Re: What is the trick for getting lower unit off 1978 80hp Mariner
yes but there should be a pin in there to hold it in. Again I'm not familiar with that particular motor but that's usually the case.
The big Mercury/Mariner motors have a spline connector for the shift shaft without any retainer.
Depending on the configuration of the studs and nuts, I have sometimes placed a spacer just above each upper studs with nuts, and the small cavitation plate above. For example holding the flat surface of a 1" open end in that gap. Then backing off the nuts slightly, this will put some load on separating the lower unit-- maybe enough to get a little space in there.
Use a good pentrating lube (blaster? Gunk?) and time and patience- on all of the studs. Eventually you can tap the back of the gearcase just above the trim tab on either side with a wood block and dead blow hammer and get some movement. When lower unit is off, replace impeller, pump plate and pump insert cup before reassembly.
At reassembly polish all studs and coat with Merc/quicksilver anti-corrosion grease or a "neversieze" that is compatible with aluminum and steel.
Also at reassembly do NOT put any grease on the top of the drive shaft spline. Only on the sides. The top must maintain an air space else you will put a loading on the crank beargins-- a bad thing.
Don't know what is wrong with your motor. It is possible to plug up exhaust where you are looking if some animals made their home in there. Otherwise this is unlikely. I suggest taking the time to check compression in all cylinders and also that stator is fully active in low rpm mode and high rpm mode. It is possible to have an engine that fades out over 3000 rpm because the hot-high rpm ignition is out of commission due to stator failure. Also it is possible to have stuck rings, scored cylinder, burned piston, cracked reed--- any or all which would show up in compression checks to point you in direction.
If you suspect "carbon" blocking ports, the most likely place is right at the exhaust ports--- remove the cover on the side of the powerhead to access this area. If it is plugged up, this is almost always due to inferior lubricants. I had a 1964 85 hp six, run about 100 hours a year, every year (on salt water-- and moored all summer) 'till I replaced it in 2005 (with a 1976 I6 model) just to get "power trim" with the integral transom bracket, and electronic ignition. It always used quicksilver or mercury oil and did not get carbonned up at all. Even when I replaced the shift shaft around 1999, and lifting the powerhead could see down into the housing, and up into the exhaust chamber there was only a black sooty surface, no gummy tar like residues. Thats an engine that was run for more than 40 years without disasembly.
