Editing Carb vacuum port ID

==='''ROCHESTER Q-JET'''=== You will want to be able to identify a '''ported''' vacuum source from a '''manifold''' vacuum source. A ported vacuum source will be either absent or nearly so at idle, and will register vacuum only when the throttle blades are opened. A manifold vacuum source will be there all the time- at idle and when the throttle blades are opened. Then decide- either by prior knowledge or by trial and retrial- which type of vacuum (ported or manifold) you want to use for the distributor vacuum advance. The PCV is connected to the front large vacuum port in the carb throttle body, the rear vacuum port of the throttle body (usually threaded with pipe threads for a fitting to connect a steel line) is used for the power brakes unless you use a port off of the intake runner for this. Once everything that needs vacuum is plumbed, plug any remaining vacuum ports w/good quality caps- the parts store "HELP" aisle caps are junk for the most part, try to find either OEM plugs or good quality/silicone plugs. 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, I cannot say exactly which port this will be on all carbs, but it is often the port on the front, passenger side in the throttle body, nearest to the idle mixture screw. 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. The EGR port will sometimes be a larger diameter than the other vacuum ports- test those larger sized ports for being an EGR port first. 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. It should be noted that Q-jet carbs prior to the introduction of the vacuum operated EGR system obviously will not have an EGR vacuum port. These carbs having an EGR vacuum port are mainly going to be from about ’71 or so and newer. Carb numbers that may have an EGR port: • 1971- 7041xxx • 1972- 7042xxx • 1973- 7043xxx • 1974- 7044xxx • 1975- 7045xxx • 1976-’79- 1705xxxx • 1980-up- 1708xxxx [[Category:Engine]] [[Category:Good articles]]

| Editing help (opens in new window)