Almost anyone who messes with outboard motors, knows that synchronizing an outboard is timing the precise spot where the ignition stator (or a lever from the ignition) begins to open the throttle valve in the carburetor. Most leave it at that and their motor runs OK. This is what the book teaches and it is all the book teaches. I’ve messed with many old outboards since they were new and I’ve learned that without a doubt, each and every outboard is an individual (like us) that likes to do its own thing. Each can do better if its owner hears and understands what it is trying to tell him.
I’ve always thought that the main difference between me (an old school professional mechanic) and a modern mechanic is that they are trained to use their instruments and I’m trained to use my ear. I wouldn’t know how to hook up those instruments anyway. I do use a voltmeter and a timing light, but that’s about it. I’m not one of those old-timers who’ll try to tell you that 50s-60s outboards are superior in any way to the stuff you can go into the boat store and buy today. But I will say that properly set-up, tuned, and synced, an old motor will perform in a shockingly modern manner. The uneducated at the local boat ramp are constantly amazed at how well my classic outboards run.
For this discussion, I’ll assume the ignition is fresh and the fuel system is rebuilt. Most of you are good at setting up high and low speed jets on your 50s vintage classics, so we’ll assume the carbs are adjusted correctly. (If any of you need help with your carbs or ignitions, PM me and I’ll be glad to go over it with you.) We’re down to synchronizing which is seen as a black art by many.
I know that when you had your carbs apart, you noticed that the choke shutter and the throttle plate are very similar only are mounted on the front and the back of the carb. I know that you understand that when the choke shutter is closed, air isn’t drawn into the engine, but raw fuel is. In order to start a cold engine, we need the raw fuel. The throttle valve will do exactly the same thing, but it will do it at a speed well off idle. It is designed to begin to open at a precise point called the synchronization point. There are frequently marks somewhere that indicate where the sync is to be set and when the engine was new, they worked well. Now, 40, 50, or 60 years later, those marks will put you in the ballpark, but probably not at the precise point. Slop, or wear of the parts involved will frequently move the correct points one way or the other from the marks. How do you set sync from here?
Remember that the throttle is another choke. If it stays closed too long, the engine will want more of the correct fuel/air mixture, but it will only get fuel and the engine will run rich. If, when you advance throttle off the sync point, the engine smokes a little or hesitates, you are synced too late. If the throttle valve opens early (before the sync point) it will suck more air than fuel and the mixture will be lean. When advancing with early synchronization, the engine will sag off for a second or two before it accelerates cleanly.
Fine tuning synchronization is done on the water under load. Idle is perfect and high speed is all set and the boat is hauling. From the idle, bring the throttle up enough to raise the bow a little (1200 to 1500 RPM if you have a tach). This will be close enough to the sync point to adjust it if necessary. From this nose high attitude, punch hard to wide open throttle. You’re interested in the immediate response. You’re looking for perfectly even acceleration all the way to WOT. If the motor sags and sucks air, you are synced too early. If it hesitates and misses a little, you’re synced too late.
The final adjustments for my customers are always done on the water under full load. Idle and high speed adjustments are double checked. Then I spend some time checking acceleration from high idle to not quite on plane. If synchronization needs adjustment, this is where it will show up.