Can I teach you to tune and balance your carburetors in 40 minutes? No, but I can set you up for success. We exclusively use a Weber Redline Synchrometer, STE-SK and if you have a bigger flow carburetor you can use the STE-ST. If you do not have this tool you cannot use our method shown here.
First Things First
When it comes to tune and balance, you need to start with a good engine. There is no point tuning a set of carbs if the engine is clapped out. We use a leak-down test to determine the internals of our engines. We prefer a leak-down test over a compression test. Read more about the differences here. Leak-Down Vs Compression Test
Ignition before Carburation
Once you have determined you have good leak-down numbers, the next system you should look at is the ignition system. This means that the timing is set on your engine. Timing should be set at the top end, high revs, which means the rest of the curve will be correct. Sometimes it can feel like a catch 22, carbs not working, can’t rev the engine, can’t set the timing correctly.
I consider the other electrical parts of the ignition system too. Spark plugs, ignition wires, alternator, and battery all play a part in a good running engine.
Bench-Set your Carburetors before Tune and Balance
So you are not stuck with the dilemma of which came first, ignition or carburetors, you can bench set your carburetors. By bench setting, you will have the best chance of the carburetor starting and running enough to get your timing set.
If you are building your carburetors along with us, we know you have set up the essentials. If not it would be worth going through a bench re-set.
- Check throttle plate position in relation to progession ports
- Measure Pump Discharge Volume.
- Set Float Hieghts
- Seat air-bypass screws
- Turn out Mixture Screws 2 to 2.5 turns
With your carburetors on the engine, turn on the fuel pump with the ignition. Work the throttle and pump until fuel squirts out the pump jets. You can check for leaks and or drips while under fuel pressure. Fix anything that comes up.
We do not have the throttle linkages hooked up. When we start our tune and balance, we need to work on each carburetor independently. Run the engine working the throttles until the engine will idle without assistance.
Time to Tune and Balance
Use a synchro meter to see what airflow you have. On this style of a carburetor, there are two throttle shafts and a coupling. Earlier, we left one side of this coupling loose. Now is the time to balance the flow within each carb.
You can do both sides.
Now is the time to listen to the engine. Work each mixture screw slowly in or out, listening for the engine to slow down. Sometimes in real life you can hear a puckering sound. Drop down, then slowly wind out until the engine picks back up. As you work the revs higher you can knock them down with the throttle plate stop on each carburetor.
You can use a timing light to do the same thing if it has an rpm function. A digital timing light makes it a little easier. But remember, the light is a tad behind reality. So if you use this method, you may swing up and down. Just make your movements slower, and wait for the light to catch up.
After we have done an initial tune and balance, we can accelerate the carburetors to see how that circuit feels. Part of our bench set was to set the pump discharge volume. This is just a bench setting and will need to be adjusted on the engine.
There are two types of accelerator pump cams. One looks like an ax, and one is more rounded.
You want the pump cam that looks more like an ax. It has a faster fuel delivery than the rounded one. To make changes to the pump cam, mark a screwdriver so you know how many turns in or out you make. Turn the adjustment one way or the other, and make sure you do the same amount on both sides. Accelerate the engine and note the feel up and then return to idle.
Just remember to make changes to both sides equally.
Putting on the Throttle Linkages
We have put off putting on any linkages until the end. Now it is time to set up the cross-bar and drop-down links. During your tune and balance, you got the airflow around 5 to 5.5 KG of air per hour. When you set up the linkages you need to keep this number the same.
You now need to check that the throttles are opening at the exact same time. To do this you will have the engine off idle, you can use your hand throttle. Or, we use a clamp to accelerate off idle. The amount is not specific, but you will need to be able to measure it with your synchro meter.
Make adjustments to both dropdown links until the airflow is perfect. Do this at idle and off idle to finish your setup.
Time and Practice will help you Tune and Balance
This will not happen overnight, but with time and practice, you can tune your carb set. We do not cover engine changes, jet, choke, or cam changes here. This is the first step, getting stock carbs to run after a re-build.
We could spend many more hours on jet changes!
The biggest lesson we can give you today is, don’t work too hard. If you spend hours doing this tune and balance you and your engine will be tired. Take a break, take a test drive. Make a change slowly, but do it quick!