Carb Synchronization
By: Dave Beckstrom

This article gives an overview of how to mechanically sync your carbs without special tools. It also includes a good tip you can use. Enjoy!

Remove the air filter.

Next, look at both carbs and make sure they are parallel to each other and perfectly vertical when the head on the engine is level. If one carb is tipped or they aren't level, loosen the clamp on the rubber boot between the carb and the engine and rotate the carb(s) until they are square and level.

To begin synching, start with the idle adjustment screws. The idle adjustment screws are tapered. As you turn them in (clockwise) the taper begins to lift the slide which increases the engine RPM. We want to make sure the idle adjustment screws are not lifting the slide.

So turn them out (counter clockwise) until the slides don't move. Next, slowly turn the idle adjustment screws in (clockwise) watching the slides for the barest of movements. Just as you see the slide start to lift, stop turning. Do the same with the opposing carb. You now have your idle adjustment screws in synch with each other.

At this point the taper on the idle adjustment screw has just begun to touch the slide. Your idle will be set way too low for proper engine idling RPMs. Pay attention to where the slot is in the head of the idle adjustment screw--this slot position is your frame of reference that you'll use when you make changes to your idle settings.

Count your turns in 1/2 turn increments and turn each idle screw in 1 1/2 to 2 turns. When you've turned them in, the slot in the head of the screw should be oriented at the same position it was before you turned it. You were counting, right? This should be a good starting position for your idle.

Now synch the carb slides. These are what are going to affect your engine EGTs. You want your EGTs within about 40 - 60 degrees of each other. However, using this technique you can sometimes get them as close as 5 to 10 degrees.

Take a look at the photo at right. As you can see, the rounded bottoms of the slides do not provide a decent frame of reference by which you can compare the slide position of one carb to the other.

Click to Enlarge

Click to Enlarge

My trick is, open the throttle so that the notch on the bottom, back side, of the slide is roughly even with the top of throttle bore. (see the red arrow on the photo at left) Its okay if you see a sliver of the throttle bore showing where the top of the notch is, as depicted in the photo at left.

The position of that notch, in relation to the throttle bore, is your frame of reference. If you can see, for example, 1/16" of throttle bore exposed at the top of the notch, then what you want to do is make sure the other carb also has 1/16" of throttle bore exposed at the top of the notch.

You adjust these by loosening the locknut on the top of the carb and turning the fitting attached to the throttle cable. It is better to lower the slide that is high than it is to raise the slide that is low. Why this is important is explained below.

Once you have both slides displaying the same amount of exposed throttle bore (that sliver of metal showing at the tip of the red arrow in the photo) your carbs are synched! Put a pair of pliers on the throttle cable fitting to hold it from turning while you tighten the locknut with a wrench.

Now here is why you don't want to adjust the fittings on top of the carbs to raise the slides too high... Open the throttle wide open. Stick your finger into each carb and try to manually lift the slide higher into the top of the carb. You should be able to raise the slide another 1/8" or more with your finger when your throttle lever is wide open.

This is important because you DO NOT WANT the slide hitting the top of the carburetor when you go to full throttle. The reason is, you can apply enough throttle pressure to pull the ferrule off the end of the throttle cable. When this happens, the slide will drop to idle even though you are at wide open throttle. It usually happens to one carb, and you'll have a forced landing if it happens in flight. By making sure the slides don't contact the roof of the carb, you can't pull the end(s) off the throttle cable.

Slide your air filter on, lightly tighten the clamp and run the engine. Check the idle and EGTs. Chances are you'll need to turn (while counting) the idle screw in further to get your idle around 2000 (my preference). If idle and EGTs look good, tighten the clamps on the filter and safety wire the filter.

The photo at right shows the results of my recent carb synching. The EGTs are 815 and 818 degrees at 2990 RPM.

Click to Enlarge