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Carb float setting tool

steveg

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Thought this approach might be easier than using a 7/16" drill bit or bolt to set the float level. Opening is 7/16" + .035" thickness of float lever:

SUfloatHeightTool.jpg
 
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Nice! Where can I order one?

So, the needle valves are spring-loaded, do we measure with the spring compressed, or uncompressed?
 

red57

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Great question Bob - I have often wondered that myself. I decided years a ago to set mine with the spring uncompressed, but don't actually remember hearing that was the manufacturers intent.
Dave
 
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steveg

steveg

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Great question - My picture is with the lever as resting on the Grose jet with the spring uncompressed.

The drawing in the SU HD tuneup manual shows the lever resting on the pin protruding from the needle.
 
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Steve,

The Grose Jets I have don't have any springs, just a (larger) ball bearing sitting on (under?) a smaller ball bearing; are you sure yours have springs? It would make a difference either way, depending if you set the height with the bowl lid upright or upside down.

My memory's vague, but I once asked a 'pro'--think it might have been David Nock but I'm not sure--and IIRC he said the spring should be compressed. I think it makes a fairly significant impact on the mixture setting.
 
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steveg

steveg

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Steve,

The Grose Jets I have don't have any springs, just a (larger) ball bearing sitting on (under?) a smaller ball bearing; are you sure yours have springs? It would make a difference either way, depending if you set the height with the bowl lid upright or upside down.

My memory's vague, but I once asked a 'pro'--think it might have been David Nock but I'm not sure--and IIRC he said the spring should be compressed. I think it makes a fairly significant impact on the mixture setting.

Bob - never seen anything about setting the float with the cover anything but inverted. Pretty sure mine don't have springs. Why would they with gravity doing the work on the ball bearing?

Now I'm confused enough to check out the spring or lack of same.

Addendum: No spring. Pressing on lever is same as lever sitting on top of ball of it's own weight.
 
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"... My picture is with the lever as resting on the Grose jet with the spring uncompressed."

... is why I asked :playful: Pretty sure your GJs won't have springs.
 
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Take the following statement with a grain of salt, considering how long it's been since my car's had SUs on it, but I too feel that the spring should be compressed while setting the float height.

The spring's tension is minuscule, IIRC, and the buoyancy of the float in a pool of fuel would be enough to compress it, raising the level of fuel in the jet well. I suppose it is theoretically possible to *tune* for the difference, by lowering the jet height, but that would leave less reserve in the float bowl(s), and engine smoothness under severe braking, acceleration and cornering may be compromised.

Pure speculation, of course!

For a comparison, Weber floats are set hanging vertically__pivot at the top__and measured WITH the gasket in place and the spring not compressed. Again, it takes very little to compress the spring-loaded pin in the inlet jet assembly (available in a variety of flow capacities) so that must've been taken into consideration when specifying the recommended float heights.

A final note on setting the float heights with multiple carbs: it is far more important that the float heights be equal for all units, than to worry if they're a touch high or low. Instantaneous throttle response depends on them all picking up at the same time, and the throttle-plate synchronization and float heights dictate this for optimum results. If the pairs__or trios__of cylinders had separate cams and crankshafts, then synchronization wouldn't be such a big deal ;)
 
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steveg

steveg

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Looking at my old HD6s,they have the spring-loaded needles.

Blowing into one of them and observing the following. The spring is strong enough the needle closes off the flow after the lever barely touches the pin. Also by depressing the spring all the way, the lever continues through about 1/4" more of travel. IMO if you set the 7/16" to be with the spring all the way depressed, the fuel would shut off quite a ways down in the bowl.

Someone else can try this out, as my grose jets do not have this phenomenon.
 
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steveg

steveg

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After setting my floats with the tool per above, a couple of observations:
Float weights: F - .7oz, R - .75oz
Fuel level down from rim, dial caliper: F - 1.639", R - 1.652"

I checked my old carbs and found another float at .75oz and replaced that into the front carb.

Will report back in a couple of days how that affected the fuel level.
 

Patrick67BJ8

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I just emailed a very knowledgeable SU carb guy and asked him “Should the float be adjusted with the spring compressed with the original type of float needle?”

His reply, “Yes”.
 
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I just emailed a very knowledgeable SU carb guy and asked him “Should the float be adjusted with the spring compressed with the original type of float needle?”

His reply, “Yes”.

IIRC, I concluded that the spring was there only to damp the float movements a little.

For contrast, I worked on this monstrosity off my dad's '55 T-Bird this weekend:

https://www.hemmings.com/magazine/hmn/2011/07/Holley-Model-4000---Teapot--/3701441.html

It renewed my respect for the simplicity and elegance of SU carburettors. Oh, and the needle valve on the Holly comes in at a 45deg angle (it's under the big brass nut).
 
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