I recently replaced an old Holley with a new/rebuilt Rochester 4MV on my 1981 305 in order to pass the smog test (when registering a car from out of state - all equipment must be original) here in CA. Once the engine warms up, when given gas the Rochester bogs down, especially noticeable from a dead start. Before I start tinkering with it or the emissions systems - any suggestions?

On a quad that usually means its time for a rebuild. In your case I would make sure you don't have an air leak .

These carburetors are very sensitive to vacuum leaks and missing/faulty smog system components. For instance, a vacuum leak in the EGR vacuum line will cause the ecarb to run lean. I once saw a vehicle with an ecarb that had the EGR valve removed and the engine ran so lean the vehicle was undriveable until the EGR valve and hoses were reinstalled. If the problem persists after you are certain there are no vacuum leaks or faulty smog system components, I would seek out a carb. rebuilder (technician) who has one of those Rochester ecarb tool and gauge kits, specifically designed and sold by GM, to set the carb's lean authority, rich authority, idle air bleed valve, etc. While at it, get him to adjust the Throttle Position Sensor voltage.

Good luck.

Good advice - it may very well be a faulty component, since I got all the emission plumbing, EGR, MCV, air manifolds etc. from wrecked Elky's invarious junkyards ( a previous owner had stripped all emission equipment from the engine). I have taken it back to the mechanic who installed the Rochester - he tells me it is not out of adjustment. I think it might be an air/vacuum leak just from the sound. In addition, when the car is first started, before it warms up - it runs awful, sounds likeit is mis - firing, and stalls when I give it gas after a stop. Thanks for the advice, will let you know how it turns out.

Turns out the hose from the charcoal cannister wound up on the exhaust manifold and melted in a spot that I couldn't see. When I finally dug around a little bit and I realized what happened, I cut the hose and spliced it back ( the heat from the exhaust maifold had melted the hose solid) together. The engine is running a HECK of a lot better, and what I assumed was a carb problem, was in actuality an emissions problem.

Glad you found it. Vaccum lines can be a pain.

It's hard to believe that a vacuum leak could cause such a dramatic difference in performance, but since repairing the line, I haven't had any problems. I'm beginning to understand why the previous owner pulled all the emissions plumbing off the engine! Thanks again for the advice, guys.