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Float level advice, please.


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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.

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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:

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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).

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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.

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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!

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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 by 240260280
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On 6/16/2021 at 10:01 AM, duffman said:

Looking forward to some live on the road testing to validate the float weight gain!

Me too, but the on-road analysis might take a while until they end up on a car. You want me to send you the "heavy-duty" floats and you can try the whole thing out on your car?

I'm actually comforted by the fact that someone thousands of miles away with different gas is having the same issues. Just another data point aiming at changes in the gasoline formulation.

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17 hours ago, siteunseen said:

With that old Jetson's phone you have take an extra battery  and charger along for the testing. Maybe some flares and a blanket or two. You can keep warm with one and send smoke signals with the other.

ROFL  Hahahaha!!! As a matter of fact, I've been informed (by the wireless carrier) that my old phone won't work for very much longer and they are sending me a new phone. And get this... The new one is a 4G flip phone!! LOL!

I read the reviews for the new phone and they're terrible... Everyone says the screen is really small and the apps that you can put on it are crappy. Well duh. Sounds perfect.

And speaking of my old phone... Now it's getting hot when I charge it and the back is a little bulgey. So I won't need any blankets to keep warm. The lithium fire will be plenty warm. Now I just need it to last long enough for the new replacement to get here.

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17 hours ago, duffman said:

CO, I would be more than happy to test your new “bloats”, but I need the temps to drop below the 117 it is today!!  Let me know if you want to send my way.

Bloats. Love it!

So yeah, I'd be happy to have you test the bloats. It would add one more data point as to the validity of the modification. I'll send you PM!

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  • 4 weeks later...

I have been away for a few months but really enjoyed this discussion. Adding weight to the float is a really interesting idea that I will keep in mind. With added weight, you could trim the side of the float so you eliminate the problem with the float hitting the bowl on the front long ear carb. I have shaved a float before but didn't think of adding weight. FYI, I shaved it by sanding it. Solved the clearance problem but...reduced weight. I know we all end up solving our float adjustment problems in different ways but here is what I have settled on.

I use two rear lids and two short needle jets. The long ear, long needle valve 72 idea for the front carb was....to me... a mistake.

I don't pay any attention to measuring how many mm down from the top of the lid that the fuel level should be. I remove the piston domes, pistons, and drop the nozzles down 10 full turns. Then I set the float tabs so that the fuel sits at or just below the top of the jet.

I tune with a colortune. O2 sensor would be better and I will try that some day.

One last thing. Take a look at some of the photos of needle jets earlier in this thread. Notice that the stock original needle jets have a needle that is larger in diameter, is nicely rounded at the tip, and for me at least moves up and down more precisely. New needle jets are cut straight at the end, are thinner diameter, and rock around a lot. I think this, combined with the curved tab come together to make it really easy to have new needle jets get tripped or stuck on the curve of the tab. Just my theory.

Anyway...great read. The fuel density theory and idea to add weight are really interesting angles to think about.


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  • 2 months later...

CO was kind enough to send me a trial set of "bloats" to test on my Z.  I have a 2.8L engine rebuilt with flat top pistons, standard cam, N42 head and block, 6 in 1 header, short ear fuel bowl lids on both front and back SUs, etc.  I have some experience playing around with SUs and Webers, so this was fun for me!  My standard floats weigh around .4 oz, and CO sent me a set weighing .7 oz.  My initial efforts to get the fuel level set, using my set of FloatSyncs, proved to be most difficult, and after multiple failed attempts, went back to CO to discuss options.  He thought going to a lower add on weight might prove to be a solution.  So, he redrilled the "bloats", took out the heavier weight, and added a lighter aluminum plug.  The weight of the mini "bloats" was .5 oz (25% more than my standard float).  This proved to be a much better solution, as I was able to quickly dial in the appropriate fuel levels in each carb, while having the tabs of each mini "bloat" set at a level where I don't have to worry about the float hitting the side of the fuel bowl to reach a full level.  I will leave it to the industrious Captain Obvious on how he created the mini "bloats", but I believe it is a great solution for those with a similar problem!!  Thanks, Bruce!

Now, on to finding the perfect SU needle for my Z ... 

Edited by duffman
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  • 10 months later...

Very interesting read and right on target for I’m dealing with . I’m a little intimidated to boring holes in my SU’s , but might have to do that . I’m building a high performance engine for a customer and I can’t get the floats to comply . I’ve been at for days - literally . My floats are slammed against the lid to get proper level using a float sync. 
I can’t afford for this engine to go lean . 
Instead of a fancy slug I was thinking a coarse screw into the side of the float . Maybe a brass vs an aluminum would give me options as far as weight - even steel . 
Any thoughts on this approach ?

I am using two short lids hoping I could duplicate my  approach easier 

Edited by madkaw
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My thoughts? I spent a bunch more time trying to get the 72 floats to work without adding weight and I failed. Nothing I tried could get them to work, so I added weight. Again.

Here's some pics of my latest incarnation. This time I made a threaded insert so I could change the weight in the future if necessary:






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So my inserts were made on a lathe (because I have a lathe and I'm not afraid to use it), but you could probably find something store bought that would perform the same task.

My thoughts on that are... Brass has higher density than steel. And steel has higher density than aluminum. I used brass because it was the highest "bang for the buck" in the density department meaning that the insert could be made smaller to get the highest mass (as compared to steel and aluminum). If for some future reason, the ballasts I put in there are deemed too heavy, I can switch out to steel or aluminum in the future to lighten them up.

I'd be happy to talk on the phone about your frustrations. If you want, send me a PM and we'll do some of that. I feel your pain.  LOL

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Adding my 2 cents, I believe that CO created a great solution for the wayward float!  The "bloat" works great on my setup, and I have no calibration issues like you are running into, Madkaw.  I highly recommend the concept!  

Bruce, I like the new screw idea!

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Just got off the phone with my car friend and PHD physics guy. His thought was as long as you had a reasonable amount of fuel in the float bowl, the Venturi will pull the fuel pass the jet fine . It’s more critical that the jet and needle are the same on both carbs than it is to get max amount of fuel in the bowl . I like the idea of the floats being more level , but it doesn’t add any more fuel to the bowl ( volume ) . I think getting the float needle to sit in the float tang properly is important too - especially with these flat tipped float needles

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I believe the importance of getting the float level-ish is to prevent it from hitting the underside of the lid.

Here's a description of the problem I've had. This is what was happening that made me add some weight.

Level is too low, adjust the tang.
Level is still too low, adjust the tang some more.
Level is still too low, adjust the tang some more.
Bam! Carb overflows.

At that point, the float is probably hitting the underside of the lid, or maybe even or the inside of the bowl if the angle is extreme enough.

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