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SU Sightglasses available (see fuel bowl levels)


Jarvo2

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Don't forget the front bowl level needs to be 2 mm lower than the rear to compensate for the engine tilt (so the fuel level in the nozzles is the same). Checked mine last December with the micrometer and got the same 3 mm from the ceiling to bottom of the lid. Set the rear 23 - 3 = 20 mm from bottom of lid, front 25 - 3 = 22 mm down. My front bowl was 9 mm too low before I fixed it with the sight glass.

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Nissan made the ears on the front lid and the front needle valve 2 mm longer than the rear one, at least on my 3-screws. IIRC, the Ztherapy video has something about that, and says you can use extra washers if you don't have the longer valve.

Used a digital angle meter to get slope of the driveway, the flat part under the door, and the motor, measured between the float bowl outlets and the nozzles. Did the trig - it rounds off to 2 mm difference required to get the same fuel level in each nozzle. BTW, if you're setting the floats on a driveway that slopes very much, it would throw it way off - got that from these forums, and it figures. If your car is raked that would throw it off too. Wonder what happens if you're driving a Z at Pikes Peak?

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Don't forget the front bowl level needs to be 2 mm lower than the rear to compensate for the engine tilt (so the fuel level in the nozzles is the same). Checked mine last December with the micrometer and got the same 3 mm from the ceiling to bottom of the lid. Set the rear 23 - 3 = 20 mm from bottom of lid, front 25 - 3 = 22 mm down. My front bowl was 9 mm too low before I fixed it with the sight glass.

Since the carbs get fuel from separate fuel bowls, the fuel level should be the same for both float bowls....engine tilt is not a factor.

I think the slosh of fuel to the back of the bowl under acceleration may affect the fuel float height and this may be different for front and back carbs because of the location of the float pivot however I never measured this.

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On the front carb the bowl is in front of the nozzle, so the tilt of the engine makes the fuel level higher in the nozzle. On the rear carb, the bowl is in back of the nozzle so the tilt makes the fuel lower in the nozzle - if it weren't compensated for.

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So, what is the right answer on where to measure the 23mm from? Is it the bottom lip of the lid, or the bottom of the inside of the lid? That appears to make about a 3 - 4mm difference.

According the the factory service manual diagram(pictured in post #2) It is measured from the inside of the float bowl lid.

When this thread(multiple forums) came up I decided to adjust my float bowls with a site glass. Before I was going to look into finding bungs that would screw into the drain plugs. Then I read arne's post and decided to use his method which is basically the same way the OP set his site glass up, only his sight glass is graduated making it easier to see where the level is. I used what I had from my miracle whip jar brake bleeder set up, which was a piece of clear tubing about 12" long and I marked the float bowl with a pencil where the level needed to be at. For the first time in 10 yrs when I move the mixture screws I can actually hear a change in idle unlike before when I had to turn the knobs down at least 3 turns. I just kept the mixture knobs at 2 turns down. It seemed to work, no pinging, good mpg and still a lot of fun.

When adjusting floats using the 9/16" dimension float to lid, it turned out I was 5mm too low compared to the 23mm dimension using the site glass. With the site glass I am within 1mm of stock specs. I still have work to do, reading plugs, figuring out idle settings when checking mixture, but at least I am close to be able to even use the mixture knobs.

The light colored plug was from the back carb at 2 turns down, the black one was from the front carb at 3 turns down, that was when the idle changed, both idle front and back sounded the same. To sync the carbs I used a hose and my ear. When I checked that against a unisyn, there was no difference.

back-frontplug_zps5617a224.jpg

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I think its natural for many to use the float bowl lid seam as the reference point, since you can actually see it, but the FSM (Engine Fuel, Fig EF-43 in the 71 FSM, picture in post #2) is pretty clear about it being 3mm above that at the ceiling of the lid. The diagram shown in post #1 has the reference I believe incorrectly showing at the lid seam.

Why they would pick that spot is a mystery, maybe its because the reference for the static float measurement is from there too? Fig EF-44 is a bit hard to interpret, but I think it uses the same reference.

post-12190-14150824445441_thumb.png

I was reading in one of the old posts I studied recently about this that Bruce says its doesn't matter too much if the float level is perfect, you're just adjusting the jet tube height to match anyway. Within very small limits this is true, but we are trying to get the fuel level at idle to sit at a very specific spot on the needle.

We don't have control of where the needle sits, its position is determined by the piston position which is vacuum determined. This is why Bruce and Blue and others have concentrated on seeing and setting the fuel level in the nozzle. This is the most important and accurate method, but hardest to actually do and judge, so the more practical (ok, lazy) of us tend to use the external methods with the site tubes.

