Synchronizing Carburetors

| 6/24/2015 8:12:00 AM

Tags: July/August 2015, Keith Fellenstein, tech corner,

Motorcycle Classics tech expert Keith Fellenstein

Editor’s note: If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as the subject.

Synchronizing Carburetors

Q: I have a question regarding “syncing” carburetors. Some gurus can synchronize them by ear, and I’ve met mechanics who are just incredible and use all their senses to accomplish the extraordinary. How does syncing contribute to performance? Usually, I measure the slides with the carbs on the bench with drill bits, selecting a size that just fits between the horn and the slide, and average the two (or three or four). Are there unique factors with individual cylinders that require a “power on” vacuum test, or am I good to go? I suggest to your readers to start a scrapbook, as I have with your articles. I look through the table of contents when I’m stumped and lo and behold, you’ve already done the work for me. — Mike Peterson/Chetek, Wisconsin

A: In a multicarburetor setup, synchronizing ensures that each carburetor is providing the same amount of fuel and volume of air at the same rpm. That in turn keeps the cylinders working in unison, with no cylinder working harder than the other. This, along with engine timing, contributes to an engine that runs smoothly and accelerates at its best. What you’re describing is generally referred to as “bench syncing” and it is a necessary first step whenever you have disassembled a multicarb setup. Depending on whether you’re dealing with CV carbs or direct slide carbs, the technique is a little different. On direct slide carbs such as the Amals on British Triumphs and Nortons, you can use the wire-gauge method you outline, using the idle stop screw to get the initial opening the same. Assuming you have the carbs off the bike, you can try this. For CV carbs, I keep a few 1/4-inch ball bearings on hand. I place them in the throat of the carburetor and open the butterflies. If the balls drop simultaneously, the butterfly valves are synced. Once that’s done, you need to make sure the cables are pulling identically for all the carbs. You can check at wide open throttle to see if the slides are all the way up to the same degree, but you can’t really tell if the intermediate range is matched. This is the range where you’ll spend most of your riding time. The easiest way to do that is with vacuum gauges. MC

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