Re: Info on a 75 Merc 7.5 Lightning/Comet?
[edit - I see Laddies answered while I was typing. Oh well, I'll post it anyway]
Should push it fine. Old Merc 7.5 and 9.8 (same head, different other parts depending on HP) are great engines with good reputation.
If you don't know the history of the water pump maintenance, you should do water pump service, or have it done. Even if you're getting flow from the telltale. Old, worn water pumps can cause headaches which are unnecessary because it's so easy and cheap to maintain them. It's a fairly straight forward do-it-yourself with a repair manual, but tricky the first time (learning curve). It's generally cheap and quick for a boat shop to do it.
You should also drain and replace the lower unit oil and re-install the screw plugs with fresh gasket/washers (specific for those screws - get a supply at a dealer). Bad screw seals is the most common cause of water getting into the lower unit oil. If you see water intrusion there after running for a while after doing that, you might have to get into a seal kit for the L/U drive and prop shafts. Or, just make sure you change the oil regularly and before fall freeze-up so the water instrusion doesn't harm the L/U.
Unfortunately, one flaw in this vintage Merc is that the drive shaft is subject to corrosion and this can affect the seal into the gear box.... and therefore be a point of water intrusion. It can be helped somewhat by dressing the shaft and getting a new seal on there. There's also some other more involved ways to address it too (some sort of sleeve that is pressed on?). As an example, mine has a chronic slight water issue even after doing what I could to clean it up, but I have just accepted that I need to change the oil more than once per season. I don't get obvious water, but the oil gets milky.
A couple of things I did with my older 9.8 (a '74 - nearly identical to your '75 7.5) were: address a rough idle by clean/rebuild carb with kit, and then I ended up replacing a coil and ultimately a switchbox to chase it down. If yours is idling and running well, I wouldn't recommend buying those parts unless you have to because they're expensive.
