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The Doctor

Dear Doctor, What is the best way to synchronize multi cylinder bikes with two (or more) carburetors? I have always struggled with trying to get my CB 77 running properly and am about to finish restoration of a tasty 75 Honda CB550. Do I really need something like a TwinMax? What do you recommend for gage to help take the difficulty out of the job? Help me Doctor. Signed Carlos in Carlsbad


RX Hello Carlos: You have one of my favorite bikes. I always liked the Honda CB550. A tasty bike indeed. Synchronizing carburetors is not particularly difficult, but you need special tooling and a good understanding of the process. Since you have mentioned both a twin and 4 cylinder motorcycles, a TwinMax electronic synchronizer will not suffice. I have owned a TwinMax, a MotionPro mercury filled 4 column manometer, and recently bought a 4 column Morgan Carbtune Pro. Additionally, I have used a 4 gang mechanical vacuum gage.

Lets focus on eliminating the least desirable types of synchronizers first. The TwinMax will only handle the twins, and I never really warmed up to the small dial or having to fiddle with the sensitivity knob. However, TwinMax has a good reputation in some circles. Mechanical vacuum gages were never my favorite, reading the 4 pointers spread out across a 2 foot wide panel seemed tedious. My ideal tool is a manometer. The 4 columns are a fraction of an inch apart, they are very easy to read, sensitive, and do not require calibration like electronics or vacuum gages. Frankly, I like the Mercury filled manometers, but they are fragile and spilled mercury is a big concern as are mercury vapors. My favorite is the Carbtune. It is a mechanical equal to a mercury filled manometer. There are four clear plastic tubes with precision metal rods inside each tube. Pulling a vacuum causes each rod to rise in the tube, and you can readily compare the height of each column is a single glance.

I love the Carbtune Pro. Very easy to use! The principle of synchronizing is to ensure each carburetor is opening and closing the same amount as every other carburetor on the engine. Idle mixture does come into play, but lets look at the basics. Most all multi-cylinder engines have some sort of port allowing one to connect a gage and measure the vacuum for each cylinder between the inlet valve and throttle slide/butterfly, I.E. the manifold pressure. When the manifold pressure (vacuum) is identical for each inlet tract, the carbs are synched. An additional mixture adjustment may be needed.

The key to obtaining a well running and synchronized bike is good tools and good practices, as well as all parts in proper working order. There are no short cuts. A brand new top of the line carb synchronizer hooked up to worn out or dirty carbs will not produce a great running bike. With a bit of practice, you can synch a twin in about 10 minutes. Each of the vendors I mentioned has excellent detailed instructions for their synchronizer at their web sites. Morgan Carbtune is made in England and has a very low shipping rate to the US. I have no business interest in any of the vendors mentioned.