TUTORIAL: How to measure carb float heights on 308

Hi Everyone,
Time for another exciting tutorial! Many of you will remember my carb synch tutorial from a while back. In there someplace it says "make sure your floats are set right before you synch your carbs." But since that was a seperate operation, I kind of ignored it. Now I'm doing the tutorial on how to set your float heights. This is EASY can be done by anyone in about 3-4 hours, including removing the airbox.

WHY?

The floats in your carbs are very simple devices. They are just like the float/valve in a toilet that shuts the water off when the level is high enough. In the toilet, if your float valve didn't eventually shut off the water, the level would rise until it overflowed onto the floor. Likewise, if the float shut the water off too soon, it wouldn't be high enough for a proper flush and NOBODY wants that!

The float/float valve in the carburettor does the same thing. It controls the level of fuel in the float bowl. Too much or too little and the car will not run right because fuel will not flow as designed.

Before you begin, you need a carb rebuild kit. I got mine on eBay for about $60 (enough to rebuild all 4 carbs). You don't actually need the entire rebuild kit, but you do want to replace the gaskets under the carb tops before you put them together. While you have the tops off, you might as well replace the float valves too. Both of these items come in the rebuild kit.

The first step is to remove the airbox so you are looking down on all 4 carbs. This has been covered elsewhere, so I don't think you need a run down on this. (Basics: Remove the filter element, unscrew the (16) 8mm nuts, pull the trumpets off, and then remove the airbox, being careful to unhook the breather hose on the underside).

Now you are looking at 4 beautiul Webers in all their glory. If they look like mine, they are dirty. If they are clean, you need to drive your car more.

PICTURE 1
Here is your typical (dirty!) carb with the airbox removed.
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PICTURE 2
Next, here is a shot of the 5 screws you need to remove. What you are about to do is remove the top of the carb. The top comes off, complete with the float assembly. It does not require that you mess at all with the linkages connecting all the carbs. Nothing will get messed up doing this.

First remove the fuel lines to this particular carb.

Next, take out the 5 screws.

Finally, just lift the top off. Be gentle with the float.
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Here is what the carb top looks like. The float is hanging down from a pivot.
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Looking down into the carb (gasket removed) you can see the two throats (note that the left one is discolored from the engine "coughing" back through the carb...a common sign of running lean).

At the back, you can see the reservoir of fuel controlled by the float height.
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Here is a close up of the float valve. What you have is a little plunger with a cone-shaped point resting in a seat. The seat is the part with the hex head on it. The plunger is the part with the round top being pushed on by the tab on the float.

If you wish to replace the valve, you need to remove the float. Just push the pin that holds the float out. (In this picture, you would push the pin out away from you). Once the float is free, you can shake it to be sure that is hasn't leaked and allowed fuel inside. If it has, it's toast and must be repaired or replaced. Next, use a 10 mm box end wrench to unscrew the valve seat and replace with a new one, plus a new sealing crush washer. The washer is important to not only seal it but provide the right clearances, so you need it.

Once it's back together you can adjust the height. (next step)
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To measure the float height, you must hold it vertically as shown, so that the float tab is resting LIGHTLY against the plunger of the valve. You will see that when you hold it vertically, the tab rests lightly, but if you hold it upside down, for example, the weight of the float depresses the plunger and throws the measurement off.
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With the unit held vertically, use a clear ruler marked in mm (available at your local stationary supply place) to measure from the bottom of the float to the bottom edge of the carb top, as shown. (Much easier to use a clear ruler than an opaque one!)

It should be 48 mm. If it is not 48 mm, bend the tab that presses against the plunger until the measurement is right. Bear in mind that minute changes in the tab make a much larger difference in the float height.
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Now hold the carb top so that the float hangs down. At this point it is resting on a second tab, pressing against the side of the valve seat. This tab just keeps the float from hanging too low and hitting the bottom of the carb. This tab should be adjusted so that the measurement is 58.5 mm. That gives it a travel of 10.5 mm from top to bottom.

DON'T ask me how they decided to use 58.5 mm instead of a nice round number like 58, and GOOD LUCK getting it exactly 58.5. I'm sure that anywhere from 58 to 59 is fine.
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That's it! Your float is now adjusted. Replace the gasket on the carb top, and gently put the top back on the carb, being careful not to bend the float, and screw it back down. Now move on to the next carb.

A few helpful hints: when you remove a carb top, there will be fuel in the passages and valve. You might want to take it outside and give it a gentle squirt of air to blow the fuel out so it doesn't drip all over the garage.

Wearing nitrile gloves will keep you from smelling like a gas station attendant when you are done.

Be careful how you lay the fuel lines when you unplug them as they will drip too. You don't need them dripping on anything important like plug wires.

Happy adjusting!

-Birdman

 

I come from a motorcycle carb background where the needle valves are part of the "butterfly" so I thought I would call it something different. Anyway, you are right, it is a needle valve.

 

Yes! Thanks for reminding me! I forgot to mention that. On mine, the gaskets just fell right off, but I can see it being an issue if they stick to the tops. Remove the gasket before making the measurement or you will be off about ~1 mm. Not sure if that is a big enough difference to matter much, but the manual says to be sure to measure without the gasket.

Birdman

 

Just to re-emphasize what pma1010 said....never tighten anything too much on these. The metal is soft.

Also, another tip...

When you put the whole thing back together, turn on the fuel pump and open the butterflies while looking down all the carb throats to be sure that the valves actually work when the bowls fill. The last thing you want is a bad float (needle) valve letting fuel dribble down the intake manifold. (At the least the car will run 5000 times too rich. At worst, you will hydrolock the engine.) The chances are slim of this happening, but it's a good safety precaution. Just let the fuel pump run a minute and fill up the carbs, then peek down there with a flashlight and make sure there isn't a stream of fuel flowing!!

FYI, just checked the GT4 manual and they call them "needle valves" as well.

Birdman

 

Thanks guys, it's my pleasure. I just have to remember to take pics when I do something, then writing it up is the easy part.

Docweed, definitely sounds like your carbs need a proper synch and that should fix it. However, my experience is that a carb car always has a slightly wandering idle depending on how warm it is. My car has the cold idle device disconnected. It idles at about 500 RPM when stone cold, then goes up to about 1,000 when it's not quite hot but pretty well warmed up, then drops back down to 800-900 when fully hot. Going from nearly stalling to 2000 RPM is a bit out of whack!

Birdman