SU HS6 Tuning

[woodward]

Hi,

I'm looking for some advice on tuning HS6 carbs on a TR4.  I've always been concerned the car was running rich, so when the front bowl began to leak from the jet connection at the base I decided to check the settings on the carbs (following re-setting the tappets, timing and points).  My question revolves around the level in the jets - I've set the float height to 3/16" from the top (plastic floats) when the lid is upside down in both carbs.  However the fuel level in the front carb is probably 1/8" or so below the bridge, and in the rear carb greater that 1/2" below the bridge.  I assume that a large part of this is because the engine is on a slant and the front float bowl is above the carb, whilst the rear one is below.  First question is whether this matters or not and whether I should strive for them both to be at the same height?  And secondly, does this mean that the jet in the front carb should be set at a different height from the rear?  Or are these two things independent?  The standard tuning method the Tuning book recommends having the jets at the same height so assuming the latter?

Thanks for any advice from the SU experts out there.

Myles

[Lebro]

I went to great lengths to get the fuel height the same at the jet, this did require different float heights, as you say because the engine is on a slant, & with the HS6's one float chamber is in front of it's carb, & the other is behind. Having said that I don't believe it makes a huge difference, & altering the jet height will easily offset the difference in fuel height in the chamber.

This from the Bulin web site:

Float chamber fuel level

The fuel level on an S.U. is not critical, and need not be treated with meticulous accuracy-the normal level is 3/8 in. under the rectangular inner facing known as the jet bridge, but this is rather difficult to observe even with the suction chamber and piston removed and the jet fully dropped. However, a simple mechanical check can be made, and this consists of sliding a certain diameter of check rod between the lid face and the inside curve of the forked end of the hinged lever when the needle valve is in the 'shut off' position. The size of this rod for both the 1 7/8 in outside diameter smaller float chamber and the larger one of 2 1/8 in outside diameter is 7/16 in. On the HS type of float chamber a 5/16 in. rod is used with a brass float, and a 1/8 in. rod when the hinged nylon float is fitted. If the hinged lever fails to conform within 1/32 in. of these check figures it must be carefully bent at the start of the fork section, in the necessary direction for correction, taking care to keep both prongs of the fork level with each other. It must be emphasized that it is not advisable to alter the fuel level unless there is trouble with flooding; and although a too high level can cause slow flooding, particularly when a car is left ticking over on a steep drive, it should be remembered that flooding can also be caused by grit in the fuel jamming open the needle valve, or undue friction in the float gear, or excessive engine vibration, or a porous float.

and:

Bob.