Mercury 7.5hp Outboard Water pump problem, need help

hotpickle

Cadet
Joined
May 30, 2015
Messages
6
Hey Guys. I'm hoping I can get some help with my merc. I'm brand new to this, so if I'm in the wrong spot please just let me know.

A couple months ago I picked up a boat/motor/trailer combo for quite cheap. The motor wouldn't start, and I'm no boat mechanic, but I've always wanted to learn how to work on an outboard so I figure I'd give it a try and see what I can learn.

I came away with a 1978 Mercury 7.5 hp motor. Here's where I'm at:

To get it to start, I cleaned the carb, replaced spark plugs, re did some wiring that corroded through. After some ether, fiddling with the choke, and a few hundred pulls, it comes to life.

Next problem to come up, the water pump isn't working. I pulled it apart, the impeller was in 100 pieces. Replaced the impeller, oil seal and o-ring, and did a deep cleaning of the line. I pulled a lot of pieces of that impeller out, and when I opened the flush a ton of sand came out. When it was all put back together, I gave a blast of compressed air through the intake, outlet, and flush and had air running out of the other end, so it was clear and good.

When I started it back up, still no water coming out, however now there was a solid stream of exhaust coming out of the outlet. Fiddled with it, tried to stick the hose on the screen on the bottom and onto the flush as well to try to prime it, that didn't work. I can get water pouring out of the flush when it is running, but it just won't come out of the outlet of that water line.

Can anyone help me with what to look at next?? I would really appreciate the help. It's taken a couple months, a ton of learning and frustration to get to this point and I really wanna get that bad boy in the water. Thanks.
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
39,235
Obviously the motor overheated for the previous owner.-----I would remove the narrow plate on the side of the motor ( 3 screws ) to inspect pistons and rings.------Now these motors had a plastic washer at the top of the water tube that melts on severe overheat.-----It then blocks water flow to the engine.----Powerhead is removed to inspect that.------Just 4 screws to remove the horseshoe cowling under the motor .--Then 6 nuts and a few more screws and fittings and the power head is off.-----Simple to do.
 

hotpickle

Cadet
Joined
May 30, 2015
Messages
6
Oh man that's wicked, I had been searching for water pump problems and hadn't heard of this.... thanks a ton! I'll give it a try.
 

hotpickle

Cadet
Joined
May 30, 2015
Messages
6
One more question, I don't think I put the carb back together perfectly. Where can I find the tuning specs?
 

emckelvy

Commander
Joined
Jan 16, 2004
Messages
2,506
Before you tear the powerhead all apart, pull the L/U again and check your installation of the impeller. It's very easy for the impeller key to fall out during installation. Then you'll get no water pumping.

You could check for pumping by sticking the L/U in a bucket of water and then spinning the driveshaft clockwise with a drill motor. Just use a length of rubber hose as a drive coupler, fastened with hose clamps at each end. You should get a good water stream from the pump outlet.

Next thing to do with the L/U off is to run water up the copper water supply tube, checking for circulation thru the powerhead. Since you indicated you had a lot of silt, you might need to remove the cyl block water jacket cover (the cover over the spark plugs). This is not a cylinder head, just a cover with a gasket. If the 1/4" bolts are stuck, don't force them or they'll break. Heat from a MAPP or propane torch will help free the bolts.

Clean out any silt/trash/debris inside the cooling passages, then flow water up the copper tube with the cover still removed. This will wash any debris still trapped, and show you whether you've got good water flow to the powerhead.

Attached is the pic of a 7.5 carb, to help you identify parts. For initial idle mixture, lightly seat the needle fully CW then turn CCW approx 1-3/4 turns for initial adjustment.

HTH............ed

http://www.marine engine.com/parts/mercury_marine_parts/static/img/MERMAR/67175/2058.gif

(copy, paste into browser, take out spaces)


 
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