on city streets.  The problem got progressively worse; 45, 35, 25, 15,  until it wouldn't go at all.  I called a friend and trailered the bike home and parked it. Over the winter, I pulled the gas tank to see if it has any junk or rust in it, looked at the spark plugs and wires and examined the points.  I didn't see anything that appeared broken or worn too badly, but did not "test" anything.  I was able to start the bike and it idled just fine, would rev to 4000rpm without hesitation (but I didn't ride it anywhere). I noticed that the 2 topmost mufflers got very hot, one lower muffler slightly warm and the 4th one stayed cool.   Last weekend, the bike started up just fine so I took it for a ride to a friends house a couple of miles away.  It ran great as far as I could tell.  On the way home after lunch, however, it developed the same symptom that it had last fall.  It happened while I was riding about 35mph and had just started out.  There was a noticeable change in the sound of the engine: kind of a lower pitch to the sound.  Any ideas where to start? Or do I just start at the beginning and keep at it until it's finished?  Should I even be trying to do this on my own? I have NO experience. I can do the simple things, I know. I think I understand how to check and adjust timing but have never done it. I've never taken apart a carburetor.  I have a Clymer's manual for it and basic tools.   Thanks for any advice! 
--Bill  (Rochester, NY)

 

[canam73]

From your description it seems that at least 2 of your cylinders are firing intermittantly at best.  The first thing to do is make sure that each is cylinder is getting the 3 things that it needs to fire: gas, air and spark. 

If it will start and run normal for a while it could be something simple like a clog somewhere in your fuel supply that lets gas trickle slowly into the 2 problem carberators but so they fill up in between rides but cut out after a period of use.  If that is the case cleaning your fuel lines from the carbs back to and including your petcock should get the bike running normal again but you will also need to eliminate the source of the blockage by using inline filters or giving the inside of your tank a cleaning to remove rust and possible coating.

Check the condition of your air filter and clean or replace it if need be.  Also check to see that nothing has blocked the air flow to you carbs.  And finally, learn how to adjust the mixture screw on you carbs.  You may know this, but the factory setting was screwed all the way in and then backed out 1 3/8 turns.  Your bike may need adjustment from that, especially if anything else has been changed from stock.

Now for the spark.  Even if your plugs look good they may not be firing.  Try swapping the plugs in the good cylinders for the others, or just buy, gap and install a new set if it has been a while.  Also make sure that the wires are delivering a charge to each plug.  If one or more is not, you know you have an electrical issue.

All of what I have said above is fairly easy and cheap to do with your bike, and should be checked before you look into anything more expensive and drastic. 

Good luck,
Grendel.

[SteveD CB500F]

The "changed engine note" and cool exhausts sound like you could be losing 1 (or 2) cylinders.  The two top mufflers are #1 and #4 which are fired by the same ignition circuit, so this could be an ignition problem (rather than fuel).

If you've had it a couple of years, it's time to give it a tune-up!  Get your spanners out and read your book again.

Start with the timing, then the tappets and cam chain tension. You might like to change the oil and the plugs as well.

Get back to us with the results.

Steve

[TwoTired]

The bike is supposed to have a tuneup every 3000 miles.   Maintenence schedules were quite different back then compared to modern bikes.  Make adjustments to the carbs last, after all tuneup items have been addressed.   Especially since the clymer doesn't cover your carb type, only the earlier carbs.  So, it will  leave the unknowledgeable bewildered.

As others have said, it is important to know which cylinders are misbehaving.  You can flash touch the head pipe to feel the relative temp.

It is a good idea to check for fuel contamination, as bits of material larger than .016 inch in size can block fuel passageways.  At the bottom of your carbs is a bowl, which is a local reservoir for fuel at eacg carb.  At the bottom is a nipple and tubing that goes to the bottom of the bike.  Temporarily remove the tubing.  Place a shallow, clean, pan under the nipple.  At the side of the carb bowl near the bottom is a drain screw, lefty loosey and fuel should drain into the pan.  (Turn off the gas tank petcock.)  Open the screw a couple turns.  Examine the drainage for bits of stuff larger than .016 inch.  If you find any, you may have a fuel contamination issue.  Check all the carbs.  Or, at least, the cylinders with cold head pipes.

Keep us posted on your progress.

[Tim.]

My advice:

www.mctangen.dk/CB500-550_Manual.pdf

 

[Philly550K1]

take a close look at those 2-3 points (they are thusly marked, or should be).  clean them up in between with a points file or emery board.   maybe run the bike with the points cover off while it's acting up, and see what behavior you observe as you wiggle or lightly pinch those 2-3 points.  riding not suggested during this test.  closed course, professional hack.

set everything else properly, too, of course, but call it a hunch....

-jc

[cb550k-boy]

Wow, thanks for all the quick replies!  I'll be trying all the suggestions this weekend and will let y'all know what I find.   

--Bill