In this month’s column, we will have a look at the mystical, magical carburetor; probably the single most intimidating part of an outboard to the novice.

It need not be; It is not necessary for you to design, cast, and machine the carb.; just to take it apart, clean it, install a few new parts, and put it back together the way it is supposed to be.

Again we will be looking at OMC-built outboards from the mid fifties until the early seventies. For those who think that I am concentrating on this series of outboard motors, they are correct, and for good reason. As I stated in my long-ago Duckworks article titled, “Primer on Old Outboards” , I am of the opinion that this series and manufacture of outboard motors are the best candidates for “cheap, reliable power” because they are relatively easy to work on, commonly-replaced parts are readily available and relatively cheap, and these engines still exist in great numbers. In the not-too distant future, I intend to address other brands of outboards, such as Mercury’s and the air-cooled Clintons and Eskas.

Most of the carburetors on OMC-built outboards of up to 40 hp, and from about 1953 until about 1972, are basically very similiar, differing mainly in size and in whether they are fitted with fuel/air mixture adjusting needles, or with fixxed jets. In some cases, the same basic casting is used for different size engines, the differences being in the internal passages.

On most of the smaller engines, a choke activated by a pull-knob is used; on the larger motors fitted with electric start, the choke may be operated by an electric solenoid, with a pull-knob back-up. A few 40’s were even fitted with an automatic choke, but that did not work too well, and anyway a manual pull-knob was also present.

The basic design of these carbs is such that they have a portion of the carb for use at “high-speeds” and a portion that comes into use at “low speeds.” The older versions of the outboards will have both High-speed and lowspeed mixture adjusting needles. Later, the high speed needle was replaced with a fixxed jet which requires no adjustment, but the low speed adjusting needle remained.

One thing needs to be said about mixture-adjusting needle valves; they should NEVER be tightened down hard on their seats. The needles are usually soft brass, and tightening down hard will usually damage the end of the needle and render proper adjustment impossible. It only takes a very slight groove on a needle to render it useless. Be very careful when tightening down the needle valves.

Before tearing into the carb, be sure the problem does not lie elsewhere. If the engine appears to be starving for fuel, did you check the fuel strainers? The old OMC’s that used remote tanks usually had a strainer bowl on the bottom of the carb, or remote-mounted on the side of the powerhead. There was also a strainer on the foot of the pickup tube in the remote tank. Are the quick connectors making a good seal and not leaking?

If the engine is hard to start, are you sure the choke plate is closing all the way? If your engine has adjustable mixture needle valves, Setting them too lean can also make an engine hard to start.

If your old outboard uses a pressurized remote tank, you might check into the column on pressure tanks for some background on the problems that they present.

Finally, those smaller engines with power head-mounted, gravity-fed fuel tanks usually had a strainer incorporated into the fuel outlet on the bottom of the tank.

If you believe that your have isolated the problem to the carb, then you can consider removal and disassembly.

Removing and rebuilding a carb is no big deal. The carbs on the engines in question are held on by only two nuts, but usually one must remove the cowls and recoil starter in order to gain access to the nuts. On some carbs it is necessary to disconnect the linkage that synchonizes the carburetor to the magneto. Usually a screw on a bell crank can be backed-off allowing the linkage to disconnect.

Once the carb has been removed, you can disassemble it, usually by removing the 5 or 6 screws that hold the bowl on. Once the bowl is removed, the float and float needle and seat will be visible up under the upper half of the bowl. Be sure not to lose any small parts.

Although in the past it was recommended that the carb be soaked in cleaner, I have noticed that the factories have backed-off that recommendation of late, and now generally recommend using an aerosol cleaner. That is what I use. I remove the float (usually it is varnished cork and carb cleaner removes varnish) and needle valve, and generally a small plug on the bottom of the bowl. Most of these carbs also have a brass high-speed nozzle extending vertically up into the center of the upper half of the carb, and this too can be removed. One will notice numerous welch plugs (metal plugs) that are stamped into various places on the carb body. These cover holes that were necessary in order to properly machine the carb, and although some recommend removing these metal plugs ( which will destroy the old plugs) in order to clean underneath them, I rarely if ever disturb them. The Sierra rebuild kits that I use do contain new plugs.

If your carb. has a fixed high-speed jet, it probably is not necessary to remove the jet itself, but one should remove the plug that covers the jet so that cleaner can be sprayed in.

In case you are already lost, be not afraid. A good manual with a few pictures will help, and I suggest the manual that I reviewed for Duckworks, or a similar manual, as the minimum to get you by. Also, the carb rebuild kits that after market supplier Sierra has available through boat dealers of any brand outboard,( and also through NAPA auto parts) come with excellent instructions. I have to admit that I don’t know if OMC factory carb rebuild kits come with instructions; I have never used one of their kits.

Once the carb is apart, you can use the plastic tube that usually comes with the aerosol carb cleaner to shoot cleaner into all the exposed passages of the carb. Remember not to get any cleaner on the varnished cork float. If it’s varnish is peeling off (usually because some idiot ran automotive “varnish removing” carb cleaner in the gasoline in an attempt to clean the carb while the moter was running) you might have to remove all the old loose varnish, allow the float to completely dry, and lightly sand and then re-varnish. I have used fuel-proof model airplane dope for coating cork floats.

I should point out here that you should be wearing eye protection when spraying cleaner into passages on the carb. Many times I have shot cleaner into one hole on the carb, and had it shoot back out of another hole right at me. Safety glasses are not enough; you need chemical hazard safety goggles and maybe a full face shield. Carb cleaners burn exposed skin pretty bad; it will really eat-up your eyes.

Once the parts are clean and dry of cleaner, you can begin reassembly. I really suggest that you use a rebuild kit when you put the carb back together. It doesn’t cost that much money and may save you from having to do the job all over again. Keep in mind that the old “soft” parts of the carb (gaskets and such) may date from the days before alcohol was common in gasoline, and these parts may be harmed by exposure to alcohol.

Some things to be mindful of as the carb goes together: the float needs to be set to the proper “level,” so as to maintain the proper amount of gasoline in the bowl. The sierra rebuild kits provide a nice drawing of what you are trying to achieve

There is a little cork “doughnut” that goes over the brass high speed nozzle that some of the carbs have extending vertically up into the upper half of the carb. Be careful when installing the doughnut that you don’t tear it.

Another area to be aware of is the “packing” which seals the mixture needle valves. Although tiny, this is basically the same sort of “packing gland” (stuffing box) that seals inboard propeller shafts and also valve stems. New packing is supplied in the rebuild kits. I usually just add new a new ring or two of packing on top of the old stuff, since it can be hard to remove the old packing, but be careful that none ends up being pushing down into the fuel passages. The proper way to do the job is to remove all the old packing.

Once the carb is back together, you can reinstall it on the engine. Make sure the linkage to the magneto goes back together correctly, and you may need to reset the carb/ mag synchronization if it was off to begin with, or if you disconnected the linkage in such a manner that it could not be put back together exactly as before. On most of these engines, “synching” is no big deal , being about a 5 minute job needing no special tools.

I always replace all the fuel hoses as the old ones may be rotted and if they are not rotted, alcohol in the gas may soon rot them. Use good clamps on the hoses. I like tiny little worm-screw clamps (I think the British call them “cheney clips”) when I can find them small enough. I also always install a small plastic “in-line” fuel filter on the engine, even if it has it’s original strainer/filter. If I am going to spend some time and money cleaning and rebuilding a carb, I want to make sure that only clean gasoline is running through it.

And, of course, the clean 2-cycle oil that should be mixed with that gasoline.

Later

Max