View Full Version : Carter YF Fuel Pressure Issue; Restoring Ham's '48

04-26-2016, 11:38 AM
As we have gone through the ’48 Willys 2WD restoration, we’ve invented the wheel a number of times, so I am going to write this story so the next guy won’t have to do Wheel 101 like we did. Now, this isn’t really about a wheel, but you understand what I mean.

A whole bunch of problems have faced us as we have tried to get the truck running. A task that should have taken a couple of weeks has had issue after issue – one problem hiding another. One of the consistent problems that has dogged us from the first time we tried to fire the engine was the engine running rich – or having a fuel flooding issue with the carburetor.

Our original approach was to get a rebuilt carburetor. I had done a rebuild on the original, but I didn’t trust myself completely and shop time gets expensive so we ordered a rebuild. In the meanwhile, we also needed a new fuel pump. When we installed the new fuel pump and the rebuilt carburetor, we still had flooding issues.

One aspect of the problem was fuel pump pressure. The new pump delivered about 12 PSI at the carburetor – way too much. We exchanged the fuel pump for another unit, thinking we might have had a weird pump. The next one did the same thing.

We tried the trick of shimming out the pump on its mounting base, but we couldn’t get it far enough out to bring the fuel pressure down. (The Willys Manual specs about 3 1/2 PSI at 1800 RPM). At this point, with the second pump delivering ‘way too much fuel, we decided to try a fuel pressure regulator from the local parts shop, but it wouldn’t handle 12 PSI input, so we went to a more robust regulator. This brought the fuel pressure down, but there were still carburetor issues. Two trips back to the rebuild shop found “oops” in the rebuild, so on the third try, with the second pressure regulator, the truck was running fairly well.

But, not willing to leave well enough alone, I decided I didn’t want a big regulator on a 1948 truck, so we rebuilt the original fuel pump. It went together nicely and when we installed it, it delivered about 3 1/2 PSI to the carburetor. That cleaned things under the hood up nicely, and I took the truck for a tour of the town and gave my bride a ride in the “Limo”. After the tour, the truck needed to go back to the shop for punch list things, so I started out for the shop the next morning and got about a half-mile down the road before it quit on me. It went back to the shop in disgrace, on the back of a flatbed.

Once again, the issue lay in the carburetor. The problem we had seen over and over had been that there was liquid fuel dripping from a port at the top of the throat, causing very rough idle and even flooding. We had a long serious talk with the carburetor guy and, bless his heart, he built us a totally new carburetor.

We reinstalled the new carb – and the same problem was there. We did some checking and fuel pressure had gone up from 3 1/2 PSI to about 5 PSI at idle. Now, 5 PSI vs. 3 doesn’t sound like much, but its 50% over spec. And, that’s at idle – not 1800 RPM. There is just too much fuel being pushed into that carburetor!

Our first experiment was to put the little regulator back on. It would handle 5 PSI inlet, just not 12 PSI. We also took another Carter YF apart to see just what the little port that leaked fuel into the throat really was.

As we looked at the Carter YF we were studying, we found that the weeping port was the accelerator pump port. Fuel should only come from that port when the accelerator pump piston pushes a slug of fuel into the engine for acceleration. In normal operation, the passage to that port is blocked by a needle check valve that just floats in the fuel passage. It gets lifted by the surge of fuel from the accelerator pump. There is no spring to seat it, just gravity.

By now, the regulator was patched into place and we quit thinking and went to work. We fired up the truck and set the regulator for 2 1/2 PSI at idle. After a bit of idle mixture adjusting and letting the engine warm up – it ran as nicely as any F-134 ever does. Under acceleration, the regulator holds a constant 2 1/2 PSI (as it should – that’s plenty of gas for that engine). There is no drip of gas down the throat. The fix worked – now the question is “WHY???!!! “. What makes this carburetor so sensitive to fuel pressure?

So, we went back to the carburetor we were dissecting. After a bit of looking, here is our theory.

Fuel in the bowl of the carburetor is at the pressure of the pump output. This includes the fuel under the piston of the accelerator pump, fuel that normally just sits there waiting for the piston to squirt it into the accelerator pump channel. Under normal pump pressures, the brass needle check valve holds the accelerator pump passage and the port closed. (The float may not be able to close off the inlet needle valve at too high fuel pressure either).

BUT, if fuel pressure in the bowl gets too high for whatever reason – that pressure will lift that needle check valve and let fuel flow into the accelerator pump passage and port – even at idle. (Think about the jiggler on top of mother’s pressure cooker). We believe that when we set the fuel pressure down to 2 1/2 PSI, there isn’t enough pressure to lift that needle check – life is good.

I’ve not seen this problem described before, and our theory for what is happening may be wrong. But – I do believe the lesson is:


UPDATE: May 17, 2016:

My theory I so publicly offered is wrong. There is a vent that keeps the gas in the bowl only at atmospheric pressure once the float seals off the fuel inlet. Oh well ....

But, the YF is sensitive to inlet pressure for a number of reasons. In my looking I found what well may be the textbook on YF's and a lot of their quirks. I highly suggest these two as reading if you are considering rebuilding a YF for any Willys or Jeep vehicle.



Good luck - we're on the third carburetor for the '48..