Technical Float height and fuel pump pressure

Maybe I'm overthinking this. Installed a new fuel pump and carburetor (2100) on the 292.

Set the dry float height as per the shop manual. But, the actual fuel height in the bowl immediately after use (never mind the pic here, fuel evaporates after a couple weeks) was way too high. The spec is 29/32" from the top of the carb to the fuel itself, about the width of a quarter.

The fuel height is what matters. So I bent the tang on the float a little bit at a time to get the fuel height in the bowl to spec.

But, in order to get to that point there's now hardly any swing in the float at all, maybe 1/8" or less, basically as it sits in the picture, (carefully!) checked it while idling and while the fuel height now stays in spec, the float doesn't really move up, it just sort of spews fuel constantly past the needle and seat. That can't possibly be correct is it?

Before tearing into this sort of figured the float acted similar to the tank and float when flushing a toilet. Fills up, and drains down after several seconds, and repeats. What I'm thinking is this new pump is way out of spec in terms of pressure and this is the result, it's blowing the needle off the seat?

 

Anybody? Bueller?

 

Yes, check the pressure. IDK about Ford pumps , but GM pumps vary from 1 1/2 psi to 15 psi and even if it's the "right" pump for the car, it can be crazy pressures. And I have not been sucessful with the round Offy style regulators.

But on the other hand, the fuel height is what matters. and the fuel level remains constant with fuel use, so yes it dribbles in as fast as you use it.

 

That 2100 looks a little different but there is a goofy little spring clip that holds the floats' axle or pin, it clips over the pin and locks it against the seat part of the N&S. If the spring clip isn't there then the entire float will rise (instead of pivoting on the pin) and fuel level will be way too high.

 

There is no argument that new mechanical fuel pump rarely meet the original pressure specs and cause havoc with carburetors. So, you can add a pressure regulator in the fuel line or do what I do and shim out the fuel pump.

Specifically, when fuel pressure is too high on my vintage vehicles I add fuel pump gaskets to shim the fuel pump lever further out thus lowering the pressure. This is a trial an error method (unless you have a fuel pressure gauge), but has worked well for me. I believe I learned this method from a VW repair book.

Recently I helped a friend with his Hudson and his fuel pump took 3 gaskets.

Just something to think about.

 

Thanks fellas, some things to think about. As a practical matter if the fuel height in the bowl is maintained it won't cause trouble? Shimming the pump, that's slick as hell, thanks for the tip! Easy as pie, cheap. Perfect.

It will bug me though if it isn't correct. Are inline fuel pressure gauges standardized fittings? That kind of stuff kicks my ***, open a catalog and there's 5,000 to choose from. All the gauges seem to be set up for rubber hose, I finally got away from that and bent up some brake line from pump to carb.

 

Good point, no the clip is installed, I just think the pressure (volume?) is way too high so the float and seat are overwhelmed. It's within specs but the float is basically submerged while in operation.

It may not matter a great deal, but fuel height in the bowl is important. It just seems to me at idle anyway, there should be a positive ON and OFF of the needle and seat, the float should ride high and sink, and the process start over. Like a toilet bowl for lack of a better description.

The 2100 is a neat carburetor, it is interesting to study operation, all the manuals are available, and sites like this to gather more info. View attachment 3287412

 

You can leave the top off of the carburetor and start it up! ( leave something over the needle and seat to control the "spray") ....you will see a steady flow, NO RISE AND FALL OF LEVEL!

 

The carburetor float does not work like a toilet bowl float. It doesn't fill up and then sink. It settles at a height that keeps the needle valve just open enough to dribble gas into the bowl at the same rate gas is exiting the bowl. This float level adjusts as you demand more or less fuel while driving.When you shut the engine down, the float will rise to completely close the needle valve and stop fuel flowing in.

 

OK.... I did that, as mentioned in the first post.

Just seemed strange that there is all this possible swing range in the float available and I had to really limit the travel down to almost nothing. The fuel level stays in spec though, and that's all that matters I suppose. Thanks for the information gents!

 

Gonna try this today, just spinning motor over cold for a few seconds it shoots to 8 psi. Spec is 3.5 to 5.5 psi

Should I aim for the low side or high side? Doesn't fuel pressure usually drop off a little after everything warms up?

 

Are you checking the pump dead head or are you checking line pressure?

 

Not sure what that means. Connected a vacuum gauge/pressure tester direct to the fuel pump outlet fitting. Cranked it over with remote starter for about 5 seconds, long enough to see it's out of spec a fair bit.

 

That would be dead head pressure.
Line pressure you need a T fitting in the fuel line, then check it running.

 

Hm. OK. What I could do is reconnect the hard line to the pump, start the engine let it idle on the fuel left in the bowl, and measure the fuel pressure exiting the line.

The engine idling PSI isn't going to change much versus just starter cranking is it? If anything, it would be even higher at idle, I'd think.

 

My way of thinking, you want to check fuel pressure with the engine running, so you need the gauge inline.
Doing it this way you can check pressure at any RPM.
When you dead head the fuel pump you are checking maximum pressure.

 

Ok, installed 3 thick gaskets. Didn't change output a lick, measures 8.5 psi at the line exit to the carb inlet. $@$&!!

