What vacuum source should I use- manifold or ported? - Alfa Romeo Forum
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Location: Staffs, UK
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Had a 33 1.7 QV

What vacuum source should I use- manifold or ported?

There seems to be compelling advice on using a manifold vacuum advance.
Has anyone used one of these instead of the more modern ported type?

In many cases the vacuum advance should use a full manifold vacuum source on the carb- but this is not written in stone. On almost any carb, there are vacuum ports that provide manifold and ported vacuum. Using a manifold vacuum source will in many cases- depending on the cam and compression- allow you to close your throttle plates a little and still maintain the same idle speed. This does a couple things: First, it will cure nozzle drip and a smelly, poor quality idle caused by the butterflies being opened too far at idle, which allows fuel to be pulled from the transfer slot. It will also deter engine run-on, or "dieseling". Also, you may find that the engine is cooler running around town in traffic and has much better throttle response. It will have no ill effects at WOT because there will be no vacuum at WOT (no vacuum = no vacuum advance added to the timing) so you will be running exclusively on mechanical advance.
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manifold , ported , source , vacuum

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