Background
Thanks to gravity and atmospheric pressure, water (and other fluids including gas) seeks its own level. What this means is if you take a tube and hold it in a “U” shape and fill it partially with water, no matter how high you hold one end of the tube, the water will be the same height on both sides. This is the basic function of the sightglass. The level of gas shown in the sightglass is the same height as the fuel in the float bowl.
Don't forget to reference the FSM (Factory Service Manual) which can be found at:
http://www.xenons30.com/reference.html
Setup Procedures
1.
Disconnect the coil plug going to your distributor and put the car in neutral, parking brake on, and chock the wheels. This will allow for cranking of the engine (powering the fuel pump) without starting the engine and without the car moving.
2.
Remove the air cleaner assembly, including the air horns (leaving just the carburetors showing)
3.
Using a caliper or accurate ruler, measure down 19 mm from the lip of the float bowl top and draw a horizontal line. You can use a pencil which is erasable, a permanent marker, or even a piece of blue painter’s tape depending on the quality/condition of your carburetor. Although the Factory Service Manual (FSM) states it should be 23mm, that is from the roof of the inside of the float bowl lid. 3mm is subtracted from this FSM figure to allow for easy measuring on the outside of the float bowl. This newly drawn line is what the fuel level “should be”.
4.
Place a dish or tray or disposable rag underneath the carburetor bowl
5.
Disconnect the fuel hose coming out of the fuel bowl. Gasoline (~4oz) will drip out. Dispose of the rags and empty the tray/dish
6.
Connect the Sightglass to the bottom of the float bowl and tighten the hose clamp. Do not over tighten as you will crush the nipple!
7.
Bend the sightglass up and tuck it behind the carburetor flange/body
8.
Adjust the sightglass up & down to line the mark on the sightglass with the line on your float bowl
9.
Repeat steps 1-8 above for the other carburetor (unless you’re just measuring one at a time)
Usage/Measurement Procedures
1.
Once the sightglass is setup as per the instructions above, activate your fuel pump
- For mechanical fuel pumps (stock 240z), you must crank the engine
- For those who have upgraded to electric fuel pumps, either active a hot-wire
switch or crank the engine
2.
Confirm nothing is leaking, if it is, retighten the hose clamp on the sightglass
3.
Continue to activate the fuel pump until the fuel level in the sightglass no longer rises;
this is the current gross-mixture fuel level setting for the float bowl.
- Fuel above the 22mm line indicates a rich gross-setting
- Fuel below the 22mm line indicates a lean gross-setting
4.
Once the current setting is identified and if not to your desire, you must remove the float bowl lids (4 screws)
- 3 screw SU’s require the float bowl lid to be rotated before they’ll lift off.
- You may need to replace the float bowl gasket if its old or you tear it while removing the float bowl lid
5.
Once removed, adjust the tang on the float itself to allow for more or less fuel in the float bowls. Additional detail can be found in Appendix A (FSM)
6.
Reassemble the float bowl lid and repeat steps 1-5 above and then move onto the next carburetor.
- If the fuel level was too “high” (rich) during the prior test, you will need to empty some fuel
from the sightglass/float bowl. The simplest way is to angle the sightglass down & let a
little bit of gas drip out into your dish/tray.
- I’ve found that by leaving the float bowl gasket out while fine tuning the float bowl
tang reduces the chance of damaging the gasket and/or wasting a lot of new ones.
Just remember to reinstall the gasket prior to full usage of the car.
7.
Once completed, ensure tray/dish, rags, etc. are properly cleaned up.
8.
Reattach the coil wire and proceed to fine tune the carburetors as per the factory manual.
I just saw this device which someone created on classiczcars.com. Very ingenious device! Basically its a 12 oz water bottle which has been cut with a line drawn parallel to the top and then rest the float bowl lid on top. This way you can see the fuel level and modify the float tang without having to mess with the gasket and pulling it out & back in.
