Re: 1975 "200" 20 hp cooling
Your 20 hp should have a rubber hose coming off the powerhead, somewhere towards the rear, going to a telltale fitting in the lower cowling. Typically on the Stbd (right hand) side of the motor.
If nothing comes out, it may be plugged. A puff of compressed air up the telltale may be all that's necessary to clear it out. Or a thin wire if you have no access to air.
Failing that, pull the hose off the fitting screwed into the powerhead and flush it out.
Another place where water will exit (but only when the motor is submerged in water to its normal operating level) is from the exhaust relief ports. Water is discharged out of the block, down the exhaust tower (aka center section or 'mid'), and exhaust pressure will carry some of the water, in a spray, out the square relief ports at the back of the motor, underneath the lower cowling.
You won't see much, if any, spray coming out the relief ports when running the motor on a flusher, since there is no backpressure to force the water out. In that case, most of the water will just go down and out the center of the prop. BTW if you feel that water stream coming out of the prop, it should be noticeably warmer than the water going in the motor.
A good way to check for water circulation thru the powerhead, in the absence of a telltale, is to feel the exhaust cover. It'll get hot real quick if there's no water running thru it. The exhaust cover should stay fairly cool while the rest of the block may be a bit warmer.
If the exhaust manifold cover is hot, shut 'er down right away and check the impeller. If you don't know when it was last replaced, it's definitely due for a new one!
The only caveat with changing out the impeller is to make sure the shifter is in Fwd or Neutral and don't move the shift handle or the shift shaft in the lower unit, with the lower unit dropped down. The most common mistake made when working on these is to get the shifting mechanisms out of synch with each other, and usually it'll fail in Fwd gear. Just take a look at all the postings (day in and day out) about not shifting right after an impeller job and you'll see what I'm talking about.
Nothing to be scared of, though, a pretty basic job that can be done with common hand tools.
Anyway, that probably is a lot more info than you were looking for, but better a cool Merc than a burnt-up one!!!
HTH & Happy Boating.........ed
