Mid 80's 30 HP will only run wide open with cover off

Never seen this before. Will only reach a fast idle with the cover on. Can't find any problems with gas line, wires, plug wires....... Carb is clean, engine has always been very dependable. Cranks fine and goes into gear and just seems to starve for gas/air at about mid throttle. If you take the cover off it runs great! I've checked the vent area on the back of the cover and it looks like it's supposed to. I was going to put some type of vent on the top of the cover, but figured I'd get you guys to give me any ideas you might have before I start drilling holes in the thing. Again. It's in fantastic running condition with the cover off and I see no problem with any wires being grounded or hoses being cripmed when the cover is on.
This one has me scratching my old bald head a little.

Perhaps a sparkplug wire is shorting to the metal brackets on the cover.------------------Other thing is an exhaust leak.----------Seems like you have looked carefully everywhere.

Would exhause in the cowling cause a problem?

Look where air is supposed to enter the motor and see if something is blocking it.

Jeff

Sounds feasable. Drop the L/U and check to see that the rubber exhaust gasket is ok and sealing properly.