Carb vacuum port ID
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If you have an automatic transmission that uses a vacuum modulator, this also uses a manifold vacuum source. Often the factory used a fitting with a restricted orifice that was fitted to the intake manifold for this but you can use a port on the carb. | If you have an automatic transmission that uses a vacuum modulator, this also uses a manifold vacuum source. Often the factory used a fitting with a restricted orifice that was fitted to the intake manifold for this but you can use a port on the carb. | ||
| − | There is one vacuum port on the carb that was used for the EGR valve. It is not suitable for use as a manifold or ported vacuum source. The problem is, | + | There is one vacuum port on the carb that was used for the EGR valve. It is not suitable for use as a manifold or ported vacuum source. The problem is, the EGR vacuum port is not always in the same location on all Q-jet carbs. It is often the port on the front passenger side of the carb, in the throttle body, nearest to the idle mixture screw. The EGR port will sometimes have a larger diameter than the other vacuum ports (except the very large PCV and power brake ports)- test those larger sized ports for being an EGR port first. |
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| + | If you don't have a port there, or you want to check it (as I would), one way to tell is to use a vacuum gauge- which you should be using anyway, come time to set the idle mixture screws, etc. The EGR port will not have as much vacuum at idle as a manifold port- but it will have enough vacuum to easily register on the gauge, unlike a ported vacuum port that will show less or no vacuum at idle. Because of where the EGR port picks up its vacuum signal (from the venturi) it will tend to show more vacuum on the gauge as the engine speed/throttle angle increases. | ||
Yet another way- though a crude one- to ID the EGR port is to blow smoke through the vacuum ports until you find the one that has smoke coming from an orifice that's in the venturi area of the carb. You'll need a cigarette and a length of vacuum hose to pull this off. | Yet another way- though a crude one- to ID the EGR port is to blow smoke through the vacuum ports until you find the one that has smoke coming from an orifice that's in the venturi area of the carb. You'll need a cigarette and a length of vacuum hose to pull this off. | ||
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