Installed NOS Fuel Pump--Oddity Observed

[A45WV]

The fuel pump in my A2 failed last week and I managed to procure a NOS pump. Removal of the non-op pump and installation of the new pump was completed without any issues. The engine starts, idles, revs and runs at speed--no problems.

But I took a look at my non-GI see-through fuel filter while it was running and noted that the filter is not filling (with a few bubbles only) as before. It seems to fill to about 1/3 full, and the fuel flows with a rhythmic pulse that's obviously in time with the engine. Here's a link to a video of what I'm seeing:

https://photos.app.goo.gl/bJkzTGGpukgtCkx38

Is this an issue? If so, any ideas on what may be causing this?

PS: yes, I need to adjust my valve clearance...long story.

[Horst]

looks to me as if the pump is not delivering enough fuel. Apparently enough to feed the engine but not enough to fully fill the bowl in the carburetor.

I have a similar filter installed and it is almost full when idling.

[Mark]

There is a volume test for the fuel pump, see if you can blow thru the filter.

[A45WV]

Agree. I believe the filter should generally be full. Before my previous fuel pump died, it would keep the filter full while running. Probably makes sense to check the line from the pump to the carb for blockage.

[Mark]

I believe they do get air locked, so loosen clamp on filter hose carb side, you should get gas coming out, I've had the same thing happen to me.

[Horst]

per the manual the test is for correct pressure. Disconnect the fuel line in front of the filter, connect a pressure gauge and start the engine (it will run with the fuel in the carb). You can of course also keep ignition off, in this case a second person should crank for about 10 seconds.
Pressure should be 5-6 psi.

[m3a1]

Even the best pump can't do a thing if its supply line is blocked. Verify that first.

[svramselaar]

hi
this is normal no problem at all
you will see if you block the return line it will be better at the filter
if it runs good with driving full throttle it is not a problem
no worries see below

https://encrypted-tbn0.gstatic.com/imag ... Q&usqp=CAU
https://t2.uc.ltmcdn.com/en/posts/3/0/3 ... 3_600.webp
https://vehq.com/wp-content/uploads/202 ... -view..jpg
https://forum.ih8mud.com/attachments/005-jpg.682435/

[rickf]

Those filters do not normally fill completely full due to the position of the inlet/outlet inside of the filter. It is set into the filter and not out at the very ends so are will normally be trapped in the filter. A lot depends on the position of the filter. And Horst, what manual are you seeing those pressures in? The normal pressure called for on the Zenith car is 3-5 lbs. and at 5 lbs. I have seen a lot of flooding issues on rough ground.

[Horst]

Rick,

Please find below. This is out of a newer version of the -20, the older version still talks about starting the engine but looks for the same pressure.

72E0F504-519F-4BE7-891F-9FED21531E18.jpeg

[rickf]

Interesting, Here is the blueprint for the 13660 carbs and you can see the max pressure is 4 1/2 lbs.

Carb print1.png

[Horst]

Haha, like in any other big development organization. People forgot to inform each other about a design change

[svramselaar]

hi
below the pressure from the 34-1 from 1982
greetings george

[rickf]

And here is the blueprint from the first series of Zenith carbs, the 12848. Same fuel pressure. I think what you found Horst was another example of a mistake in the military manuals! There was one in one of the early manuals stating the wrong float level on the Zenith carb that caused a lot of problems for people in the collector field.

carb print2.png

[Horst]

Of course there might be a mistake in the manual. However it is both in an old version and a much newer version of the -20.

I wonder what the pressure drop of the in-line fuel filter would be. That filter sits between the point of measurement and the intake port defined at the carb.

I think we all agree that in the case of this thread the pressure is most likely not on the high side but maybe on the low side. So I believe it is still worthwhile to measure the actual pressure and report back.