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Understanding the operation of the vacuum advance unit
with the PORTED VACUUM unit connection.
Not applied “to manifold type” vacuum units.
Location is critical for proper operation.
Interpreting this ported vacuum graft.
The vacuum unit produces additional advance, as shown by the dotted line, when the engine revs are high, but the load is light. That is when there is high vacuum in the manifold due to small throttle opening. When the engine is pulling hard on full throttle the vacuum in the manifold is low and the vacuum advance becomes wholly or partly inoperative. The spark timing is solely dependent on the centrifugal advance. The general effect of the vacuum advance unit is to increase the liveliness of the engine and a noticeable improvement in fuel consumption. Important is how and where the pipe leading to the unit is attached. Location is critical for proper operation. The vacuum source for the vacuum advance unit is extremely important to be located directly below or above the throttle butterfly to properly regulate vacuum advance at high RPMs and light load. At idle the vacuum advance unit does not operate. The vacuum pipe should be half sealed when the throttle butterfly is closed.
Early carburetors ported vacuum may be just under the throttle butterfly.
Later carburetors ported vacuum is over the throttle butterfly.
Jaguar and Triumph Note: with Zenith carburetors.
Your carburetor may not have ported vacuum!
Your Zenith carburetor may or may not have the threaded port next to the rear top mounting flange over the throttle shaft for ported vacuum. Some with the threaded hole may or may not have the small vacuum drilled through to the throttle from the factory! Zenith-Stromberg may have drilled them or may have not completed the process.
This must be done with precision.
We now have the tooling to complete the process on your complete carburetor or just the body.
Cost $50.00 / $25.00 with unit purchase. Call or Email for information.
Non-Vacuum advance Distributors!
Not good for street use. Should only be installed when involved in high performance racing or under constant wide open conditions. Anyone replacing a OE vacuum advance distributor with a performance mechanical advance distributor doesn't understand the operation of the vacuum advance unit. The original OE distributor with the correct vacuum advance unit, enhances cooling, engine efficiency, throttle response and fuel economy.
Any improvement after installing a new mechanical advance distributor only indicates
your original distributor had poor mechanical and vacuum advance.
British Vacuum Unit, which is not affiliated with ZF Friedrichshafen AG, the owner the US trademark for Lucas®.