Float level advice, please.

[Captain Obvious]

2 minutes ago, duffman said:

I am having similar challenges to you, with my front carb needing to be severely tilted up to allow appropriate level of fuel in the bowl. 

I think it sometimes sticks on side of front bowl and fuel overflows.

duffman, Your results are about the same as mine. Able to bend the tab to get it "to work", but the tilt is uncomfortable and runs the risk of hitting the lid or inside of the bowl. Not to make it sound like I'm glad you've had problems too, but it's nice to not be the only one dealing with this.

I should be able to spackle the ballasts over to day and give it a day for the epoxy to fully cure. I'll run final results then and let you know!

[Duffman]

Look forward to your results!

[240260280]

I don't think the shape of the float that is submerged is an issue unless part of it binds. It is all about height of fuel below the bridge in the throat of the carb.

Is there a problem with binding that needs the weights?  Denser fuel will cause the float to sit higher.

 

 

[Captain Obvious]

3 hours ago, duffman said:

Look forward to your results!

Spackle is curing. We'll see how it all turns out!

So in the meantime, I had a pair of old round tops here. History is I rebuilt these a number of years ago (in 2011... My how time flies) and ran them on my car for a little while before swapping them out for a pair of flat tops. The point is... I had the float levels on these in spec ten years ago and they've been sitting dry in a box since. In theory, they should still be in spec.

Well guess what... Here's the front:

and here's the rear:

The floats have a neutral tilt when the valves close, but the level is way low. The bottom line is that the only thing that could have changed is the fuel.

So I guess that brings up a new question... "OK, so it looks like fuel has changed over the last ten years. Is it going to be like that from here on moving forward, or is it going to switch back at some point in the future?"    

[Captain Obvious]

2 hours ago, 240260280 said:

I don't think the shape of the float that is submerged is an issue unless part of it binds. It is all about height of fuel below the bridge in the throat of the carb.

Is there a problem with binding that needs the weights?  Denser fuel will cause the float to sit higher.

The only thing the shape of the float submerged matters much. I would theorize that you want the tapered nose because it would more gradually slope the pressure on the valve up and also not allow any trapped bubbles stuck under the float. But other than stuff like that, I can't see it mattering much.

The problem is, as you mentioned, denser fuel will cause the float to sit higher. A higher float causes the valve to close sooner, and a valve that closes sooner results in a lower level in the bowl. And the issue is that I've had troubles getting the level high enough without the float actually hitting the underside of the lid.

You remember this discussion you and I had almost ten years ago? I think this was all related to this same issue:

https://www.classiczcars.com/forums/topic/43889-fuel-bowl-level-and-bending-tab-not-working/

[Captain Obvious]

So I tried my ballasted floats today. Short story? They work great.

I filed the high spots off the spackle. Not that I really needed to, but OCD does things like that:

Then I set to trying them out... After a couple very simple, completely predictable tab bends, here's what I got. I set the level just a tiny bit low because 1) it will come up a little bit when there is a fuel pump pushing fuel, and 2) it will come up the width of the bowl gasket. Front looks like this:

And here's the front tilt:

And here's the rear after one completely predictable tang bend adjustment. Again, a little bit low on purpose:

Here's the rear tilt:

Based on the tilts, it looks like I could actually cut down on the weight a tiny bit. I'll make notes for if I ever have to do this again.

[Captain Obvious]

And just to make sure that my "in a glass" test method was valid, I tested again using the more traditional method with a piece of clear tube off the fuel nipple at the bottom of the bowls.

Here's my rig:

Here's the result for the rear bowl::

 

I did have a WTF moment with the front bowl... I put the fuel in, and it overflowed. I discovered that the float was stuck against the bottom of the bowl casting. Yet another fantastic feature of the long ear 72 design. So I emptied it and refilled while tapping on the bowl with a plastic screwdriver handle. That dislodged the float, and here's the result:

[Duffman]

Will be interested to see how they work in running car, though should be great.  I will use my Float Sync tool to see how level adjusts in running car.

Good job!!

