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Float level advice, please.


siteunseen

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7 minutes ago, Captain Obvious said:

Well I guess I'm going to test that statement about the new aftermarket valves being the same front and rear. I just put in an order on rockauto for rebuild kits for the 72. They call out two different part numbers for the kits and make it clear there is a different one for front and rear. So I'm going to soon see why they spec different kits for 72 F and R.

If it's not the needle valve length, then I don't know what it's gonna be. I guess I'll let you know in a little while when the kits get here!  LOL

Here's some aftermarket needle type.

20210418_093942.jpg

 

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59 minutes ago, Captain Obvious said:

 

@Terrapin Z, @siteunseen

When you guys get a chance, could I trouble you both to take a caliper measurement of the short valve going all the way to the top of the valve?

Also, do your short valves have the "R" stamped on them? Looks that way in the pics, but just checking.

👌

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Terp, Awesome. Thanks for the great pics. And your number matches the Hitachi valves I have here.

So assuming site's 72R valve comes in at the same dimension as what's in your pic, here's a summary of what I have so far. Most of this is old news, but just to put it into a form that my little brain understands...

The float valve for the 72F should be 20.5mm. That is 2mm longer than the previous years and Datsun adjusted both the length of the float ears and the depth of the valve recess of the 72F lid to accommodate that change. The intent seems to be "They wanted the front carb float level to be approx 2mm lower than it used to be in previous years."

The float valve for the 72R should be 18.5mm, and it's interesting to note that this is the same length valve Datsun used on the previous round tops in 70 and 71, but a different part number*. And again, Datsun adjusted both the lengths of the float ears of the 72 R lid as well as the depth of the valve recess to change the float level. The intent seems to be "They wanted the rear carb float level to be approx 2mm higher than it used to be".

*An interesting side conversation would be... So if the float valve for 72R was the same length as the previous years, why did Datsun bother to change the part number? Why not just spec the 70-71 valve for use in the 72R lid? I've got two theories about that:

First is that there's a note in the EF section of the 72 FSM about how they changed from a hardened steel tip to a rubber tip for the sealing of the float valve. Second is that I've got (what I believe are) early 70 valves here and they have a different through hole. The 70 valves I have 2.0mm through hole, while the later 72 valves have a 1.7mm through hole.

So while the length may be the same, they may have sprung for a new part number because of those changes. However, despite those changes, it seems to me you could use the same valve for the 70-71 round tops and the 72R. The 72F is still the outlier.

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  • 1 month later...

Arrgh... Stupid floats!! These things are working me over! I'm finding it difficult to get the fuel level high enough.

Is it conceivable to think that maybe today's gasoline has a higher specific gravity than the gas they were using forty years ago? Heavier gas makes the floats more buoyant and that drops the fuel level? Maybe some sort of EPA thing to slow down evaporation?

Or am I just losing my touch?  LOL

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A while back I looked through older SU (including Hitachi) books and forums about the float level heights. 
Many of the sites said the same thing. "The float levels aren't critical but the fuel level should be near 3/8" below the carb bridge". That would place the fuel level 9.525 nozzle turns down from the top.

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Well maybe it's extra floaty aftermarket floats or something. I'll post a pic of what I got going on when I get a chance.

The problem I'm having is that the level is too low. So I bend the tab down some. Level comes up some, but not enough. And then when I keep bending the tab down, I eventually run into the situation where the float mechanically bumps into something (underside of the lid or inside of the bowl chamber) before the valve closes, and it overflows.

Basically, I can't bend the tab down far enough to bring the level up to spec before I run out of compliance. It's like the float is too floaty.

I got tired of messing with it before and went ahead and bough all new parts... New aftermarket valves. New aftermarket floats. I was hoping it would all magically come together with the new parts, but no joy.

I've got some antique OEM (not aftermarket) floats here... Maybe I'll see how they float.

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I’m certain you would have considered this but is there any way of adding mass to them? For example a small self tapping screw at the very top? Or if not enough, making the hole slightly larger and adding lead shot, then sealing up with a bung, screw or silicone sealant?

