Topic: 1981 CB650C Carb Tuning Guidance

[kstylian]

Hi All,

Long time lurker, first time post.  Hoping to get some help on setting up these carbs correctly on my 1981 CB650C.

Background:

Engine has ~19K miles.  PO installed new rings prior to selling it to me and never got it to run correctly.  After ordering/rebuilding a set of 1982 CB650 CV carbs (removable pilot jet) the bike starts up, idles and generally runs pretty well.  It pulls strong from low to mid RPM, but feels a bit anemic/sluggish from ~5K up to redline.  It does pull to 9K, but it doesn't have me "death gripping" the bars.  I just feel like it should have more to give.

OEM 4-4 Exhaust.  OEM Air box, cover removed, with K&N filter.  38 Pilots (stock 35) and stock 110 main jets.  Stock needles jets/position (non adjustable) and float heights are spot on (also non adjustable).  Compression 110-120 PSI cold/dry and 120-130 PSI cold/wet (WOT CV carbs).  Elevation 2000 ft.

I'm currently going through the tune up procedure again (value lash, timing, cam chain, carb sync, etc) to make sure all is correct before I start trying to dial in these carbs. 

Pictures below are from the plugs that have been in since initial test/tuning and have about 200 miles on them.  Any thoughts regarding their general appearance?  They do look a bit sooty/rich, but don't know what circuit may be contributing to this.  I know that specific circuits will need to looked at individually via plug chops.

I have a fresh set of plug in now and have another spare set for the initial plug chops (may need more).  What is the general consensus for tuning carb circuits?  Do I work from the main jet down; WOT chop (main jet) --> 1/2 throttle chop (needle) --> 1/4 throttle chop (pilot circuit), or vise versa.

Thanks for any help and I'd like to keep this thread going with progress/questions until things feel/look good.

Cheers!

-Charlie
 
 

[tomkimberly]

When you rebuilt the carbs, did you use Honda parts or an after market kit?

Tom

[kstylian]

When you rebuilt the carbs, did you use Honda parts or an after market kit?

Tom

All jets (pilot and main) are genuine keihin from jetsrus.  Originals were drilled out.  Needle jets, floats and float valves appeared to be in good share so they were cleaned/reused.  Pretty much the only items used from the rebuild kits were the gaskets/O-rings. 

[cb650]

Try new plugs.  Try without a air cleaner.  Is the exhaust blocked somehow? 

[kstylian]

Try new plugs.  Try without a air cleaner.  Is the exhaust blocked somehow? 

New plus are in.  Exhaust doesn't appear to be plugged in any way.  All pipes equally hot, no rattles/noises and seem to have equal exhaust pulse/pressures.  Running better w/o air cleaner would indicate a rich mixture.  Couldn't this be verified more precisely (which carb and circuit) by reading the plugs?

I think I know the answer, but just looking for some confirmation(s).

Going to recheck/set all the tune-up parameters and jump into plug chops, starting with WOT (main jets)?

[tomkimberly]

I never saw any improvement with removing the air cleaner cover.

When was the last time the valves were adjusted, and by who? If the PO, I would redo them myself.

Tm

[kstylian]

I never saw any improvement with removing the air cleaner cover.

When was the last time the valves were adjusted, and by who? If the PO, I would redo them myself.

Tm

The PO did a fine job to factory specs.  I readjusted to Hondaman's recommendations about 200 miles ago.  Will recheck/adjust if needed.  Bike didnt came with the air filter cover.