Fuel Pump

reddog · Post by **reddog** » Mon Sep 29, 2008 10:19 am

Does any body know if there is a difference between an M38 and an M38-A1 fuel pump? If so what are the differences?

Thanks Bruce

Post by **Ryan_Miller** » Mon Sep 29, 2008 7:06 pm

There does not much of a difference from the outside.

Some M38A1 pumps came with a priming handle. I am not sure which ones or how many. Some more knowledgable people here could tell us.

I do think the priming handle at the pump was a much better idea than the primping pump at the dash as on the early M38's.

Post by **wesk** » Tue Sep 30, 2008 9:58 am

There are many differences in pumps. The very early pumps required a vent check valve modification and can still be found either way. When ordering parts overhaul kits you must have the pump's part number from the flange. The hand priming handle was an early M38A1 goodie and served only to help fill the fuel bowl when the jeep sat a long time.

As for the pro's & con's of the two entirely different pump systems one must understand each system's purpose. Keep in mind that pumping the gas pedal on a YS series carb does nothing since the accelerator pump function is vacuum operated meaning the engine must be producing a healthy vacuum to charge the system and then allow it's discharge.

Handle on fuel pump system serves only to fill the bowl in the carb. This does nothing for adding fuel in the cylinders during the start sequence. This is a carb bowl primer.

The hand pump on the dash is a real engine primer since it delivers a charge of fuel directly into the intake manifold. It is used to aid starting when the engine is cold or when cold weather starts have a tendency to turn the engine slowly which delays the ability of the carb's accelerator pump system to work. This is an engine fuel primer.

Willys stopped putting the dash primer pumps on as a cost saving move. The Army still field installed them when the jeep was required to serve in a harsh cold environment and then removed them when it returned to a warmer climate.

Army spares usually issued the latest Carter pump version for field replacement in either of the model jeeps. The depot overhauled pumps were usually overhauled to the latest configuration.

Post by **Ryan_Miller** » Tue Sep 30, 2008 1:03 pm

That is why I usually let Wes do the answering. I am more specialized in M38 stuff.

Wes as always has sage advice.

Cacti_Ken · Post by **Cacti_Ken** » Sun Jan 03, 2010 4:46 pm

I would like to bring this back up for discussion.

Are the pump parts ( diaphrams. oil seals, seal retainers, valves, gaskets, etc.) the same in the pump with the priming lever and the pump with out the lever?

Post by **wesk** » Mon Jan 04, 2010 12:50 am

There are several part differences including the fuel half diaphragm and the hand prime lever linkages and seals. The only safe way to buy a kit for these pumps is to order from a quality vendor supplying the pump number form the mounting flange.

I'd avoid NOS military kits and there are several different part numbers for those kits as well.