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Yup. Absolutely considered it. That and/or taking material off the bottom of a floaty float.

But I guess my first order of business was to try to find out  if it's just me.   LOL   If it's just me, maybe I need to go back to float school.

I think today I'm going to try to measure the density of an old OEM float alongside a new aftermarket version. I've already weighed them (and they're the same to the resolution of my scale), but I haven't measured their volumes.

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Yup. Absolutely considered it. That and/or taking material off the bottom of a floaty float.
But I guess my first order of business was to try to find out  if it's just me.   LOL   If it's just me, maybe I need to go back to float school.
I think today I'm going to try to measure the density of an old OEM float alongside a new aftermarket version. I've already weighed them (and they're the same to the resolution of my scale), but I haven't measured their volumes.


In that case we shall follow your floaters with interest! :p
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I bought fresh gas (just in case the two week old stuff had started to evaporate, thereby driving up the specific gravity) and then I took some pics of the situation.

Here's my basic quick thrown together test rig. The carb lid is sitting on top of a small glass. The masking tape is just to provide a little stability to the lid so it doesn't slip off the glass:
P1190388.JPG

Here's and aftermarket float right out of the package. Tab looks like this:
P1190392.JPG

P1190393.JPG

And here's what the fuel level looks like with that float. This is for the front 72 (long ear) carb, so the target is 22 mm down from the bottom of the lid. I'm way low:
P1190389.JPG

So I bend the float tab to adjust the level. And after a couple tries, it looks like this:
P1190400.JPG

P1190399.JPG

The level is close, but still a little low. Big problem, however, is that I've run out of compliance. You can see in this pic that the float is so tilted that it's hitting the underside of the lid. A put a little extra pressure on the fuel inlet and it pushed past the valve and kept going:
P1190398.JPG

The other problem with the float being this tilted is that you can actually end up on the downhill slops side of the valve tang.

So I also tried an old OEM used float and got similar results. About that only thing I can come up with is the gas is higher gravity than it was forty years ago.  Fresh premium grade from the local station?

Surely it can't be just me. Can it? I've done countless floats in the past. Am I just losing my mojo? 

Edited by Captain Obvious
typo
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Nope. 72 Long valve. Right on spec.

And the rear set-up does about the same thing. For the pics, I just grabbed the long ear lid 50/50 chance.

On edit - But that's a really good question. Because what started this whole thing in the beginning was a mixmash of incorrect parts including a short valve on the long ear lid. I was hoping that with all new parts in the right positions, all the problems would magically go away.

Edited by Captain Obvious
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46 minutes ago, siteunseen said:

Run both on rear tops. 

I've got some Grose valves I will send you thanks of our hero @240260280

Thanks for the generous offer, my friend! If I can't figure this out I'll take you up on that.

I've thought about running rear tops, but they're giving me the same problem. Level is higher, but not high enough. Arrrrrgh!!

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I was thinking the same thing; maybe having the short ear version of the fuel bowl lid on the front carb would help get the desired fuel level.  But, you are saying that won't make a difference, right?

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If I get a chance, I'll put the rear lid (short ear) together and take a couple pics.

Both lids have complications. The short ear lid has the float pivot point closer to the underside of the lid. That means there is less float tilt available before the float hits the lid.

The long ear lid allows more float tilt before the float hits the lid, but there's another problem...Since the whole system was designed originally with short ears, the shape of the float was designed to never bump into the inside of the bowl. But if you go extreme tilting of the float (further than it was ever designed to tilt), it WILL contact the inside of the bowl.

The bottom line is... Too much tilt is a bad thing.

And as an anecdotal piece of info... If you look at the pics in the old FSM's you can see where they "expect" the fuel level to be on the float when there is enough pressure to close the valve and the relative tilt on the float. Note that the float is shown as pretty much no tilt. Float is "neutral" and from a design standpoint, I think that makes a lot of sense:
float level.jpg

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