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Obsolete Outboards
by Max Wawrzyniak

Column # 2
Pressure Tanks

Pressure tanks. Those two words cause more problems for the novice obsolete outboarder than about any other. Recently I became aware of an individual who had read my Duckworks article, “Primer On Old Outboards” where I mentioned that one should always be sure to get a pressure tank and quick-connect fitting if one buys an outboard that uses a pressure tank.

This gentleman bought an old outboard that used a pressure tank. The seller handed him a tank and assured the buyer that the motor had been run using that very tank just last year.

Turns out the tank was not a pressure tank, and there was no way that particular outboard could have been run using it. I won’t speculate as to the honesty of the seller.

With that experience in mind, I have decided to “ed-ju-kate” ya’all in the ways of pressure tanks. I will assume that you know nothing and start from there.

There are basically (2) types of fuel tanks used on outboard motors; the smallest engines generally use tanks mounted on the powerhead and feeding gasoline to the engine via gravity. There is little to go wrong with this system short of a clogged tank vent or plugged fuel filter.

Usually all outboards but the very smallest will utilize a remote tank; i.e. a gas tank sitting somewhere in the boat and a rubber fuel line which connects to the engine. All modern outboards, and many old ones, are fitted with a fuel pump which draws the fuel through the hose and up to the engine. Usually the fuel hose is fitted with a squeeze-bulb for manually pumping fuel to the engine for starting.

A pressure tank is a remote tank as well, but the engine does not use a fuel pump in order to draw fuel through the hose. Instead, the tank is pressurized though one passage in a two-passage hose, and the gasoline is forced by this pressure back up the other passage in the hose to the engine. In order to give the engine it’s initial charge of gasoline for starting, the tank is fitted with a manually-operated pump which is pumped a few times before starting, just as the squeeze-bulb is squeezed a few times on modern engines. These tanks operate at a pressure of about 2 to 3 PSI.

Because the pressure tank is, of course, under pressure, there is no vent on the tank, and if you remove the lid in order to check the fuel level, you will release all the pressure and your engine will stop.

The first OMC-built outboard to use the remote pressure tank was the 1949 Johnson 10 hp . The last to use the pressure tank was probably the 1960 5 ½ hp. In between, virtually every Johnson and Evinrude outboard over 3 hp used the pressure remote tank ; the exceptions were a very early Evinrude 7.5 hp that had a neutral clutch instead of a full gearshift; some 1959 models (it took two years to complete the change-over from pressure tanks to fuel pumps) , and some models that were fitted with optional fuel pumps (OMC did offer conversion kits).

The first pressure tanks are referred to as “high boys” because they were relatively tall. After a few years, the “low” tanks were introduced in 4- and 6-gallon capacities, and these low tanks look fairly similar to outboard tanks used for fuel-pump engines.

Oddly enough, the Gale division of OMC, which sold engines that were supposed to be “economy” models, had fuel pumps standard on their remote-tank models as far back as 1955

So how does one identify a pressure tank? The pressure tank will always have attachments for two hoses , usually hose barbs, with one labeled “fuel” and one labeled “air.” Also, generally an OMC pressure tank with have a large, removable ‘tank top,” usually painted black, whereas most “fuel pump” tanks have a relatively small fuel-pickup assembly mounted on the top of the tank. However, remember those Gale engines that came with fuel pumps back in the mid-50s? Often there were sold with remote tanks that looked very much like pressure tanks, with the large removable tank-top, but with only one hose barb outlet on top. Although one rarely sees these old Gale fuel-pump tanks anymore, they are still out there, waiting to trap the unwary. Always look for the two hose connections on the top of the tank.

Do not be confused by “fuel-pump” style tanks; these have a single hollow fuel-conducting pin with check valve on the tank connection, along with a “locking” pin which holds the connector securely in place. Pressure tanks always had the hose clamped to the gas tank; the hose was not attached to the tank with a “quick” connector..

By the way, the caps for these pressure tanks are something special. If your tank is missing the cap, it might be difficult to replace.

A very difficult piece to replace is the quick connector that attaches the hose to the outboard. If you do not get this connector with the tank, you will have a hard time finding one for any kind of reasonable price. Several years ago when I was desperate for one, I paid $20.00 for a used one fitted with new “O” rings; lately I have seen prices of $60.00 quoted for these things. Just for a fuel-line connector. I have seen prices of up to $100.00 quoted for a useable pressure tank, hose, and fitting. Which is why you should think hard before buying an ‘50s OMC outboard that needs a pressure tank but does not have one.

Of course, once you have your tank, what if it don’t work? Well, if gas leaks out of the tank top when you try to push on the primer-pump button, chances are you have a bad diaphragm. These are sometimes available through Johnson/Evinrude dealers if you can find a parts man that is willing to look. I have posted a parts breakdown with parts numbers elsewhere on this column.

Be very careful when disassembling the tank top itself, as there are a few little parts inside that tend to scatter.

