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Subject: SU carburettor float levels

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I was changing the old cork seals in my V12 carburettors. When having the carbs on the table I was thinking about the two different heights of the float chambers. The one with the starting enrichment is higher than the other one (more than 1/2"). But in instruction manuals you read how to set the forks very precisely so that the level is always the same relativ to the top of the float lid.
Is this height difference neglegible or should the forks be set differently ? Or is the lower float chamber not the correct one?

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Subject: Re: SU carburettor float levels

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Peter, from our experience and observations some motors had the feature that yours has, others did not. It matters not, the critical thing is the level of the float forks should be the same so that at rest the level of the fuel in the jet is identical in each carburettor. To achieve this simply obtain a 1/2 inch bolt or rod, turn upside down the float chamber lid and place the bolt across the edges beneath the forks, then blow into the fuel inlet in the lid and if air will pass through then the needlevalve is not seating and a little more clearance betwen the bolt and forks is needed.
Mark and Jim.

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Subject: Re: SU carburettor float levels

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Thank you Mark, I agree if the float chamber lids have the same height, than the described adjustment of the forks gives the same fuel level relative to the float chamber lids. In my case this results in a fuel level on the right side (with starting carburettor) that is level with the jet and on the left side about 15mm below the jet, because here the float chamber lid is so much lower.

There are numbers on the float chambers of my carbs: 3255 HL on the right one (with starting enrichment device) and 3258 LL. I assume HL means high level or something similar and LL respectively. I will also cross check with another friend with a V12. He has sent me images showing that his float chamber lids are at the same level.

I wonder why this precise setting of the forks is described everywhere if 15mm difference are not important.

Subject: Re: SU carburettor float levels

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Here are two photos sent by yted, the left one shows the situation in my car, the right one is with equal levels which I think is correct. I measured my lower float chamber bowl again and found that the difference is in the distance from top of the bowl to top of the mounting arm.
As long as I do not find the correct bowl, (total height 70mm, distance between top of bowl and top of mounting arm 50mm, distance between middle of bowl to middle of mounting hole 60mm), I will make an extension ring to fit ontop of the lower bowl.

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Subject: Re: SU carburettor float levels

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Here is my extension ring. Now the distance from each float lid to the jet and thus the fuel level relative to the jet is the same on both sides (as far as I could check fuel level a few milimeters below the jet, which should be right, no fuel can drip in when the engine is not running)

I also attach a sketch to make it clearer.

No idea if these 10mm difference are important or not but I think it is correct now.

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