Topic: 1976 CB550K - Stumble below 3000 rpm

[Otto]

Finally got a chance to take my 1976 CB550K for a ride today. What a cool bike... Everything went very well except for a stumble below 3000 rpm. It happens off idle to approx 3k, then dissapears and accellerates nice on up to redline, then reoccurs @ approx 3700 rpm down when downshifting to slow down. The carbs were recently rebuilt by the prior owner. Prior to the test drive, I dressed the points that were pitted, reset the points and changed the plugs to some NGK D7ES that I had...... I then syncd the carbs which are dead on. Air screws are out approx 1.5/1.75 turns.

I am thinking I may need new points as the ones in there are not the best... I see lots of points for sale on Ebay, but not sure how to tell which ones to get. I also have a nice points plate with nice looking condensors and points that I just yanked from a 75 -CB750K that only had 11000 miles on it. Points look new. Cannot tell if they are Aftermarket or HONDA OEM. How do I tell?

Am I i on the right track looking at points or does the blame lie in the carbs? 

Sources for points and condensers?

Thanks in advance.

Bob 

[Sporkfly]

It sounds like it could potentially be the carbs. How did the plugs look when you pulled them?

[Otto]

I haven't pulled them since the ride. The ones that were in there when I synd the carbs were dry sooty... Wiped clean. But, had been idling a lot while I was working on it lately.

bc

[TwoTired]

What air filter are you using?

[Otto]

Twotired,

I'd hope you would chime in. I have read many of your posts....and was hoping to get some of your insite/experience on this..

You know, I didn't check the air filter but remember seeing some dark foam in the air filter box. Felt oily/sticky now that you mention it.  I will check that and let you know...

I am thinking that the bike may have ben rejetted, not sure, but there is a Kerker 4-1 on it. This thing sounds really nice! Not loud, just right! The Prior Owner told he that he only got 30mpg too. It should be closer to 50-55 mpg in my opinion.

What air filter should I be using - ? OEM right?

Ever seen this 550? I really like it. I'd use differnt bars though - too old (48) to ride hunched up like that)

http://forums.finalgear.com/post-your-car/my-honda-cb550-bike-restoration-project-19365/page-4/

Bob

[Sporkfly]

The oily filter you mentioned is likely the one for the crankcase breather. You should have a paper element above that one (I really hope the PO didn't get rid of the air filter ). If you haven't changed it personally, you don't know when it was last replaced so replace it.;

[Crusty_Butt]

Get it warmed up, take out the air cleaner and go around the block. If the problem goes away try raising the clip on the jet needles a notch.

I had a cb500 that had aftermarket pipes and discovered the previous owner had raised the needles. It had an off idle stumble like you describe.

Bret

[TwoTired]

I use the Uni NU-4055 Foam filter in the stock air box on most all of my 550s.

They still need to be cleaned and re-oiled periodically.

30 MPG is pretty bad.  S/B 45-50 range with an open low restriction exhaust.  Look for a clogged up filter or poorly adjusted carbs.  You'll likely have to go in and check for too large a main jet, drilled jets, jet needle clip positions and Jet needle orifice wear.  Checking the the emulsion tubes are clean is another check.

Your spark plugs ought to give an indication of too rich with black carbon deposits.  For tuning, you'll need a few spare clean sets (and a test track).

The mechanical slide carbs are not well understood by many.  Some people put in main jets way to large and then lean the mid and pilots to compensate.  Resulting in poor street performance.

Start with the filter.   Then find out what's been done inside your carbs.  Float height is a factor as well as the condition of main jet orings.

Cheers,

[Otto]

Will check it out tomorrow.

What size jets should I have?

Bob

[MrZxp]

It may be that what you're experiencing is related to this:

http://forums.sohc4.net/index.php?topic=7401.msg67251#msg67251

My 550 displays the symptom(s) virtually exactly as described....

Cheers, Phil

[TwoTired]

Will check it out tomorrow.

What size jets should I have?

Bob

Stock was #38 pilots and #100 main.

Cheers,

[Otto]

Ok,

Went out this moirning and pulled the plugs... Sooty. Replaced them.

Pulled the foam air cleaner, it was nasty. Sprayed some brak clean thru it , the hot water. Blew it out with air compressor air nozzle.

Went for a ride, a little better..

Put the carb sync gauges back on - still dead on.

Went for a ride. A little better.

Came home, srewed the air screws all the way in and back out 1 full turn only. Went for a ride.. Better.

All in all better, but still stumbles off the line, need to rev up before releaseing the clutch.

No where near as much stumble on deceleration. Performance is good. Sounds great with the Kerker.

Should I try turning the air screws in a little more.. Seem to help??

Also, still cant idle at 1000 rpm. Have to settle for 1150-1200. Otherwize stalls.

Ideas from here?

Thanks All,

Bob

[TwoTired]

Are all the head pipes equally hot?
Were all the plugs equally sooty?

I think you are going to have to find if/how the carbs have been tampered with.


You do know that you can't snap the throttle wide open for acceleration, right?  You don't have accelerator pumps on this bike. (or fuel injection, or CV carbs)  It should respond with good power using up to 1/2 throttle travel changes.

If your idle is too rich, you can turn the idle air screws out some more.  This won't help much if your jet needles are too rich and your main jet is too big.

Cheers,

[Otto]

Yep,

Gonna have to pull the carbs.... we'll figure it out...

Can't really tell if too rich or too lean at idle though.

Bob

[bender01]

your tach might be that far off too. Mine is.on my 75 550.

[Otto]

Question. If I remove the gas tank, can I remove the tops of the carbs and check the needles without pulling the carbs?

I hope thats not a dumb question...

As an experiment do I go up or down with the needles?

Bob

[bender01]

Yes you can. I dont @ the needles setting or if you have stock exhaust?