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Is this pipe plug on this 1966 283 Rochester Carb base the vacuum port for the Distributor Vacuum Advance?




dtm

Yes.

in my opinion the answer is no. the picture shown is the tube for the choke operation. heat tube. the vacum advance vacum comes from a port at the bottom of the carb. just my two cents. get a second opinion

Look again, the arrow is pointed toward the pipe plug in the bottom of the carb!!!

at my age i'm not paying attention, YEP, YOUR RIGHT. that can be used for vacum. sorry bout that

The vacuum advance line is in the location on my 1957 Rochester 4 Bbl.

Yesterday was beautiful here in Ohio. The sun was shining and the birds were chirping. So I set the timing at 4 degrees BTDC, set the dwell at 31 degrees, set the rpm at 475-500 ( the 1966 283 is a 4 speed car ) and had the vacuum port rubber hose plugged coming from the base of the Rochester 4 barrel carburator. All seemed fine. The 283 ran great at idle. Then I reattached the vacuum line to the vacuum advance on the distributor. The idle rpm picked up to nearly 700 rpms. Does the base of the carb really pull that much vacuum at idle to increase my rpm idle speed by nearly 200 rpm's?

DTM

Yes, the 1966 V-8 had full vacuum at idle where as most have vacuum at a fast idle. By 1968 that feature was discontinued as the engine produced more polution with the timing was advannced that far at idle.
From my experiance seting the timing at 8 Deg. peps up a 283 engine. I run my 1957 at 10 Drg. and it dosen't ping with regular grade gas.

Thanks Gene: When it quits raining here in Ohio, I will change the timing to about 8 degrees BTDC.

dtm

I have 2 Exactly the same Rochester Carburetors for my 1966 283.
One has NO threaded hole for a vacuum port on the cast iton base of the 4GC Carb. This carb has a copper tube on the very top of the airhorn on the passenger side.

The other Rochester 4 barrell carburetor is the one pictures earlier in this thread. It pulls the vacuum from the cast iron base on the passenger side just in front of the choke.

It would be obvious to me ( but maybe not that obvious ) that an engine pulls a different level of vacuum at the cast iron base of the carburetor compared to the other carb that I have that is designed to pull vacuum from the very top of the air horn.

Which carb is correct for my 1966 283? And what would the other carb be correct for?

dtm

The carb. with no vacuum port in the base is designed for the type where the vacuum is sorced from a hollow carb. mounting stud.
This was used for several years beginning in about 1959.

I thought I had a 1966 shop manual....don't have so can help.

I looked in the 1966 Chevrolet New Product Booklet and it states that 1966 4GC Carbs are carry-over items from 1965. Therefore there are no pictures in this booklet. The 1966 Chassis Service Manual Booklet show a drawing of the 4GC Carb but it doesn't show the passenger side front corner of the carb where the vacuum port is either to be located or not located. The 1966 Chassis Overhaul Manual shows a 4GC Carb that does not show any vacuum port anywhere on the passenger side of the drawing for the cast iron baseplate. A Vacuum Port is definitely missing from the cast iron baseplate for this extra carb that I have.

So, if the vacuum doesn't come from the baseplate, where does it come from? And which is correct for 1966 283?

dtm

The hollow carburetor mounting stud. Right side, can't remember if the front or rear. The stud is hollow and gets right into the manifold vacuum. A rubber hose goes from the stud to the vacuum advance. Also can;t remember what years had this feature.
Replacement studs are available from vendors.
I think the hollow stud is 3774089 and used from 1960-1964 with 2 or 4 Bbl. carb. I have a thin later book and this is what I believe is correct. 1965 Service News would have this listed in advance Info.(The switch back to carb..base vavuum)

So that means that the Rochester 4GC carburetor with NO taped vacuum port in the cast iron baseplate would have had a hollow carburetor mounting stud with a hole down thru the center of it? Amazing. Therefore, my 4GC with NO vacuum port in the baseplate would be correct for 1960-1964 ?

dtm

That would be my understandaing.