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25 hp merc compression

S

srl

Member
"My outboard quit running as w

"My outboard quit running as we crossed the lake
got it started spark on both plugs but runs very rough
compression feels very low when i hold thumb over bottom piston
whats the fix, is it easy?
Thanks,
Steve"
 
"put a compression tester on e

"put a compression tester on each cyl, unless you have a calibrated thumb. If its good check for spark again and then clean the carb"
 
I'm getting just over 100

I'm getting just over 100 psi on top cyl
0 psi on bottom cyl
is the next step replacing the head gasket?
is there a critical torque rating when replacing the head bolts?
thanks for any help
 
"the next step is removing the

"the next step is removing the side cover, which ever is easier. This would be the transfer port cover or the exhaust cover. You would be looking for damage on the cyl and piston. Both these are visible with the cover off. Best to investigate a little before buying any parts. Here is a link to your engine,http://www.crowleymarine.com/mercury_parts/3236.cfm"
 
"Steve, your motor is "hea

"Steve, your motor is "headless", so no gaskets to replace.

If your have no compression in one cylinder, then either the piston, rings or cylinder itself is shot (or any combination of them).

A 99 model will have Mercosil cylinder linings so at least it can be rebuilt and bored if necessary.

But first, as JB suggests, pull the exhaust cover plate and get a look inside and decide from there how much you want to spend to fix it.

It's very rare that a cylinder gets toasted on this particular model and the cost of repair could be greater than a replacement motor of similar vintage or newer...."
 
"Nice to here from you Graham,

"Nice to here from you Graham, and you agree with my post
kiss.gif
. Looking at his engine in the pic I had wondered if they had changed the jug design, hard to tell from the pic."
 
"Steve its common to have a ga

"Steve its common to have a gasket under the waterjacket cover. But thats not a head gasket as you know it. Take Grahams advise if not mine, have a look at the side covers. If your engine is the same as all other merc's, the head is cast into the block/case. It looks like the inside of a jug with the bottom cut off.
Either way if you pull the watercover off you will shortly be pulling the side cover. Let us know how it goes but to fix it you must pull the powerhead, split the case, pull the crank, and at least change the rings. Thats if the case/block is cracked or worse."
 
JB / Graham
thanks for the ad

JB / Graham
thanks for the advice
took exhaust cover off and things appear to look fine as i pull the cord
now what???
 
"push on the rings with a smal

"push on the rings with a small screwdriver, do they push back? If they have tension then there is a possibility the so called head has a crack or hole."
 
"Pull the power head apart and

"Pull the power head apart and check out the pistons. I'll bet the bad one has the rings stuck big time in their grooves, and smear marks down the side.

Jeff"
 
"Gents
The mechanics tell me

"Gents
The mechanics tell me get a new one
because the cost would be between 1,000-$2,000.00
to fix, one said the bores were coated

would the lower unit from this merc fit the lower unit on my 25 hp 1998 Force?
now that i have a merc w/power head problems and a force w/lower unit problems"
 
"tear it down yourself, don&#3

"tear it down yourself, don't trust the so called mechanics at the shop. They don't want to work on it, its too old. Next time at the water drop that force into the drink, maybe it will become something usefull."
 
is it much of a project to tea

is it much of a project to tear down and rebuild
with everything together its hard to see where and how it splits
 
"right behind the carb is the

"right behind the carb is the half that needs to be removed. After the 2 bearing ends are unbolted the cranckshaft, rods and pistons pull right out. Its not easy the first time but it can definatly be done at home. It really all depends on whats wrong. If the block is trashed why bother, get a replacement or overhauled unit. If its just a piston you can buy new for $100 plus a gasket set. All depends on you, learn or pay. You will need a manual for torques and assembly."
 
"Jeff, are you saying its not

"Jeff, are you saying its not worth fixing. I checked the merc site and rings are $125 but a piston is $90. I would figure Sierra makes something cheaper."
 
"Not really! What I'm say

"Not really! What I'm saying is, buy a used power head on eBay or someplace else. Lots of motors out there with smashed lower units.

Jeff"
 
"Hey, Steve: If you pick up a

"Hey, Steve: If you pick up another Merc 25 power head--and it has ignition components on it--I'll buy some of them from you. Need (at the very least) a charging coil and spark box.

Jeff"
 
"can i start this motor w/exha

"can i start this motor w/exhaust cover off?

mechanic said try spraying "Mercury power tune" in carb, may free up the rings"
 
"I wouldn't try (never

"I wouldn't try (never done it myself) - alot of "raw fuel" gets sucked right into the exhaust from the intake port (2 strokes lose 30% or more of their fuel right out the exhaust) and without the cover giving the cylinders some back pressure it would probably be worse - and could be potentially dangerous if you get a backfire with all that atomized fuel real close to the powerhead....

Give the powertune a try - not magic, but at 5 bucks a can at least worth checking out - I use it as a preventative and for cleaning out powerheads that have been winterized - I don't trust anything in a can to actually provide a "mechanical repair", but it can't hurt..."
 
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