Michalak Discussion Group
is the moderator)
Types of tanks
Tank & fittings
& fuel pumps
Connector Repair Tools
by Max Wawrzyniak
Column # 2
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
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 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
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
˝ 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
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
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
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
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
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
Service Manual” posted in the book review Section of Duckworks.
Well, that about wraps-up this month’s column.