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1988 25HP tiller model

J

jim18611865

Regular Contributor
I have a chance at a maybe 1988 25 HP tiller for $100.
I know the owner, and he had a lot of trouble for two seasons and got fed up and bought a new engine.
From being on the boat with him the two seasons he owned it, and from his description it sure seems like the only thing it really needs is a full/real carb job.
My last free merc I ended up $200 in the hole after selling it but it wasn't that bad to work on. I have several OMC's but the 7.5 1971 merc was the only one I ever worked on.
I will of course check spark and compression before taking delivery.

Any other thing I should look for before buying?
The two seasons he owned it was salt water only, before that was 100% fresh water.
I will of course do the water pump and L/U oil.
It has now been sitting for two years. Likely not even rinsed after the last outing.
 
Pull the plugs, feel it for piston roughness (seizure) then do a compression test. If 120 plus it's probably okay. Then check it for spark, If that is okay too, grab it! You can't go wrong, even if you part it out.

Jeff

PS: The cost of rings or a carb/ fuel pump rebuild kit is about what he wants for it! Seriously.
 
OK I got it this weekend.
Serial # is 0C173354. Is this a 1989?

90 PSI each cylinder by my gauge. Healthy spark as well.
Cosmetically looks really good.

I am thinking lower unit servicing, and new fuel system (pump, lines, and carb kit).

Does this thing shift with the throttle twist handle?
It looks like it does, so I may start liking Mercs.
Now where do I get a factory manual, they don't look as easy to find as OMC manuals?

Thanks again!
 
Post 86 25 horse Merc's are a really nice little motor.

Yes, the throttle and shift are both controlled by the (twist) tiller handle. It is nice in most situations except when troubleshooting. Then it would be nice to be able to advance the throttle without putting it in gear - which you can't do with the tiller handle on these - advancing the throttle, put's her in gear - no choice...

Some of the chronic issues with mid/late 80's portables (8 thru 25 horse) that you may want to check out are:

Bad gaskets on the intake covers - not the manifold itself, but the covers on the side of the powerhead (located back near the heads - which aren't really heads, just a waterjacket cover).

Bad upper main crank seal - called an oil seal by Merc. Can easily be replaced by pulling the flywheel and picking out the old seal which is a combination of metal, rubber and a spring. The spring (which snugs around the crank) pulls away from the rubber over time and you get a crankcase leak affecting the upper cylinder.

Both the above conditions tend to leave an oily film on the powerhead or a goopy mess in the lower pan (what's left of the gas/oil/air mix that leaks from the crankcase, after the gas evaporates) - so if it looks like the motor has been sprayed down with fogging oil after it has been running for a bit, look for either to be the culprit.

Also, take a good look at the coils. Remove them and wipe them down well then inspect them for small hairline cracks. Even if they check out ok on the meter, replace them if you find any cracking at all on the cases. Sierra coils are about 1/3 less expensive than OEM Merc coils and they produce higher voltages which helps keep the idle very smooth on these motors.

Do get a complete carb kit. The fuel pump is integral to the carb and a rebuild of both on a motor approaching 25 years is certainly in order.

Likewise, do the waterpump. You should pull it first and get a look at all the components. Merc has two or three different kits for these starting with a basic impeller kit all the way up to new base plate, water tube guide tube etc. The "total" kits are obviously more expensive, but cheaper than buying the individual components should you need them.

If the motor has ever seen salt water, take a flashlight and check the water tube when you have the lower unit off. Salt is very hard on the tubes and often burns holes in them. It can also clog up the exhaust tube (gets like a mineral build-up and closes off the hole), so after checking the water tube, try getting a look at the exhaust tube which extends about half way down the housing.

As Jeff notes a Seloc's manual is really more than enough for this particular motor. Selocs "Volume 1, Mercury Outboard, 1965-1991, 1 and 2 cylinder, Tune-Up and Repair manual" is more than worth the 35 bucks. It is available in on-line format, but I really prefer the printed version - both the same price, your choice.

If there is a particular part or excerpt of the factory manual you may need, you can always drop me a message here. I have pdf versions and can send you a page or two as applicable.
 
Thanks a bunch, that is really good info. It was freshwater it's whole life except the last 4. Two on salt and two sitting under a porch.
Powerhead is really clean.
 
OK either I am dense, or this site doesn't carry the parts I need.

The carb kit looks like gaskets only (18-7212) Where is the full kit?
The only fuel pump kit for 25HP doesn't include my serial number?
 
25HP.JPG
Money isn't the issue, where is it?

Which of these is my engine?
I would happily part with my money if this wasn't so confusing.
Maybe I will have to take a run to my dealer.
 
90 lbs. compression isn't very much on one of those. I re-ringed mine when it was that low and got 120 plus out of it.

Jeff
 
The 90 lbs. is likely OK. My smaller engines 6HP and 8HP OMCs read 70 psi each. They run and have run perfectly for years.
My bigger 3 cyl OMCs all read 95 and one has been perfect for 4 years.
Thanks again, I will be ordering parts (once I figure out how) once I read the manual.

Those of you who work on both OMC and Merc, is it more difficult finding what you need? (ie. just finding part numbers etc.)
OMC has a nice website where I can look up anything 1968 and up. Does Mercury have a similar website?
OK guyjg I think pointed me to the site that like OMCs.
 
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