If you pump and pump and no gas comes out of the “fuel” hose barb on the tank top (remove the fuel hose), there is a good chance that you have a stuck lower check valve. The primer pump is a simple diaphragm pump, with a check valve up in the tank top on the discharge side of the pump, and a check valve down inside the strainer on the bottom of the fuel pick-up tube on the suction side of the pump. Often the check valve down there at the bottom of the tube will stick due to old gasoline sitting in the tank . Take something like an awl or an ice pick and gently inset it through the strainer mesh on the bottom of the fuel pickup and push the check-valve disc loose- it should rattle when the pick-up tube is shaken. Then soak the bottom of the tube in cleaning solvent.

If there is fuel leaking out around the quick-connector where it attaches to the engine, the little “O” ring seals in it can be changed.

Remember that the tank must hold pressure in order to work.

Rather that go into a lot of detail on trouble-shooting these tanks, I have posted several original Evinrude Service Bulletins dealing with these tanks.

As to parts availability, a few parts for these tanks are available from Sierra, an after-market parts supplier. Sierra parts can be obtained through just about any boat dealer, and also through NAPA auto parts stores, although the kid at the NAPA counter may not be aware of it.

For example, the “O” ring seals for the quick connector, OMC part # 301824, are available as Sierra part # 18-7111; the cork gasket which goes under the tank-top, OMC part # 302557 , is available as Sierra part # 18-2887. The twin-passage hose is also available from Sierra, part # 18-8051, but you have to buy it in a 50 ft roll, list price about $90.00 A bit of calling around might find you a dealer selling it by the foot.

For any other parts, you are probably best to try a Johnson/Evinrude dealer.

The last part of this column will deal with the inevitable question; can these engines be converted to use a fuel pump and a standard (like, available at Wal-mart) gas tank?

The answer is yes, they can, but it may not be cheap.

First, all of the pumps in question are diaphragm-style pumps that work by utilizing alternating pressure and vacuum to move the diaphragm back and forth. The source of this alternating pressure and vacuum is the crankcase of the two-cycle engine, which by its nature has alternating cycles of pressure and vacuum. On multi-cylinder 2-cycle engines, the crankcase for each cylinder is sealed from the other cylinder(s), and the fuel pump is connected to only one cylinder. The by-pass cover is on the by-pass which conducts the fuel-air mixture from the crankcase to the combustion chamber

The easiest way to convert one of these engines Is to use the same parts that OMC used to convert these engines to fuel-pump versions; for example, the 5.5 hp, 10 hp and 18 hp utilized pressure tanks until about 1959; after which, they were equipped with fuel pumps. Not many parts are needed; a new “bypass” cover, a fuel pump that fastens to the by pass cover, a new motor-end quick connector, and some hose and clamps. It may be possible to order these parts new from an Evinrude or Johnson dealer, but they will not be real cheap. The outboard “for sale” boards and of course e-bay are other potential Sources.

Owners of big 25’s, 30’s, 35’s, and 40’s can use either the by-pass cover-mounted fuel pumps, or the more commonly seen remote-mounted fuel pump, which picked up it’s pressure/ vacuum pulsations via a hose attached to a hose barb on a special by-pass cover.

Another alternative is to keep one’s eyes open for a “parts engine” that has these pieces on it.

Some of these parts are interchangeable; for example the mid ‘50s Johnson and Evinrude 15 hp’s were never available with fuel pumps as standard equipment, (it was an option) but the parts and pieces from a later J or E 18hp, or from a Gale 15 hp will work, and parts from a Gale 12 might even work.

But if the special “by pass” cover (the key to an easy conversion) can not be found (or in the case of the pressure-tank 5.5 hp’s , which did not have by-pass covers) one still has an option available, but there is some risk involved. I have seen this method sometimes work and sometimes not work. The procedure is to first obtain a diaphragm fuel pump that operates on pressure and vacuum, just as the by-pass cover-mounted pumps, but one that uses a hose connection instead of mounting directly to the by-pass cover. .

One then modifies the check valves in the engine’s intake manifold that supply pressure to the remote gas tank. As previously mentioned, the crankcase of a 2-cycle engine is alternately under pressure and vacuum. Pressure tank engines have check-valves fitted to the crankcases of each cylinder to allow pressure (but not vacuum) to flow to a hose barb on the intake manifold and eventually to the remote tank.

One has to completely block the flow through one of the check valves: I would suggest using a special form of thin aluminum sheeting underneath the intake manifold gasket in order to block-off the passage. I believe that this special sheeting is usually referred to as “beer can” material.

One then completely removes the check valve from the other passage,

Now, instead of having check valves metering pressure from both sides of the crankcase, one has pressure and vacuum from only one side of the crankcase (since the engine is alternate firing, leaving both passages open at the same time would result in pressure and vacuum pulses that cancel each other out.)

The fuel pump is then hooked up to the same nipple on the intake manifold that used to supply only pressure but which now supplies alternating pressure and vacuum, and the rest is easy.

For those intrepid-enough to try this, I would suggest making all changes to the engine in a manner that can be reversed; as I have said, I have heard of this system having an occasional problem. It also would be wise to have a parts breakdown handy to help identify and locate all the parts.
For those wanting further details, I will refer you to my review of The “Old Outboard Service Manual” posted in the book review Section of Duckworks.

Well, that about wraps-up this month’s column.

Later, dudes
Max