Carburetor synchronizing is something many owners prefer to avoid. Chalk it up to a fear of undue complexity or just plain getting it wrong, it’s a job otherwise capable weekend mechanics typically decide to turn over to more experienced hands. That’s certainly a wise move if you’re mechanically challenged. But if you can confidently tune your bike — adjusting the valves and setting points and timing — then you can probably handle a carb synch.
The point of synchronizing is to make sure each carburetor is pulling exactly the same amount of air, so that each cylinder is filling the same. Importantly, tuning your bike is the first step in a carb synch, because if the valves aren’t set correctly and the ignition timing is off, trying to synchronize carburetors usually makes a bad situation worse by masking other problems with carb adjustments.
Synching a twin by ear can be done with experience, but a four is a different matter as three properly set carbs can cover up one poorly tuned unit. And if you’re synching the carbs on a multi-cylinder like our subject 1973 Honda CB350 Four, you have to have the right tools. In the old days it was common to use a mercury stick — a roughly 30-inch tall set of four clear plastic tubes in a common reservoir filled with mercury — to measure engine vacuum. Mercury was used because its high density made measuring easier and more accurate on a smaller scale.
Mercury sticks are no longer available, but you can buy synch sticks that use a non-toxic fluid and electronic synchronizers are also available, although 4-cylinder versions are pretty pricey. In between are vacuum gauges, with a good set like the one we got from BikeMaster running in the $225 range.
Our 1973 Honda CB350 Four. Honda used the same basic carb setup for all the small fours (CB350-CB550) from 1971-1977.
Assuming you’re starting with a fully tuned and running engine, preparation is actually fairly simple. Removing the tank is typically a must so you can get to the carbs, and to make things easy rig up a small 1-quart auxiliary tank that you can strap to the frame or seat. Whichever tool you use, you’ll want to make sure you have the necessary hose barbs for your bike. Our Honda required two long barbs for the inner carbs (where the manifold ports are fairly buried) and two short for the outer (which are easily accessible). Most kits, like ours, come with a full assortment of fittings to fit different threads.
Before getting to work, run your engine up to operating temperature so the bike will idle properly (Honda says 1,200rpm for the 350) without the choke. And make sure you have adequate ventilation and a fan to help keep the engine cool as you’ll be working with the engine running.
A typical carb synch shouldn’t take more than an hour or so, but budget a full morning if it’s your first time for the job. While it may seem like it at first, nothing about the process is particularly difficult. The critical issue is taking your time and working through the process methodically. As ever, have a good shop manual at hand to aid in parts identification and proper torque specs. And when you’re done, go for a nice ride; you’ll have earned it!
1. To begin, remove the gas tank and replace it with a small auxiliary tank so you can get to the carburetors. Our auxiliary tank is nothing more than a repurposed lawn mower tank — perfect because it has an outlet and a cap — with a universal petcock in the line.
2. Next, locate the adjustment points on your carburetor. Our Honda has four adjusters, with a strap between the left and right pair of carburetors. Make sure the throttle cable is loose and not pulling on the throttle linkage.
3. Remove the four screws, one for each carburetor, from the intake manifolds. They can be tight, so be careful not to strip the heads if you don’t have the proper JIS Phillips driver.
4. Next, find the correct hose barbs and thread them in place of the screws. Synchronizing kits usually come with multiple barbs of different lengths and threads, each with an O-ring seal to ensure no air gets in during testing.
5. Now comes the fun part. Run the lines from your gauges, in sequence, to the manifold hose barbs. Make sure the gauges are set “off.” Start the engine and adjust to a 1,200rpm idle. Slowly open each gauge (the knurled barrel above the hose on our gauges) until you get a steady reading.
6. Honda calls for a reading of 16-24cmHg, with readings within 3cmHg. Cylinders 3 and 4 on our bike are fine at 18cmHg, but 1 and 2 are low at 14 and 10. To adjust, hold the adjuster screw and release the lock nut, then turn the adjuster. Here we’re using a standard screwdriver and 8mm box wrench.
7. Consider buying a proper adjusting tool, which has a screwdriver running inside a long tube with an 8mm socket at the bottom, making it easy to turn the adjuster and then lock it down.
8. We were lucky we only needed to adjust two carbs, but even so it took a bit of fiddling, going back and forth between adjustments and blipping the throttle to settle the engine, before we got our Honda to achieve a nice even reading across all four cylinders.
9. Once you’re satisfied you’ve achieved the best readings you can, check the idle is still at 1,200rpm then turn the engine off. Remove the lines and adapters, reinstall the manifold screws, disconnect the auxiliary tank and reinstall the stock gas tank. That’s about it; you should now have a nice running Honda!