SU carb and vacuum wiper

[garryruck]

I have an SU carb with a vacuum tube on the manifold for a vacuum wiper wiper.  The car runs poorly/rough when the tube to the wiper is attached but OK when the tube is removed and left unblocked.  Finger on the tube and the engine runs poorly.  Any ideas?

[Hugh Barnes]

Has it ever run well on that set up? Funnily enough, I've just been working under the bonnet of my Box that has that set up. It does run fine (apart from the wiper being its usual poor performance with the engine under load..)

Can you describe 'poorly/rough' in any greater detail? Does it run as if it is over choked? It sounds like it's something to do with air volume in the fuel mix..

[garryruck]

When you accelerate using the lever under the bonnet it picks up OK but the weighted piston you can see drops back down even when revving high. With the tube removed the piston stays up and only drops back at lower revs. It used to run well but I cut a small piece off the tube as it was split and that's when it started doing it. I am sure it's to do with a mixture problem but adjusting the jet position lean or richer makes little difference. The jet was centred as per the book and the piston drops back with the requisite clunk. The needle is dead straight.

[Robert Leigh]

It sounds as though your carb is set or needled to run too rich, and the air bleed through the wiper connection is weakening it enough for it to run ok. As Hugh indicates, has it ever run correctly on the present set-up? With knowledge of your SU size and needle we could make better informed comments.
Robert Leigh

[Chris KC]

A split tube amounts to the same thing as removing the tube - i.e. air is entering the manifold via the tube connection, by-passing the carburettor.

The carburettor piston is balanced to maintain a constant depression (vacuum) at the jet regardless of airflow. That is to say, if airflow through the carb increases, the piston rises in order to maintain the same vacuum level at the jet; and vice versa (Bernoulli effect). If you get a different behaviour pattern of piston under similar conditions of throttle demand and engine speed, it means airflow through the carburettor has changed; which is what you'd expect if you are opening and closing a 'leak' path downstream of the carb.

IF it has been running OK, it is probably set too rich, as the mixture passing into the manifold from the carburettor is then being diluted with clean air from the vacuum take-off.

I suggest you need to connect (or blank off) the wiper tube once and for all, and then re-tune the carb from scratch - but heading in the direction of a weaker mixture.
I wouldn't discount that with the leak sorted out you might benefit from a different choice of needle (or piston spring, if it has one). As Robert says, it's hard to comment without knowing what you have in detail.