 

not sure where you stacked the gaskets the gaskets and spacing up will not change the pressure.that 8.5 is too high even in our race cars 5lbs was plenty that was at 7200 rpm with a holley 4412.it means some plumbing but without a regulator you will never get it right.even 30 years ago fuel pumps were not perfect as too pressure had one that had 11lbs. brand new many others also out of spec.

 

Only accurate way to test fuel pressure is with the gauge T ed into the fuel line to carburetor.
Just hooking the gauge to the pump is only max pressure.

 

Guess I still don't understand this.

The vacuum gauge/fuel pressure combo unit I have, the directions disappeared sometime in the last century, I believe it is designed to be connected to the fuel pump output though. I did connect it directly to the end of the hard steel line, and it does read the same. I found the spec sheet for the equivalent Carter fuel pump (This is an Airtex), and max pressure is listed at 8 psi. It does meet that.


"Max pressure" or not, it is roughly 2 to 3 times too high. What am I missing?

 

I ordered a NOS fuel pump, it will require new diaphragm made with ethanol-tolerant rubber, but it will be interesting to see the output pressure. This over-spec fuel pressure is something to watch, as near as I can tell the only thing preventing m***ive flooding is about 1/64" of viton tipped needle & seat, and explains why the float has to be nearly bottomed out to achieve the spec fuel height in the bowl.

 

My Yblock fuel pump is right at 4psi with a 5/16" line and feeds Carter WCFB dual quads . Unknown on the diaphragm material but I add Stabil to every tank. No problems in 7 yrs. Good Luck..

 

Truck64,

I can relate to your frustration.

I just got off the phone with Carter Fuel Systems (1-800-342-6125) to discuss the psi rating of a pump that I run on my FE Fords. This pump can be used on FE, MEL, or Y block Fords. The part number is M3528 and the specs are 4.5 to 5.5 (max) psi. The reason I called Carter was that Summit Racing is showing this pump with a max psi of 8. Carter tech support confirmed that they had not changed the pump and it will work on FE, MEL or Y block fords and the max psi was 5.5. I hope this helps.

 

OP: Did you have an actual problem in the beginning? What was the car doing?

 

If you T that gauge inline it will show running fuel pressure, not the dead head pressure you are reading with the gauge just to the output of the pump.

 

I do not totally agree with the statement about the max pressure I take that to mean it will reach that pressure period not occasionally.even if occasionally it is still to high period if system (meaning carb needle and seat) are not designed for that pressure it will overwhelm and cause flooding.other than a drag car with multiple carbs or such would require that amount of volume at wide open throttle for a sustained time.the dead head mentioned is what the needle and seat see it will over come the needle seat once that happens the flooding starts no way to recover..

 

Checked the float height in the new carb before installation. Fuel height after installation at idle though was way too high/flooding. Look at that pic in the first post. Typically the float is roughly level or horizontal at rest. That float is actually about as low as it will go, in order to maintain the correct fuel height in the bowl at idle. This is probably to be expected for a fuel pump output pressure roughly triple the factory spec.

 

Lots of good information in this thread.

Some discussion about deadhead pressure vs running pressure with the fuel pump. Since the pump is a mechanical pump where the pump operation is keyed to the RPM of the engine, the difference in deadhead pressure to run pressure should be minimal (measurable, but minimal). This statement would be totally false for an electric pump, as the pressure and the available volume are more or less constant.

A fuel pressure gauge inserted in the line should that the pressure may be continuously monitored as suggested above would give the answer. One test is worth 1000 guesses!

As also mentioned, the fuel level in the bowl SHOULD remain very close to constant. The fuel valve and float should make this happen.

On a stock 2100, the following should not be a problem, but it MIGHT be, depending on the source of the rebuilding kit used. Fuel valves come in MANY different orifice sizes. The float exerts a certain pressure (buoyancy) on the valve. The pressure required to close the valve depends on the orifice size, as well as the fuel pressure. Some of the folks that believe racing carburetor parts may be used on the street often find they have flooding at idle when they install an oversize orificed fuel valve seat.

I have not checked the not-made-in-the-USA FLAPS kits recently, so have no idea if the orifice used is one-size-fits-all and is correct-for-none.

One can easily consult their shop manual and determine the correct orifice size.

Jon.

 

Right, I have the airtex equivalent of that pump. They look exactly the same fwiw. I know it will "work"; Have you measured the output on it? Thought about ordering the Carter version, but I'm getting a little burned out generally on all the "new" **** parts these days and don't feel lucky at this point. Seen this movie before.

Edit for clarity: So we'll see what this NOS pump puts out, pic below. Should be here tomorrow. C3TZ-9350-B Has a NSN on the box too, so it's some kind of service replacement .mil surplus from the early 70s or thereabouts. Carter branded.

 

I tested the output right at the pump with a fuel pressure gauge. It's almost three times the spec - just cranking the engine over at, maybe, ~ 200 RPM. How is that guessing?? It isn't going drop in output pressure at 600 or 2500 or 4000 RPM is it?? See where I'm goin' with that? Simply eyeballing the float from how far I had to drop it, I could tell it was way off the beam.