12-03-2019, 01:10 PM
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.
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.