stlbluesbrother

Anyone know exactly why some quadrajets from the same year have 1 vs 2 ports at the rear? I know 1 port is for the choke tube. The other for OAI vacuum?

Below is a picture of a 7040250 - 350 cid

jensenracing77

all of them have the port that goes to the choke stove in the intake manifold. some cars had a thermostatic vacuum switch, if it had the switch (located at the front passenger side of the intake) it would have had the second port. these are just clean air sources, there is no vacuum on them.

stlbluesbrother

Didn't all A-body Olds cars in 1970 have a TVS? I've owned several 1970 carbs and only one had the dual ports.

jensenracing77

Not all had them. My Rallye 350 is a 4 speed car and had a TCS and no TVS. not sure what else will determine what it had.

rocketraider

One of the ports supplies fresh filtered air to the choke hot air tube. The other one is a vent port for the TCS system. When the TCS disables vacuum to the distributor vac advance, it vents it off to that port on the carb. One reason is for noise control, another so the carb vacuum port does not pull unfiltered air thru the TCS solenoid or DTVS when it's in vent mode.

I believe the larger port is for the choke. If nothing is connected to them it does not affect anything as they are simply open tubes unless the choke or vent tube is connected. No vacuum leak or anything.

stlbluesbrother

Thanks for the explanation, guys.

oddball

They should be capped if unused. Otherwise, the engine could easily suck in unfiltered air through them.

rocketraider

How? They aren't connected to any vacuum source and I doubt you'd get enough venturi effect thru them to suck in any appreciable air due to their location.

But, vacuum caps are cheap enough.

BILL DEMMER

pressure differential due to air filter and air cleaner snorkel.


bill