7.5 hp Water passage questions - PHOTOS

[wbeaton]

I've finally got back to working on my 1974 Mercury 75 7.5 hp. I bought this motor as a runner last winter. I installed a new impeller and tested the motor this spring. The motor pumped lots of water. Too much in fact as water shot out along all the gaskets (exhaust, water jacket and base) and the telltale, but none out the upper exhaust.

I took the powerhead and cowl base off the motor and inspected every passage and port. I could not find any obstructions in the powerhead. A hole in the interior of the exhaust tower leading to the upper exhaust was completely plugged with carbon. I cleaned it out and reassembled everything with new gaskets and torqued to spec. This helped a lot, but it didn't completely solve the problem. I also retightened everything to as tight as I'm comfortable doing. I'm still not getting water out the upper exhaust and water is still leaking around the gaskets, though not nearly as much as before. Water is definately getting through the powerhead and out to the exhaust tower.

Here are a few photos. Maybe someone can help me figure out why its still restricted. I could really use the help. I want to get this thing finished.

 

[emckelvy]

Re: 7.5 hp Water passage questions - PHOTOS

To answer your question in the 2nd pic, "Does all the water exit here?", YES!

I had a '76 Merc 4.5 with a similar problem. Pumped a mad flow out the telltale like I'd never seen but it just didn't seem like it was cooling properly.

Pulled the powerhead and the discharge holes in the bottom of the block were completely plugged. Had to pull the water jacket cover over the back of the motor, clean out those holes, and blow air/flush water thru to make sure the passages were clear.

After that it pumped water normally.

You're developing so much pressure that the water has no where to go but out the telltale and out all the gaskets.

I'd pull the powerhead and check out the discharge passages in the cyl block, bet you find they're all plugged up.

The flushing passage follows a slightly different route, but that's just water getting IN the powerhead and you don't seem to have a problem there!

BTW it's debatable whether the gaskets will re-seal after having been "blown out" so you might figure on replacing them. Or just running the motor after you fix the water discharge issue, and checking very carefully for leaks. Be sure to pull the spark plugs and check for water droplets, which would indicate a leak on the exhaust manifold side.

At minimum re-torque all the cover fasteners and maybe that'll help.

HTH & let us know what you find.......ed

 

[wbeaton]

Re: 7.5 hp Water passage questions - PHOTOS

I understand why water is shooting out the gaskets. Now I'm just trying to figure out where the restriction is.

The engine is cooling fine at idle.

I've already had the powerhead apart twice and I can simply find no problem with the cooling passages in the block. I've followed the flow path through the block and everything seems fine.

I stated that I've retorqued everything to the point that I'm concerned to torque any more. I've already cut new gaskets again.

Water is definately getting through the powerhead and out to the exhaust tower, but no water is coming out the exhaust.