After sitting for the winter when I first fired up my bike for the season it had a serious afterfire (backfiring through the exhaust). I attempted a basic tuneup with new plug wires, a cleaning of the carburetors and new spark plugs but no improvement was made. After some researching I found that it could be from the carbs being out of balance. I searched around but all I could find was expensive professional setups. I wasn't willing to spend the money for it since I was going to be selling my bike soon anyway so I decided to build my own.
Step 1: Materials Required
To build the carburetor balancing tool you will need:
Wooden yardstick ($0.98 at Home Depot)
Clear flexible hose (~$4 for 10ft of 1/4" I'D at Home Depot)
Hydraulic jack oil (~$4 at Harbor Freight)
A pack of vacuum tee fittings (~$4 at Autozone)
Clear Packaging tape
A zip tie
Possibly some extra bits of vacuum hose
~45 minutes of time
Step 2: Assemble the Balancer
Start by folding the tube in half lengthwise. Put the bend in the tube at one end of the yardstick. Attach the hose to the yardstick with a zip tie (this isn't 100% necessary but makes continuing the balancing tool assembly much easier). Continue running the clear hose up the sides of the yardstick and attach with clear tape about every 8 inches. Now, stand the balancer up on the end with the bend at the bottom. Carefully pour in the hydraulic jack oil until both sides of the balancer are at about halfway up the yardstick.
Step 3: Attach the Balancer to the Motorcycle
Using the tee fittings (and some extra bit of vacuum hose if you need to), tap into the vacuum system on your carburetors. Connect 1 side of the balancer to 1 carb and the other side to the other carb. Start up the bike and see where your levels are reading.
Step 4: Adjust Your Carburetor Balancing
Find out where the adjustment for the carburetor balance is on your motorcycle. On my 1990 Suzuki VX800 there is a cable with a threaded adjuster on the front carburetor. Slowly make a small adjustment. Blip the throttle (just a quick, small rev. Going crazy here can pull the fluid out of the balancer). When the RPMs settle back down to idle see where the balance level is at now. Based on what you just found out about how certain adjustments effect your balance level you can now continue making adjustments. Continue slowly making adjustments, blipping the throttle, and checking the balance until you get to an acceptable balance.
Step 5: Suggestions for Improvement
After using this this for a little bit I've found a few improvements that could be made to it:
1. Use an oil that is darker. The clear hydraulic fluid can be hard to read.
2. Use some form of vacuum restriction. the vacuum levels vibrate and move while the bike is running (about 1/4" of variance for my setup). Using a small orifice fitting would greatly reduce this.
3. Have some way to shut off the balancer in an emergency. When I first connected this to my bike and started it up the vacuum difference was so large that I had to shut my bike off within about 10 seconds to prevent the oil from being sucked out completely. Some sort of valve could help here.
If you're not ready to drop the $40+ for an actual vacuum balancing setup I would highly recommend giving this a try. My bike doesn't run perfect now but it's much better than when I had started.