fuel pump question

I hope this is not the stupidest question ever posted to this forum, but here goes.
What is the function of the mechanical fuel pump. When would I need to manually pump it? My pump handle does not move more than a half inch I'm not sure is working at all?

[Don Moyer]

Dear Unregistered,

Actually, far from being whatever you thought it was, your question gives me the opportunity to explain something that is long overdue, which is the details of the functioning of the priming lever on the mechanical fuel pump.

Anytime the engine is running, the mechanical fuel pump is operated automatically by a push rod that extends through the block below the mounting flange of the pump and rides on a lobe on the cam shaft. The push rod, as it follows the rotating cam lobe, pushes and releases a spring-loaded diaphragm which in turn moves fuel through the pump and on to the carburetor.

The priming lever duplicates the normal action of the push rod and provides a secondary method of operating the spring-loaded diaphragm of the pump. If you disconnect the fuel line from the carburetor and catch the fuel in a clean glass jar, you can operate the priming lever by alternately pulling it away from the direction of the block and then releasing it. Each time you pull and release the priming lever through its full spring loaded range of travel, a full diaphragm chamber load of fuel is moved from the diaphragm chamber into your jar.

If you reconnect the fuel line to the carburetor and work the priming lever until the float chamber in the carburetor is full, the diaphragm chamber in the fuel pump will remain fully charged (since there's no place else for the fuel to go), and at that point there is nothing else for the priming lever to do either. It will hang free from the bottom of the fuel pump.

NOTE: The priming lever imparts a bit more mechanical travel to the diaphragm than the push rod from the cam shaft. Therefore, you will feel a partial spring-loaded stroke on the priming lever if you work it after a normal shutdown of the engine, as it packs a bit more fuel into the diaphragm chamber of the pump. After one or two partial pumping actions, the priming lever will again become limp and hang free below the pump as before.

One of the more important advantages of the priming lever operation is that it provides a very convenient way to check the pump and the float valve in the carburetor. Once you operate it to fill the diaphragm chamber in the pump as in the above description, the priming lever should remain in its free and limp condition for quite some time. If you return within say 5 minutes and find that you can get a full spring-loaded stroke on the priming lever (or maybe two), you would have to wonder where the fuel has gone.

Here is a rather inclusive list of examples of where the fuel may have gone, listed in order of likelihood of occurrence:

1) A small disk type valve in the pump which prevents fuel from moving back upstream (in the direction of the tank) leaks a small amount and has depleted the fuel charge in the diaphragm chamber of the pump. This is one of the most harmless explanations, although if a valve leaks too badly it will eventually affect the pressure being maintained by the pump.

2) The float valve in the carburetor is leaking a small amount. You should be able to confirm this possibility by removing the flame arrestor and checking for the presence of raw fuel in the bottom of the intake throat.

3) There is a small leak in one of the fittings between the pump and the carburetor. You should be able to discover such a leak by running your fingers over each fitting as you would check for a pluming leak under your kitchen sink.

4) There is a pin-hole leak in the diaphragm which is allowing the fuel to bleed through the diaphragm, into the bottom housing of the pump.

Once in the bottom housing of the pump, fuel can flow through a small hole in the block designed to lubricate the mechanical levers of the pump in the lower chamber, and end up in the crankcase. This is obviously the worst case scenario. If fuel is leaking through the diaphragm, it will usually show up as droplets hanging on the bottom of the priming lever, and it will also show up in the oil, causing it to smell like fuel, and (in worst cases) cause the oil level to show a rise while the oil is being diluted by the fuel.

Hope this helps in your understanding of the mechanical fuel pump.

Don