1997 Mercury Force 75 hp overheating

Sorry guys, posted this last week but have more info and couldnt figure out how to edit post. New water pump kit, thermostat, heat sensor... boat over heats 200' from launch!!! Dropped lower unit and rigged up a hose to tube from pump, ran boat at idle 15 min and 3-4000 rpms for 15 min... no problem, engine was so cold there was condensation on it and water out pee hole was 35-40 degrees. Im thinkin its in the lower unit, could I have bought wrong water pump kit? Everything Ive read says Mercury put the lower units from the 40 hp on the 75s but I have a 40 hp lower unit from the original motor on this boat and its obviously smaller...the driveshaft alone is 5/8" vs 3/4". Do I have some kind of "Frankinmotor"???

The driveshaft splines would not allow such an interchange. I suggest you bucket test the lower unit to see if it's pumping properly.

The second photo shows what to expect when running the LU on a drill. The other shows how to connect the driveshaft to the drill.


Jeff

fastjeff, I was wondering how I could rig up a test...thanks. I ordered a new pump base while I had it off as that is the only thing I didnt replace... will try this when I get it on. I know the driveshafts can not be interchanged, I think the lower unit is not the one off the 40hp...maybe an earlier year or later. Its 2" longer than my old one as you can see in pics. My old one came with boat from Tracker and is a 1998 40hp 2 stroke, I bought this one as an upgrade and serial # comes up 1997.
Sorry, cant get pic to upload???

You have the ole nightmare motor..3 cyl 1 carb. The 40 and 75 units look close as both small diameter gearcase but will not interchange. What shape was the old impeller in? melted? formed curve?

It was curved but was still in good shape. I had to replace seals on propeller shaft so figured while I had it off??? It looks to me like the new impeller is offset in the housing, the driveshaft hole does not appear to be centered. All the impeller blades on one side are folded over while the opposite side just touches the sides of the cover... I noticed it was extremely hard to turn driveshaft when I had it together before, it took two hands just to turn it!!! I replace the water pump on my old 40hp several times without a problem and it wasn't this hard to turn... wish I could figure out how to load pics.

OK, found another pump kit and installed it making sure pump outlet and tube went together correctly. Tested in a barrel of water as per Fastjeff and it seemed to pump a good volume... Took boat out yesterday and ran it all day, even let it idle once for 15 min after running it WOT. Buzzer never came on and water out pee hole was barely warm. Thanks for the help guys. BT