25 HP Mercury outboard engine.

[MickinOz]

Anyone here have a 25 HP Mercury 2 stroke outboard engine from about 1992-1995?
Pull start, no lighting/charging coils.
I would like to ask a couple questions.

[twisted lines]

I don’t but will tell ya
Boats.net
Has break downs & part numbers.
US parts are sent

[MickinOz]

Thanks mate. I just took a very quick look, that's going to be a very useful site.
In the mean time, I need a question answered.

I'll tell my long story and maybe someone will see and have answers.
My only other experience with Mercury was a '70's model blue band 40hp that could best be described as a black anchor.
Plain steel hardware and shafts meant they were barely able to last a season in our saltwater.
It put me off the brand for a very long time.

Now, I have this lovely little 13 feet 6 inch fibreglass cat, with twin 90's model 25's on it.

These things look well used, but run sweet and the power heads are real quiet and rattle free.
So I finally put the boat in the water this morning. #2 son and I took Thumper the dog on his 1st ever boat ride.

Drove her out past the island, trolled for snook for a half hour, tried to anchor up for crabbing but too rough.
I knew that grapnel anchor with no chain was never going to hold on our sand and mud grounds, but I figured I had to try it before buying a new sand anchor.

Anyhow, we decided we would head back in, time to see how she handled a head sea in 15-20 knot winds.
I checked willyweather.com.au when I got back, we had 16 knots to 20 knots when we were running back into the teeth of it around 10:30.
Our bay is very shallow and kicks up easy, 20 knots is definitely time to go home for small boats, so I was pleased at the way the little cat was handling it.

About 10 minutes into the trip she felt pretty sluggish and I looked around to see only one prop wash.
The starboard engine was not running!

Now, these are manual start motors, literally tiller steer dinghy motors that have been adapted to forward control engines with aftermarket throttle and gear levers.
They have virtually zero electrics except for the ignition modules, with a single cable each running to kill switches at the helm, as far as I can see.
It was windy, the other engine was running, the hull was making a bit of noise as it hit the chop, etc., so I only noticed when the boat ran out of puff.

We stopped and checked her over, nothing obvious to see. Flicked the starboard kill switches up and down a few times, set the throttle, gave the primer button a couple of jags, pulled her over and she started.
So I blipped the throttle, she ran up in revs OK. All good, lets be on our way.
Except she stalled going into gear.

Did it all again, she stalled again.

Hmmm. Well, we be going home on one engine in displacement mode.
Except, we stopped off at a little side creek on the way home so we could at least say we got the crab nets wet.
After 1/2 hour of crabbing that didn't produce anything legal size, it was time to get my son home by the promised mid-day deadline.
Portside engine started no worries, so I tried the starboard as well. Fired right up, went into gear without stalling, and we went the last two miles home at eyebrow raising speed.
Up the last bit of shipping channel and into the harbour, it is a little more sheltered, and there was that sort of chop on the water that seems to help with lift and a little aeration for less drag and best boat speed. It's spooky really, the thing runs dead flat at speed. Even corners remarkably flat with two of us in the boat.

So all in all, not a bad shakedown cruise, identified a few things to improve, i.e the anchor will be fine down the peninsula in the reef country, but I need a mud pick up here.

But the starboard engine has me stumped, why cut out, refuse to run under load, then after a half hour cool off, take off like a scalded cat (pardon the pun)?
The vent valve on the tank was open.

Had no trouble pumping up fuel with the in-line bulb.
The fuel filter has done maybe 3 hours.
I honestly think I can rule out fuel starvation.

So I turned my mind to overheating. In our rough shallow bays we get a lot of floating weed and I did have to clear it a couple times.
But I can't see any sort of temperature sensor wired to the ultra basic ignition unit.

Does anyone have any ideas?
I will run the multimeter over the kill switch wiring, but what else is there to look at?

[twisted lines]

Heat related coil & condenser; both will do that!

