Motorcycle Repair/Black Plugs?


Black Plugs
Black Plugs  
QUESTION: I have a 1973 CB750K and it is all stock (air box & honda filter. exhaust with baffles, stock jetting etc)  the plugs have always shown signs of running rich, black soot that is powder like, the bike uses no oil and has never shown any sign of smoke of any kind out the exhaust.

After about 500 to 1000 miles I usually loose cylinder 1 and after cleaning all the plugs (all black) it's good to go again.

Any ideas how to get it to run a little leaner, not too lean as the headers are bright chrome and no signs of blueing.  

Also what direction is leaner on the Air / Fuel screws,  (turn out richer?)



Honda offered several jetting profiles for the carbs, so we need to know what series is stamped on the carb bodies... 657A is one of the calibrations. Main jet sizes range from 105 to 120 depending upon the year/carb series. The float level should be 26mm on all models. Air screw settings are 1 turn out from fully seated on all models.

I would do a basic tune-up... valve adjust, compression check, ignition timing/advance check and then test the spark plug caps for high resistance.. should be about 5k ohms through the cap, unscrewed from the spark plug wire end.

Turning the idle mixture screws out won't affect the plug fouling to any degree. I suspect float level issues, ignition timing retarded, incorrect main jets (could be drilled out, too) or the plug caps. A bad spark plug cap affects both the cylinder it is attached to and the opposite one on that coil... 1-4 and 2-3

Things to consider for future maintenance would be replacing the intake manifold rubbers and checking the spark advancer unit for proper advance/retard function.

Bill Silver

---------- FOLLOW-UP ----------

QUESTION: Carbs are 657A LB,  jets are 40 & 110's  I swapped out the 110's with a 110 set I had and no difference.  Floats set at 26mm,  a/f were set at 3/4 but now 1 turn.  Valves adjusted, timing set with light and advancer is working.  I installed new plug caps about 5 months ago.  Bike has always run rich since I bought it 3 years ago.  Starts runs drives fine,  pulls through all rpms.  Only usability issue is Idle has never been perfect but not bad.  Idles a little worse as it gets hotter.   Let me know what you are thinking.  Thanks

John, you seem to have everything covered so far. I see that the needle clips are supposed to be in #4 slot on the needles. What are your compression readings? Checked the manifolds for any vacuum leaks?

Have the carbs been dipped/cleaned with ultrasonic cleaner? Check the air bleed ports in the inlets for build up of varnish or other crud. The two air bleeds supply air to the idle circuit and main jet circuits. When they get plugged up you start getting excess fuel globs instead of nice metered fuel/air.
The main jet holders are cross-drilled and those little holes need to be open to help emulsify the air/fuel from the main jet. Varnish builds up through evaporation of gasoline in the bowls over time. I have seen gas caps and air bleeds become plugged up/restricted from old fuel deposits.

It is interesting that the K2-3 carbs only have one part number for the slides now, as the early bikes had left and right side specific slides which could be installed backwards, which makes them run REALLY rich at idle and low speeds.

In recent years, I have had to jet Honda Four carbs up to get them to overcome the alcohol in the gasoline which makes them run lean, but so far the ones most affected are the 350-400F models.

Bill Silver  

Motorcycle Repair

All Answers

Answers by Expert:

Ask Experts


Bill Silver


Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.


I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.

VJMC (Vintage Japanese Motorcycle Club) of North America

VJMC newsletter, as editor for two years and as contributing editor currently.

3 years auto shop in high school, teacher's aide in Automotive Technology in Jr. College, Diesel mechanic course in college, self-taught mechanic and automotive writer.

©2017 All rights reserved.