Diaphragm Fuel Pumps

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Diaphragm Fuel Pumps-for Two-Stroke Motors

Snowmobiles (and many other types of two-stroke cycle powered machines) typically draw their fuel from a remote tank. The device used to "pull" or pump the fuel is often a rather simple diaphragm-type pump. It is simple because there are essentially no mechanical parts nor electricity involved. The pump works through the use of varying air pressures and the flexing of malleable internal parts: diaphragms.

The pumps usually found on snowmachines (snowmobiles) look like these:

Fuel pump ID.jpg


The very nature of fuel systems and fuel pumps implies that fuel will be exposed. In this case we are talking about gasoline, a very volatile and flammable material. Be aware and be careful. Avoid any sources of ignition while working with gasoline!

The Parts

T pump parts.jpg

These pumps are made up of three metal castings with a diaphragm separating each casting: three castings, two diaphragms. In this picture, the casting at the top, the one with the two wings with holes for mounting, is considered the pump's bottom. The casting to the left is the pump's top. The larger casting is the fuel transfer section. While they may appear a bit bewildering to look at, these pumps are very simple. Hopefully you will understand how they work after reading through the next sections. While they have a "top" and a "bottom," these pumps can work in any position.


The pump uses a series of alternating positive and negative pressure impulses, about one hundred times per second, to pull fuel from the tank and push it into the carburetors.

The engine does this by supplying "blow" and "suck" impulses from the crankcase. (The crankcase on a two-stroke engine is unvented in order to created the needed negative pressure or "vacuum" which pulls fuel into the engine. Most carbureted two-stroke motors draw the fuel/air mixture into the crankcase before it is "pushed" into the cylinders.)

When the piston is driven down by combustion, the space under piston is compressed. This creates a positive pressure which forces fuel up through special passage-ways into the combustion chamber. A small part of this pressure is tapped off from one side of the engine's crankcase in order to operate the fuel pump. It looks like this:

Engine impulse.jpg

Once the fuel charge has been pushed out, the pressure essentially returns to zero, so, as the piston moves upward, there is a negative pressure under the piston. This is what draws or "sucks" fuel in from the carburetor. Again, since one side of the engine has the impulse tube which is connected to the fuel pump by a hose, a small part of this negative pressure is used to "pull" on the pump.

The Diaphragm

Impulse Diaphragm.jpg

The insides of common diaphragm pumps look like these. The pump at the top is a Mikuni, the lower one a Taiyogiken. Both are used on Polaris and other machines. Both work the same way. The impulse covers are on the left. In the center of each cover, one can see the hole which allows the engine's positive and negative pressure impulses to enter the pump. There is a diaphragm between the cover and other parts of the pump. On the upper pump, it is lying on the cover; on the lower, it is on the fuel chambers. The engine impulses never directly contact the fuel. (The impulses, even though they come off from the engine, are not hot since the pressure is taken off from the crankcase, not the combustion chamber. The crankcase never gets very warm since it is constantly being cooled by incoming air and fuel charges.)

Fuel Transfer Block

Main block impulse.jpg

The fuel transfer "block" is the central section of the fuel pump where fuel is moved into and out of the pump. There is a diaphragm sealing both sides of this pump section so that only fuel - no air- moves through this section - and no fuel gets into the other sections. It works on the principle that a fluid can be pushed past a flexible membrane over a hole, while the same membrane can quite effectively seal the hole when suction is applied. (Think about what would happen if a piece of paper were held against a straw you were blowing and sucking through; blowing would be easy, sucking would pull the paper against the straw's end, sealing it.) There are two transfer-ports or "holes" in the picture above. The upper transfer-port is covered on the visible side by a flexible plastic disc which prevents fuel from being pulled downward. The near port is covered underneath by the same type of disc and will allow fuel to be pulled through.

Main block back.jpg

This is the reverse side of the same pump section. It might also be considered the pump's bottom side (although this type of pump will work in any position). The fuel inlet (from the tank) is on the left. The impulse diaphragm (which is on the other side) moves out and in as the pressure in the impulse line goes low and high (relatively).

It is, perhaps, easier to visualize how the pump works when looking at it from this side. Fuel moves from one side of this transfer block to the other depending on which side has the lower pressure relative to the other. When the impulse diaphragm (on the other side) "pulls," fuel from this side of the pump to try to get back through the transfer-ports to the other side. Since the upper transfer-port in the picture is covered on this side, that cover will be pulled down more tightly. The near port is covered from the other side. Since the cover is flexible, it will be pulled downward which will allow fuel in the near section to be pulled through. In actual use, this side of the pump is covered, however, which means that the pressure will pull on the only opening still available, that is the fuel inlet (from the tank), the "inlet" tap to the left. On the "negative pressure" portion of the engine's impulse, fuel is pulled into the pump. When the pressure reverses - becomes positive on the reverse side and negative on this side- the opposite happens; the flexible cover on the near port is pressed against the port opening and the cover on the upper, visible side will open, allowing fuel to move into the upper chamber - and through it. It is pushed out the two taps on the right side of the picture. They are connected to two separate carburetors by their respective fuel lines.

There is another diaphragm needed to make the pump work well and reliably. It is a diaphragm covering these two chambers. Since fuel cannot be compressed, two enclosed, air-filled chambers provide some flexibility or "bounce" to the impulse pressures when the carburetors are full and will not allow fuel to be moved. (The pump has to accomodate the pressure pulse even when the carburetor bowl and the fuel lines are full.)

Buffer side.jpg

Cross Section Graphic of Operation

The graphic below shows a cross sectional view of the basic pump parts and their relative positions while in operation.

(The red arrows indicate fuel flow and its direction of movement. Fuel moving into the pump comes from the tank; fuel moving out is moved toward the carburetors. The blue arrows represent impulses which are generated by the engine's crankcase.)

Diaphragm Fuel Pump 1.png

Problems: Prevention and Correction

These pumps are long-lasting, require little maintenance, and can give thousands of miles and many years of trouble-free service. It is important that both diaphragms are completely intact, free of cracks or pinholes, and that stray particles or sediment do not get under the fuel port covers.

Here are a few things that will help to maintain them.

Besides causing problems for your engine and carburetors, moisture can condense and freeze or cause corrosion sediments to form in the pump. Either of those can cause pump problems.

Added regularly, a few ounces of isopropyl alcohol - two to four- will help keep your fuel system "dry". (Do not use methyl alcohol in two-strokes.)

A warm engine's parts fit well. When the pistons and rings seal the cylinders well, there will be less blow by from the combustion chamber. Burning fuel produces water vapor which you don't want being pushed into the crankcase. (If it's in the crankcase, it will also end up in the impulse line).

While it is better not to store them with fuel that may get stale and change into varnish or sludge, trying to start a machine with dry fuel lines can be difficult. Treated (stabilized) fuel doesn't seem to cause any harm to the pump parts. Neither does a light mix of injection oil in the last tank before storage. Perhaps the light oil coats and helps prevent corrosion of metal parts. Pouring a few teaspoons of gas* directly into the cylinders to fire them avoids excessive pulling or cranking, and is a good way to get empty pumps and fuels lines going again. **Make sure you wipe up any spilled fuel, and do not pull or crank the engine without the plug caps re-installed. Any spark can ignite fuel vapor!

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