[Namerow]

Here's a little math for you to ponder:

According to the Sunoco article that is referenced by CO in an earlier post, their then-current 'Supreme' fuel blend had a density of 5.95 lb/gal, while their then-current 'Regular' blend was heavier at 6.06 lb/gal.  So the Supreme weighed about 1.8% less than the Regular.

To make things easy, let's say that the Hitachi-SU float has a cylindrical cross-section and is constrained to move vertically.  In this way, a change in fuel density results in a change in float height that is directly proportional.

Let's now say that we're starting with the light Supreme fuel in the float bowl.  The top of the cylindrical float rises to a height of 'Y' mm above the floor of the float bowl.  Let's make Y = 50mm. 

Then we drain the float bowl and replace it with the heavy Regular fuel.  The float doesn't need to sink as low in this heavier fuel in order to displace its weight, so the top of our cylindrical float now sits higher (relative to the floor of the float bowl) by 1.8% and 50mm becomes 50.9mm.  That is, the float has risen by 0.9mm.

That kind of math doesn't fully explain the crazy float angles we see with the Hitachi-SU's in practice.  I think what's going on is a combination of a few geometrical factors:

1. The immersed part of the float is sightly conical, rather than cylindrical.
2. The Hitachi-SU float doesn't actually move in a pure vertical direction.  Instead, it's vertical path follows an arc (which may explain why the bottom part of the float was made slightly conical)
3. Once the float passes some particular 'tip-up' angle (relative to horizontal), I suspect that the change in the cross-sectional shape of the immersed part as a function of immersion depth begins to get really non-linear.   
4. To add to the non-linearity of the float behavior (hydrostatics?), the contact surface of the metal shut-off tab also travels through an arc (which may explain why its contact surface is curved, rather than flat).  It's not clear whether that curve accurately compensates for the arc in the travel path.  There may be, once again, a point-of-no-return, beyond which the tab-to-shutoff pin contact behavior goes non-linear. 

This would be an interesting simulation exercise for someone with the time and the curiosity to pursue it.  It's mostly 2D geometry (the conicity of the float makes it somewhat 3D), but it's hard to do on paper.  Easier with a CAD-CAE program (which I don't have).

[Captain Obvious]

Nice analysis. The geometry of the real system is definitely more complicated, but I think your simplified theoretical example is good enough for discussion here.

As for the float shape, I think the conical tip may simply be to try to assure the float doesn't hit the inside of the bowl as the float tips beyond neutral. Despite that cone tip though, you can tip the 72 long ear floats so far up that you will hit the inside of the bowl. But the 72 long ears were an afterthought.

So one might ask... "Why don't I just keep bending the tang down further to get the fuel level up?"

My non-expert answer would be twofold:

First, if you bend the tab down so far that the float contacts the underside of the lid, you're screwed.

Second, even if you don't have to bend the tab down that far, you can end up on what I'm calling "the downhill slope" of the tab. By that, I mean... You have crested the top of the arc where the needle valve tip contacts the tab. And because of that, much of the force on the valve tip is actually to the side instead of straight up. It kinda pushes the valve tip to the side instead of up.

Everything about the system indicates they wanted the floats to be close to neutral tilt. Doesn't have to be exact, but not to fat from neutral.

[Duffman]

I appreciate Namerow’s thoughtful analysis, many things to consider in the workings of this “simple” system!

CO, your points are spot on, in regards to the reality of floats that have to be raised to their max level, or beyond.  I have run into issues with both the float hitting the side and the tang/needle misalignment causing problems.  Looking forward to some live on the road testing to validate the float weight gain!

[240260280]

It's "Slosh".  When the car accelerates, the fuel sloshes to the back side of the bowl, when the car decelerates, the fuel sloshes to the front. This causes the float to change its height slightly. The acceleration also affects the float.

If acceleration of water places more at the back wall of the bowl, the water pressure will push the float towards the front of the bowl (put a rubber duck in a bucket of water then spin it on a turntable. The Duck will be forced towards the centre|).

It is a very complex physics problem and the key is that it affects both carbs in the opposite way due to hinge location asymmetry.

 

 

Edited June 16, 2021 by 240260280