Topic: cb550 ressurection hi rpm power loss

[lofijsc]

Hello,

I recently picked up a rotton 78'cb550k out of someones back yard. it was dead for decades. I have some experience with cb's in general so I had a good time getting it ship shape. But I was hoping to get some carb advice from you gurus out there.

It is miraculously running strong with a little popping on the decel but I feel like the power really drops off at hi rpm (6500 or so). I was wondering where I should start.

THANKS

[Stev-o]

First off, welcome.

Have you given it a proper "3000 mile" tuneup?
Checked compression??
Stock air box and exhaust???

I would start there...

[lofijsc]

Thank you, happy to be here.

I have:
-set the points
-adjusted valves
-changed oil
-cleaned carbs
-new plugs
-tested coils and caps
-NOS repacement gas tank/new fuel lines

It has a stock airbox but the exhaust is a modified cb650's 4 to 1.

no compression check and no carb sync.

[Kong]

Did you replace the air cleaner?  Its the only thing missing from your list and if there was a mouse nest in it that might restrict airflow at high RPM a good bit.  Another possibility, and kind of a long shot, but possibly the centrifugal advance weights are stuck.  I just put a Dyna ignition system on my aged 550 last week and while doing so found that mine were frozen solid.

[Tom in Newcastle......Ontario]

Hey lofijsc........welcome ......where are you located

My 78 550 k was cutting out in the higher revs when I was pushing hard.....I replaced the condensers with new ones purchased on E-bay(Cheap Daichi), lubed the advancer and re-set the points.........luck of the draw I don't know but the cutting out stopped,

[lofijsc]

@Kong- The airfilter is free of debris, but I did cheap out and not buy a new one. That could be the culprit. Can the filters be cleaned and re-used? I had a cb750 with a frozen advancer once. It took me hundreds of wasted dollars to figure that out.

@tjjkc- I will put the condensers on my list of suspects. As of right now the bike doesn't cut out. It just feels like the bike's power band drops off way to early. I read in the specs that peak HP was at 8000ish. Not on my bike. After 6000rpm it pulls like pig, so I'm hoping it's a tuning thing and not a geriatric motor. BTW, Thank You I am located in OC, CA

[lone*X]

Since you indicate it runs OK in the low and mid range it sound like most tune up items are acceptable.  Look at the carbs again.  Are you sure you got the main jets all clear?  Visible light thru the jets?  Sounds like clogged mains (high speed jets) to me.  You may want to strip the carbs again and make sure all passages are clean and clear.  Then get them properly synced.

[TwoTired]

The airfilter is free of debris, but I did cheap out and not buy a new one. That could be the culprit.
Can the filters be cleaned and re-used?

The paper filters can be back blown at 3 months and replaced at 6 months.  They can't ever be completely cleaned per design, plus the paper fibers collapse making the air passages smaller through them.  In short, they get too restrictive, deepen the carb throat pressure and draw too much fuel from the carb jets.  They effect is worse at higher air velocities.

If your spark plugs are coated with black soot, it's running too rich.

BTW, the owners manual outlines the filter replacement schedule clearly.

Cheers,