Float level advice, please.

[Patcon]

What about trimming the pin on the float valve? A little should go a long way

[Namerow]

I did a similar bench-top experiment a couple of years ago (I was bored and curious -- a dangerous combination).  I discovered that it didn't take much tab-bending to generate some pretty wacky float angles at shut-off.  The 'cam-to-follower' geometry seems to be pretty sensitive.  I thought about creating a little trig simulator on my laptop, but decided that I wasn't quite that bored.  Or curious.

I also tried weighting the float as a secondary adjustment strategy.  It worked, but I didn't like my chances of gluing a dime onto the float and having the adhesive last more than a week or two. 

It would be fascinating to learn how the Skinner-Union and Hitachi folks did the design layout for the float and linkage in the first place. 

The whole thing just begs for an external adjustment screw.

But were would the charm be in something that sensible?

[Captain Obvious]

Thanks for the input guys. I considered both shortening the needle valve and adding weight to the floats. At this point, it seems like I'm going to have to do something like that even though (in theory) I should not need to. I keep coming back to changes in gasoline formulation and density.

Namerow, I completely understand what you're talking about with the wacky geometry. That's one of the things that has me wanting to keep the floats "about neutral" at valve shut-off. Because if you bend the tab down so far... too far... you can actually run the risk of the geometry changing direction and opening the valve back up again at extreme float angles. the valve pin kinda "slips off the downhill side" of the tab - if that makes any sense.

I'm not sure you can achieve that extreme of an angle with the short ear lids, but with the 72 fronts you can. I need a way to keep the floats more neutral.

Next chance I get, I think I'm going to mess around with adding some weight to see if I can fix this.

[Patcon]

There is a product called "Seal-All" I believe. I once used it to seal up pinholes in an F350 fuel tank. Very fuel resistant. It might help to affix extra weight. Its like a cross between model car glue and clear caulk

Edit for correct product name

Edited June 10, 2021 by Patcon

[Captain Obvious]

I messed around a little today with additional weight on the float. I wired a chunk of scrap steel to the bottom of one of my new aftermarket floats. The float weighs about 9-10 grams and I added about 9 grams of extra weight. For the test, I just wired it into place so it wouldn't fall off. Looked like this:

I put that contraption on the 72 short ear lid. Float tab looked like this:

Since this is the rear lid, it's still one mm low, but there's plenty of compliance in the tab bending to bring it up a mm:

Here's the float tilt. Very close to neutral:

And then I took the same float, with the same tab and put it into the front long ear lid. The result was this:

With a darn near perfect tilt as well:

So it's nice that the theory and geometry agree with reality.

But still the big outstanding question is.... Why do I need to go through these lengths of adding weight to get the floats to behave properly??? Special extra-dense low volatility North-East summer blend gasoline?

[Captain Obvious]

Oh, and before anybody asks "Why did you pick 9 grams of extra weight?"

The answer is "because that's what I found in my scraps box."

[siteunseen]

Looks like a 3/4" PEX crimp ring.

[Captain Obvious]

So today I decided to add some weight in a more permanent way...

First of all... Anyone ever wonder what's inside the float? How is the mounting tab anchored? Well, it looks like this:

So armed with the knowledge that the mounting tab does not extend very far down into the float... I drilled hole in the float:

And made a brass slug to press into the hole:

Rinsed and repeated for the other float and now I have these:

In case anyone wonders... The little threaded hole is so I can run a small screw into the brass ballast slug as a removal handle. I figure that until I'm sure about the amount of weight, I might want to pull it back out. And since the hole is blind (I didn't drill all the way through), there's no way to grab it. Works like this:

Here's the results in the front carb (long ear). Level is a little high, but that's easily fixable with a little tab bending:

Long ear float tilt looks like this:

And the rear (short ear results):

With it's corresponding float tilt:

Again, it's fantastic when the theory lines up with realty. The plan now is to spackle over the ballast plugs with JB Weld since it's fuel resistant and then do the final adjustments.

I still have no answer to "Am I the only one that has this problem", but at least I have a solution.

[Namerow]

9 hours ago, Captain Obvious said:

Anyone ever wonder what's inside the float?

Your willingness to destroy an old float 'to see what's inside' has led to an important discovery:  they're not hollow.  If someone ever proves that the specific density of modern fuel has changed relative to that of 1970's-era fuel, then this process of ballasting the floats might become a standard part of rebuilding Hitachi-SU's.  I'd be curious to know what Z Therapy have to say about this, though.  One assumes that they've successfully set the floats on hundreds of H-SU's by now and I don't think I recall any discussion on their part of special tricks being used to cope with modern gas.

[Captain Obvious]

Agreed. From what I can gather from the various discussions about setting the float levels, I'm not the only one that has had difficult to explain results getting the levels correct. But i think most people just bend the tab and eventually get it close enough where it will work and they get tired of messing with it and call it a day. In fact, I have done that in the past. But this time I just couldn't get it close enough to even call it a day, so I started digging deeper for the root cause.

It could be that the original floats are now on the knife edge of being able to be adjusted into spec due to changing fuel over the years. And I'm thinking that maybe these 72 carbs (which have slightly different float geometries than the previous years) are closer to that edge than other years?

I'd be interested in having carb experts weigh in as well. I'm pretty sure they aren't ballasting the floats.

I also wonder what would happen if I checked my float levels with gas purchased in some other part of the country far from where I am.

[Duffman]

Interesting stuff, CO, I am looking forward to your final results on the weighted floats.  I have bought, fixed up and sold 12 to 15 sets of SUs over the years, always testing on my own car for appropriate fuel levels, performance, etc.  I needed to bend the float further than what directions said but still within range, and no floats hitting side of float bowl.  On my own set of SUs, 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 run same level of fuel in each bowl, no difference in front and back levels.  I think it sometimes sticks on side of front bowl and fuel overflows.  Your solution may be a great answer for this!  Good luck!

[Captain Obvious]

I did a little digging into the subject of gasoline changing over the years, and it's absolutely conceivable that there have been density changes as part of pollution reduction actions over the years.

Not my area of expertise and I don't know what I'm looking at though, but here's a small snippet of the info out there:

https://www.sunocoracefuels.com/tech-article/specific-gravity-matter

https://www.epa.gov/fuels-registration-reporting-and-compliance-help/gasoline-properties-over-time

https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100B3YI.pdf

https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100T5J6.pdf