The reason may be that your 350 cannot produce enough vacume to open a 750 carb, ive seen this before. you can get a kit to make them fully adjustible and work for you.

I'm not sure if the secondarys will open all the way or not with the vacume canister hooked up. The secondary blades have been pinned to open as the primarys by the previous owner for some reason. I rebuilt the carb and installed a light spring in the canister and also enlarged the vacume port that goes from the secondary throttle bores to the vacume canister. That should give it a stronger vacume signal to open the secondary blades. If they open to fast and bog, I'll just put in a heavier spring. I've messed around with these carbs when I used to drag race, but I usually just traded them for double pumper carbs which always seemed to work better for drag racing. I'm hoping I can get this vacume carb to work good on my boat. I think part of the low power problem was from the secondary metering plate being to lean. I drilled the jet holes out to .080, (they were .078 and the plugs were bone white after a WOT run). I would think that there should be a little tan color to the plugs, hopfully this will work.