Carb Balancing for Beginners
or the things I have learned while mucking around with bikes!
Applies specifically to Suzuki GS motorcycles and, in principle, all motorcycles with multiple carburetors.
[NOTE: I have no association with Morgan Carbtune, other than being a satisfied customer]
The bike must be running reasonably well:
Warm the bike to operating temperature -- a short ride should accomplish this. You may want to have a fan available to help cool the bike while it is stationary.
Set the bike on its centerstand.
Remove gas tank. Find a means to supply gas to the carburetors while the tank is removed. I set the tank on the rear carrier of my GS1100, or bungeed a small gas can to the rear of the seat on the GS400.
You can also make an auxiliary fuel tank from a universal coolant reservior purchased at an auto parts store. Some fuel line, a plastic hose barb, a bit of bent coat hanger wire to hang it from the handgrip, and viola!
Remove the screws in the balance ports on the carbs. On the GS1100, they were allen bolts; on the GS400: phillips head screws. Install the hose barbs in their place.
Note: for twins, replace "cylinder #3" with "right cylinder" and "cylinder #2" with "left cylinder" in the next few paragraphs
Adjust cylinder #3 to best idle. You may have to remove the covers over the idle screws. Drill each carefully with a small diameter bit, and extract by turning a screw a few turns into the hole you bored, and pop it out with a tug on the screw with a pair of pliers.
To ensure that the idle mixture was correct, I used a Colourtune, a sparkplug with a view! You can see the colour of the combustion flame as the plug fires. When you see a bunsen blue flame, the idle is correctly adjusted. When I first tried this, I could not get a blue flame. Once the jets were replaced all was well. The moral is that tuning by ear gets the best idle, but not necessarily the correct idle.
Once the idle mixture is correct, set the idle speed to 1200 RPM (or other speed as described in the shop manual) using the adjustment knob located between the airbox and the carburetors. Cylinder #3 should draw about 10 in Hg (24.5 cm Hg). This is your baseline measure. If the motor is not drawing this vacuum, double check that the idle RPM and mixture are correct; a rich cylinder will not create the full vacuum.
Shut off the motor, and move the Colourtune to #2 cylinder. Replace the sparkplug in cylinder #3. Start the bike, and fine tune the idle mixture on #2. Loosen the locknut between #2 and #3 cylinders, and adjust the vacuum to 10 in Hg (25.5 cm Hg) for both cylinders. Tighten locknut being careful not to move the set screw. You may have to re-adjust the idle to 1200 RPM if the cylinders were way out of sync. [photo]
Stop at this point for twin cylinder bikes, return idle speed to normal, remove the manometer barbs from the carbs, replace the balance port screws, and replace the gas tank.
Shut off the motor, and move the Colourtune to #1 cylinder. Replace the sparkplug in cylinder #2. Start the bike, and fine tune the idle mixture on #1. Loosen the locknut between #1 and #2 cylinders, and adjust the vacuum to 11.25 in Hg (28.5 cm Hg) for #1 cylinder. Tighten locknut being careful not to move the set screw. Most Suzuki GS 4 cylinder bikes with stock airbox and exhaust draw greater vacuum on the two outboard cylinders vs. the inner two.
Repeat the steps described in step #5 on the final cylinder. Your carbs should now be balanced. [photo]
Return idle speed to normal, remove the manometer barbs from the carbs, replace the balance port screws, and replace the gas tank.
Note: you can also use the Colourtune to set your midrange, by adjusting the height of your jet needle, or to choose main jet size.
Additional web pages on this topic: