I have a Rochester small body 2GC twin carb setup and while fooling around installing them, I discovered a small threaded hole that is open without a plug or any indication as to what should be done with them. I have attached a couple of pic's. The hole is just near to the blue tape in the first pic and is located below the fuel in line. Question is, what is it for and surely I don't just leave it without either piping it or plugging it???
OK, great and thankyou. So since I have vacuum advance from a different source, then I will go ahead and plug both carb openings.
OK, just for the record and to help make this a more useful thread to others in the future. The vacuum openings that I am going to plug, take a 3/8" diameter plug using a UNF thread. The best item to purchase to screw into these holes is a 3/16" brake line plug fitting. Generally available in brass or stainless steel from most brake suppliers.
Guys, while I have your attention, could anybody tell me from a visual of my twin carbs, which is the primary unit??? I am thinking it is the left hand one!!! If I am correct, does it matter which carb is at the front of the engine or is it all the same either way???
primary will need air mixture screws, secondary does not have them and usually will not have the power valve or ant idling parts. The sec. throttle blades must close airtight. Position usually does not matter in your application
Looking at the first photo the forward carb has a choke, and the curb idle screw on the rear carb is backed way off effectively shutting the throttle bores completely. A true secondary carburetor has thicker throttle plates that seal better.
Where are you getting the vacuum from for your advance? Unless I am mistaken you do not want manifold vacuum. You want venturi vacuum where the vacuum is gotten just above the throttle plate. That way you do not have full vacuum advance at idle. I believe you should connect your vacuum advance to one of those ports.
OK thanks for those comments, I can easily pull vacuum from there. My vacuum is currently from the rear base plate of my carb, which also supports my brake booster.
The fitting you're closing off on the carb is probably "ported" or "venturi" vacuum. But it might be worth knowing for sure in case you want to use it for some other purpose. It sounds like your distributor is currently connected to a full manifold vacuum source, which it should be if it's also supplying the brake booster. The vacuum advance can actually use either manifold or ported vacuum, depending on the rest of your tune-up. In particular it will depend on your initial timing, mechanical advance, and total timing. A lot of street driven engines seem to like manifold vacuum for better driveability. But of course there's no hard and fast rule. Experiment a bit and use whichever source works best for you.
Did you switch base plates ? Your earlier pics has one carb with a red base and the other with a black base plate.
Good observation, I've painted them between my posts. I've had the carbs for ages and only now getting the engine ready to run and install in my project. Now that I have been so involved, I am starting to question things that I have observed as being not right or not what I understand, since I am no carby expert...
While that port may appear to be ported vac, I'd look closer. Most of the early 2gc carbs with that port feed it internally from a passage way down thru the base to below the throttle plate. So I think it gives you manifold vac not venturi.
So, Jebbesen, what are you recommending that I consider in your reply, as I am a novice with carbs???
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