Bing Model 62/32/9-10 CV Carburetors
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
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
-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
-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.")
"Answering questions, questioning answers.”