Re: Powerhead base gasket ?
The sneezing at idle is caused (usually) by a carburetor that is running lean, normally due to a slightly too large idle air bleed jet which is located at the top front portion of the carburetor. That engine uses a #33 jet (.033).
With the carburetor face plate off and the engine idling, one carburetor at a time, stick a couple fingers into the carburetor throat, acting as a manual choke. When the sneezing stops and the engine smooths out, you've found the malfunctioning carburetor.
If the above cures the sneezing problem, I'd suggest that you thoroughly clean and rebuild all of the carburetors first to see if this corrects the problem. If it does not, then you can replace that air bleed jet with a smaller size..... try a #31 (.031) or a #30 (.030). The smaller air bleed jet restricts the air flow resulting in a greater fuel flow.
If there is an exhaust leak due to a failed PH gasket of any kind, it should be obvious when running. Remove the small covers that exist under the powerhead to inspect for leaks..... should be a excess fuel/oil residue escaping at any leakage point.
What is the compression on that engine, and does it have a strong spark on all cylinders?
Also..... Check the hood to make sure that the air intake portion of it isn't clogged, also check the air intake under the bottom carburetor.