[Pmullen503]

It wouldn't be a bad idea to replace the impellers for the water cooling pumps on both engines. They don't last forever.

As stated already; Coil, points and condenser. If you can't find new parts locally, you can at least start swapping parts to see if the problem follows a particular part from motor to motor.

If you have a contactless thermometer (IR), You can run both motors and start comparing temps at various points on the motors to see if one is running significantly hotter.

(PS. Mercurys are built right here in Fond du Lac, WI where I live.)

[MickinOz]

No points and condensers in these motors, chaps. They aren't that old.
CDI's I expect.
The serial numbers do indicate they are US built, not Canada or Belgium.
They are listed as 1995 motors though the serial numbers suggest they were built 1992/1993.
That's not unusual here - plenty of motors don't get sold the year they are built.

There aren't that many things it can be, to my way of thinking.
I've had it happen on other motors.
One was a blocked fuel filter, it would idle all day, but when I opened the taps, the carb would gradually drain and the motor would run out of fuel and die.
That engine was gravity fed from an under cowl tank.
Pump up fuel, repeat.

The other got blocked water intakes and the CDI shut off as soon as the engine got too warm.
But the thing is, that motor had a temperature sensor that shorted the ignition when hot enough.

I will start with the easiest thing I think. I have a spare new filter, I'll put that in.
I had already thought about new impellers, and I will, but it still confuses me that I can't find any sort of temperature sensing.
Unless the ignition module picks iy up direct from the block? It is mounted right next to the exhaust plate.

[twisted lines]

water flow from both engine's
Compare flow & temp by Hand.

[MickinOz]

Just had a thought - I might get a couple of those cheap tachos that pick up signal from a spark plug lead. The primer set up on these motors is a bit how's your uncle.
You turn them full clockwise before you can pull them out to operate the little primer pump. This opens the throttle a little for a fast idle.
After the motor is started and is warmed up you push them back in and wind them anti-clockwise to set the idle speed. But you are only judging it by ear.
A tacho would be handy. It might be that I simply set the idle too low.

[MickinOz]

I must say, I dipped the tanks when I got home. Very satisfying fuel economy.
Very satisfying indeed. I thought the port engine would have used significantly more fuel, as I ran it for a couple of miles further than the starboard engine, but barely noticeable on the dip stick.

[twisted lines]

About Y2K company I worked for paid for 4 two stroke 125 merc’s they ate coils.
Didn’t hate to blow one up

[MickinOz]

Found myself with a couple hours spare, so I did a little trouble shooting.
Filter looks filthy, and unlike the other motor its aftermarket. So that will be going.
But I also found a couple of smallish problems.
One, the clip that secures the outer sheath of the starboard throttle cable to the engine was installed wrong and not doing its job.
Two, the block that the cables pass through to enter the engine cowling was loose, as the securing bolt was not present.

Other than that, not much.
Started each motor, ran them until they were warm, then disconnected the fuel lines and let them idle.
Both seemed to idle same speed, both were about the same temperature, measured at the thermostat housing with an IR thermometer.
In any case, I still cannot see any means by which the electronic ignition would be able to detect high temperatures and switch off.

The starboard engine very easily started multiple times, and ran like a clock ( but with less ticking noises ).
I'm going with operator error to be honest. Perhaps with a little bit of loose bolt thrown in.

I trimmed the motors in another notch while I was at it. In the really choppy stuff, there was a bit of ventilation going on.
When coming out of the hole I had to have more speed on than I wanted to have, to get the stems down into the water.
I had to go pretty quick to keep her over the hump and on the plane.

So I figured I could afford to trim her in a bit. It might slow it up a little bit, but I'm not too concerned.
In my experience on our nasty wind driven chop, it's more important to have a boat that's good at going slow, rather than able to go extra fast.

Tried measuring hertz at the primary side of the coils with my multimeter.
But looks like it's too sensitive, and every little harmonic counted as a hertz. If I was to believe the numbers, the engines were doing 24,000 rpm at idle!