Vacuum or bad Sensor

[Cybidium]

I have a 76 280z with an L28 and am having some strange issues.

1/2 the time when I start my car, doesn't matter if it's cold or hot it runs really bad while the other 1/2 it runs just fine. By running bad I mean the idle drops to around 500 and black smoke that smells like gas comes out of the exhaust.

Because my brakes don't work very well I started to think that it might have something to do with my brake power booster. I went to autozone and got a vacuum tester. While my car is running poorly, with the tester hooked up to the brake booster vacuum line it reads around 5psi. When I rev the engine it goes up to about 10-15 psi and drops right back down to 5 and tries to stall.

If I cover the vacuum line with my thumb as it's running and slightly take it off, the cars RPMS increase and the black smell goes away and if I totally remove my thumb the RPMs increase exponentially.

I've searched both the hybridz forums and these but haven't found anything that exactly touches this problem.

Any help would be great

-Tim

[cali260Z]

yep i have the same shi#@ going on with my 260