About the tang length difference, Bruce uses the term "It's out and out bafflement!!".

http://www.classiczcars.com/forums/carburetor-systems-s30/45214-72-su-float-level.html post #14

I agree. Good thing it doesn't matter! Read on.

What he does make clear is to set both carbs at 23mm fuel level no matter what. What is never clarified in any post is the upper reference point for that measurement. I'm sticking with the FSM for the moment and my top edge of retangular side-thing position.

Other posts (looking at hybrid and zcar) mention the long ear vs short ear controversy. It is TRUE that you use different float level settings for the long verses short ears, but not different resulting fuel levels! I think this simple confusion of terms has lead to most of the confusion and angst about this subject.

Float level setting are the lid-to-float measurement you make with the lid upside down or right side up, however you want to do that, and they vary from 13.5 to 16.5mm depending on the tang length you have, (see last post here su identification/ parts wanted - Fuel Delivery - HybridZ) and this is a good starting point, but the resulting fuel level in the bowls is what matters, and is ALWAYS the same target, 23mm. Put your floats whereever they need to be to accomplish the fuel level target.

Now if only we had a way to ease the fuel level adjustment other than bending float tabs and all that goes with that process....

Edited by zKars
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Thanks Stanley,

At first glance I thought the drawing finally explained it however after some thought I think the process is more complex and more dynamic. Ultimately the fuel pressure at the bottom of the bowl summed with the venturi negative pressure drawing fuel at the jet are the two forces that must balance between the front and rear carb.

Here are some details to consider further:

- The fuel height in the jet is set by the pressure of the fuel in the bowl. Since the acceleration forces are nearly perpendicular to the surfaces, there is very little change in fuel pressure at the bottom of the fuel bowl.

- The fuel height in the middle of each fuel bowl will not change drastically as the fuel sloshes forward or backward thus the pressure on the exit at the bottom of the bowl's center will not change significantly.

- The floats will be affected greatly by the sloshing due to acceleration: the front float may close due to more sloshed fuel to the back of its bowl and the rear float may drop due to more fuel sloshed to the back of the bowl. If this happens, and the pendulum swing of the floats due to the acceleration forces are also factored in, the front bowl fuel level in the front bowl will not be replenished due to the closed needle valve and the jet will draw the bowl empty. Likewise the rear bowl will replenish too much due to the needle valve being held open.

- The vacuum force pulling the fuel at the jet due to the raised needle and associated high CFM venturi will probably pull the fuel with a force much greater than any pressure changes due to sloshing fuel in the bowl.

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I think I'm siding with Stanley on this one, at least in the static case. He's not illustrating the effects that dynamic motion have on the system, he's showing static fuel level changes due to simple rotation.

If you rotate about a point in the center of the carb, the jet tube physically rises (or falls) but the fuel level has to stay constant, so the fixed needle in the jet tube has to move away from or get closer to the fuel level! This is shown clearly in the drawing and I believe it represents reality.

Remember the critical thing that affects how the car runs is where the fuel level contacts the needle, so carb rotation angle seems to be VERY important. The entire tuning process is trying to get that relationship correct.

Technically you have to move the front and rear float fuel levels up and down to get back to a compensated average.

But the more important question that comes to my mind is, why does a Z work well when driving up or down a steep hill if angle is so dang important? Well there is a very good reason.

Here's where we move to the dynamic running case. The fuel does not sit a nice static level 10mm down the jet tube when the car is running. Fuel is being continuously drawn out through jet tube and is metered into the venturi by the diameter of the needle that is currently positioned at the jet tube top at the bridge, based on current demand. The float levels really don't matter at this point other than being in a spot that lets the bowls stay full, without the level getting too high. You still have to have fuel control to keep the static level BELOW the bridge or fuel will just pore out at low demand levels.

The exact flow rate/mass profile of fuel in the jet tube during running is a complex thing. I'm pretty sure that how much the carb is rotated doesn't have that much affect on how the car runs as long as there is fuel supply below the top of the bridge. Until the car is upside down in the ditch that is....

At this is my take on it. Static float level/fuel level settings are one thing. Dynamic run time flow regimes are quite different.

Longer tangs on the rear float bowl must have something to do with controlling the float motion or preventing it from binding in high angle situtations perhaps. Maybe someone took it up a steep hill and it quite running. Something that would be different in the back verses front due to the mirrow image configuration . I am pretty sure it was done for an engineering reason, not because the supplier screwed up and started supplying float bowl lids with longer tabs and they didn't want to waste them..... Ain't speculation fun.

Edited by zKars
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