Bing Model 62/32/9-10 CV Carburetors

Ready References


From: Tom Cutter 
Subject: How to Adjust your Carburetors WAS: High Idle Diagnosis and repair
Date: February 25, 2012 8:57:40 AM EST
To: ... airheads list 


I hear this complaint most often in spring and fall when daily 
temperatures fluctuate most.

Here's how to diagnose whether it is a carb or ignition problem: Take 
the motorcycle out for a brisk ride of at least 10 miles, to fully 
heat-soak the engine and gearbox to operating temperature. If you don't 
do this step, don't touch any settings. Revving the bike in garage, 
basement, driveway or convent will NOT work, riding works.  Do it.

Return to your work area, leave engine running. Put the bike in fifth 
gear, SOLIDLY hold both brakes and SLOWLY release the clutch JUST until 
the engine "bogs" down to 500-700 RPM or just on the verge of stalling. 
Pull in the clutch and shift to neutral. Does the RPM go back up to 
2200, or does it now stay low?

If the RPM drops down and stays down until the next time you rev the 
motor,  you most likely have a sticking advance issue or incorrect 
ignition timing. Diagnose and repair the ignition advance system. 
Check/Set the idle timing at 6 degrees BTDC.

If the RPM drops down and slowly rises back to 2200 RPM as you let it 
sit, you probably have an intake air leak of carburetor 
setup/adjustment issue.

Shut off engine and get ready for some methodical, planned process work

Before you attack a carburetor problem, make sure the engine is good. 
Check and adjust valve clearances, check leakdown, verify plug 
condition. Check the ignition system for cracked coils, damaged spark 
wires and caps, (Test them)  Once your motor is known to be good and 
ready to run well, THEN look to the carburetors.

You need to start with a BASELINE SETTING, especially if you have just 
done carburetor repair work:

-All jetting and needle position should be in OEM-recommended 

-All screws (cap and choke housing) must be installed an secure.

-The choke housing gasket must be in good condition and fully 

-Carb diaphragms must be intact and pliable, correctly aligned in the 
dome and bore.

-The throttle butterfly must be properly centered in the carb throat. 
Back off the idle speed screw and look through the carb at a bright 
light to see any gaps or misalignment.)

- Float adjusted so that it totally shuts fuel flow when the seam on 
the side fo the float is parallel  to the gasket surface.

-Idle speed screw tuned down against the butterfly actuating lever 
until FIRST contact, then one full turn more (opens the butterfly a 
tiny bit.)

-Idle mixture screws lightly seated in, then back out 3/4 turns. (THIS 
IS NOT "the setting" for the mixture screws, it is a base position from 
which the bike will run long enough to make a careful dynamic 

Carburetor adjustment and synchronization:

-take another ten mile test ride ("Sorry, honey, I can't go flatware 
shopping with you, Cutter says I have to take a ten mile ride." You can 
thank me later,)

-Put fans blowing on each cylinder head.

-Shut off engine and set up your spokes for cylinder shorting.

-Re-start engine, allow 30-45 seconds for the motor to stabilize.

-Do a preliminary readjust of the throttle cables (allow 2-3 mm free 
play in each cable temporarily during this process, adjust choke cable 
so that it CLOSES the arm fully.)

-Then set the idle speed and mixture. Go back and forth: Speed/mixture. 
Speed/mixture, until you have the correct mixture (by ear, strong 
running) at the desired single-cylinder speedof 600-650 RPM with the 
opposite cylinder shorted out using the spoke-shorting method.

-Then turn the mixture screw counterclockwise 1/16-1/8 turn. (This 
allows cleaner off-idle pull.)

-Balance your idle speed screws to achieve balanced 1000-1100 RPM. Use 
your spoke-shorting method to verify that both cylinders are working 
the same.

-Lastly, set the throttle cable free-play back to 1 mm (0.40") again 
(It changes as you move the idle butterfly screws.)

-Make sure the cables are relaxed and seated in the adjusters.

-Check cable synchronization at 2200 RPM, shorting each cylinder and 
LISTENING to the exhaust cadence and note (lean is "ringy" rich is a 
duller "thump, thump".

-Adjust The slower cylinder to meet the faster until shorting either 
cylinder at 2200 RPM drops the other cylinder to the same speed. You 
can cheat and look at your tachometer, but that will not give as 
accurate reading as your own ear. LISTEN to the beat of the engine. You 
can easily hear a 50 RPM difference once you listen.

-Check and recheck the synch from pulling off idle, on that initial
cable take-up, because that point will most affect your riding
pleasure. Check the synch at 4000 RPM (do this fast, it makes a lot of
heat) If your throttle cables are in good condition, the off-idle and
4K settings should be the same. If necessary, fudge the adjuster a
bit, to get both settings pretty close.

Take your bike for a ride, come home and touch-up the idle if it
changed after a full cycle of thermal normalization. (That is what you
are doing when you ride ten miles. "Dear, I need to get some thermal
normalization for my bike, See you just before cocktail hour. Put on
that little French maid costume, I might be really happy when I get
home wink wink.")

Tom Cutter
Yardley, PA
"Answering questions, questioning answers.”