Synchronizing Carburetors

By Dr. Motorhead

Hey Doc!
Arnie Torgelson here – up north here on the Range ya know. Say, I seen your articles in the Boathouse, but I never read nothing about having two carbs on an engine. I finally got rid of my canoe and I think me and the Misses want a speedboat. My buddy Mercel has this Chris Craft boat with two carbs on it. He says he’ll sell it cheap ‘cause he don’t know nothin’ about what ya call syncromeshing these things so the engine runs good. How come there ain’t just one carb? How come ya need two? Is it because like my dad says, if one is good, then two must be better? Should I be a scared of this boat like my pal Mercel? 

Me and the Misses got a big laugh from the last Boathouse BSLOL magazine. My grandpa built this cabin on Burntside lake up here in Ely. That’s where I spend most my summer hanging out and fishin and canoin and stuff. I know Bob Johnson. He is a real Doctor ya know. He comes up here with his pal Beth. They got this white boat and boy is it fast. He’s got only one carb on that one. But any who, Big Bob, we call him up here, can’t be Dr Motorhead. Here is why. Have you seen him try and fix something on an engine? Two left hands and all thumbs. Poor guy, I guess he was put on this earth to make folks feel good when they get sick. And besides, have you seen his messy handwriting? If he answered all these questions, no one would be able to read it. Oh Doc, I think you were pulling our leg. Besides, I think you and me met sometime back on Lake Vermillion or something like that.

Hey Arnie, Fred Motorhead here.

Good memory Arnie. You are correct. We have met in the past. It was many years ago when I was still in college. I was on summer break, paddling through the Boundary Waters. We were on the portage from Trout Lake heading back to Vermillion. We had conversations about all the beautiful old wood boats that were once on the lake. Remember, before “what’s his name” came and bought them all up. That was a long time ago.

You are also correct in that, I am me, Dr. Motorhead, and not Dr. Bob. I had a little fun with Mr. Hoffa’s letter last month. Smart guy that you are, you saw right through it. I’ve seen the boat Mercel is selling. Buy it, it’s a nice clean boat at a fair price. Don’t worry about the two “carbs”.

O.K. let’s get right down to your carburetor questions. Remember, in the past I have said that engines need to suck in oxygen and fuel, then push out all the exhaust gases. Twin carburetors help in two ways. Each carburetor will feed the fuel and air mixture to three of the six cylinders — at least that is the case with Mercel’s boat. Some boats have eight cylinders – the fuel gets distributed more evenly and directly – thereby increasing horsepower. So yes, in this case, your daddy is correct, two is better than one.

Two carburetors are the same as one with respect to the maintenance and the adjustments to idle, low and high-speed jets. You will have to read my previous articles to catch up on how to perform these tasks. The difference is that they need to be synchronized not synchromeshed. You’re talking transmissions when you say synchromesh.

Here is what happens to your engine if the carburetors are not working together. This is really only noticed at low speeds, not high. If one “carb” is set at a higher idle than the other, your engine will suffer from an identity crisis. Half of your motor wants to go faster than the other half. I liken it to the left side of you wanting to run down the street while the right side wishes to walk. The normally smooth operation of walking becomes a rough and confusing situation. Remember the Steve Martin movie, “All of Me?” Your engine will have this same awkward and confusing gate that Steve Martin and Lily Tomlin had while they occupied the same body.

So how do you get things to run right? Here’s how. You can purchase a synchronizing tool that fits over the intake end of the carburetors. In essence, this measures the vacuum pressure and allows you to adjust the throttle plates on each carburetor. You see, they need to be in the same position in the upper throttle body. All of you should remember previous lessons and now what parts I am talking about. But perhaps for most of you, you don’t need the expense of this tool, so do as the Doctor does. Remove your carburetors. If they are out of whack they probably also need cleaning. Get them cleaned up (previous lesson) and set each one upright on the bench. One at a time, install a small spring to the throttle linkage arm which will pull the throttle plate into the closed position. Turn your idle speed adjustment screw — not the air mixture screw — in until it just starts to move the plate open. Turn this screw in and out a couple of times so you know that you have the screw just at the spot where it will begin to move the plate open. Now, do the exact same thing to the other carburetor. Look at them both. The very small gap between the throttle plate and the throttle body or the side of the carburetor should be the same.
Now it’s time to install the carbs. With the springs still attached, install the linkage that connects the two together. Remember, and this is very important, adjust the linkage to the carburetors and not vice versa. You do not want to move the throttle plates at all while completing this task. This task now complete, set the throttle linkage to the carburetor. Remove the springs, making sure everything stays put. You should be set to go. Your idle may be a little low at this setting, but once you have the engine started you can adjust the idle speed as you normally do.

I should make mention, adjusting the idle mixture is a little trickier. When you attempt this, get down close to your engine and listen closely as you adjust. The whole engine will not slow down as you screw in the mixture screw, only half of the cylinders will respond, so you need to listen well, but you will get it, as sure as I am Dr. Fred Motorhead.

Until next time, happy boating, see you at the boat shows this summer.

Dr. Motorhead