that hole is part of the choke heater set up. The way it works is that air is drawn by vacuum into a tube that is in the exhaust cross over in the intake and that then heats the air, the hot air opens up the auto choke valve by unwinding the thermostatic spring in the choke housing. I haven't seen one of these on a boat but on cars when they used them (yeah I'm old enough to have been driving then) there was usually a tube coming off the air filter hooking up to this, so the air drawn in was filtered, but there may have also been an orifice to limit the amount of extra air being metered in. I have to look at a parts break down to see how it was originally set up. Had something similar on my 1970 Ford Torino 302 cu in with 2bbl Autolite carb.
On my '88 which is nearly identical to yours I have a Quadrajet which uses a well type manifold heated choke.
BTW you should check those manifolds before running it in the water, I have heard they can fail and let water in the engine, I had 3 sets of them on mine though and never had a failure, the last time I had to change the exhaust (salt water we have to change every 7 years or so) I changed it over to the conventional 2 piece design that they used on the later models.
OK took a look at the parts diagrams, that's a Rochester 2bbl you will have to look at a manual to figure out what's wrong with the linkage. It does not appear that any additional steel line runs from the flame arrestor to that open port. If closing it over makes the engine run better then your carb is probably in need of cleaning and is running too lean. Because when you cover that opening up in effect you are removing air from the mixture.
Also the engine is not a 1992, that would have had a Holley carb with an electric choke, the newest it could be is a 1989, that's the last year they came with Rochester carbs.
1989 OMC Stern Drive 4.3 [432APRMED] - Parts Lookup - Crowley Marine
432= 4.3 engine, 2 bbl carb A= 1st run of model P= power steering, R=right hand rotation the rest is a date code I think.
I spotted the OMC model # sticker on your valve cover above is 432APRMED
this is what you need to get the right parts.
