I have a 1971 corvette 350/270hp with th400. When I first got the car it had low manifold vacuum (13-14"), poor idle below 700 rpm and a very strong gas smell in the exhaust. I just finished completely overhauling the engine. Everything has been replaced except the castings. With ALL vacuum ports plugged ( no tranny, brake booster, or any external vacuum equipment connected, I still only get 13-14" of vacuum at idle and the engine will not idle below 700 rpm. I have checked for vacuum leaks at the carburetor and the intake manifold gaskets. Is it possible to have an intake leak anywhere else?  

What are the specs on your camshaft and where do you have the base ign. timing set?  

egr or non egr ?
egr could be not seated if it has one.was the  intake manifold  magnafluxed for cracks ? also look at the little pipe that screws into the manifold where the brake booster gets its vacuum from. ive  seen them have cracks in them from being wrenched on too hard.also are they the original heads and manifold? obviously you checked the carb if its a qjet they leak around the butterflys when they get old .
hope that helps  

I put in a compcam 260h grind. According to compcam this is a very mild 300 hp cam and will produce plenty of vacuum at idle. The base timing is 8 degrees btdc.

Non egr. I put a plug in the manifold where the tee goes to eliminate that as a potential leak point. The carburetor has been tested on another engine and functions perfectly. The machine shop says they checked the manifold and heads for cracks.

Is it possible for the heat riser passage to be cracked and open to the plenum? (might be hard to see visually, I would think)  

Got any pics to show us what you are trying to show.  

How big is the carb?