What should the compression be for a 1987 merc 150?
Is compression a standard nember or will it vary from different types of engines.
Also when buying an older engine what should I be looking for or asking questions about to “hope” I get a solid engine?
I don’t know the exact spec for a 125 merc, but usually 2 strokes are in the 120-140 psi range. The important issue is that all of the cylinders are close to each other. What are you seeing?
For an “acceptable” motor, minimum allowed compression I “believe” is 115lbs.
All cylinders need to be within 10% of each other. Not 15% above and 15% below……..15% total within each other (all four or six cylinders).
What questions:
Basically, 2 strokes are “silly simple” operation. Air/fuel gets sucked in and blown out all through the action of the piston. 4 strokes are complicated, 2 strokes are simple…….
As for what questions: As long as the motor hasn’t been torn apart and the compression is good……….There isn’t much more to dig into. Basically, a 2 stroke works or it doesn’t……..
Make sure your compression is acceptable within all cylinders and then you can have it “super-tuned” at a good shop that will check the resistance of the ignition and ensure that the carbs are in good shape.
However, if on a test run, she runs strong………there isn’t much more to really look at.
The only concern you really, honestly need to have is that she isn’t running lean and you are operating within the rpm specifications to ensure you don’t burn a piston or burn up your bearnings.
Again, if it runs strong, compression is good, and the motor hasn’t been torn apart (via new powerhead or pistons), rock and roll!!!!!
I would also ask the previous owner about the water pump/impeller. How long they have had the motor, if they ever changed it?
After 40+ years of fixing fishin motors, here’s what I start with. It usually saves a lot of time in the long run.
First, check the fuel bowl. If there’s ANYTHING BUT clean gas in it, clean it AND all the fuel lines clear to the tank.
Next, check for good fuel pressure. Fuel pumps aren’t expensive compared to the headaches they can cause. Replace it if you have any doubts about it.
Reed valves (2 stroke only) will also cause all sorts of wierd problems. Replace them too, with Boyesen 2 stage reed valves and you won’t believe the difference in low end smoothness AND in high speed power.
Replace the water pump. They’re also cheap insurance. Don’t run the motor more than 3 seconds without water in it or you’ve damaged (maybe ruined) the water pump.
Always change the spark plugs, and then go on to the electricals and other stuff that you suspect needs work.
Check compression. 140 or more is great, but more important is that they are within ± 5 pounds of eachother.
Remember, God does not deduct that time spent fishin. But you can loose a lot of points with him when you’re on a lake with a motor that goes teats up. Not many of us can walk on water, so fix it good when it’s in your nice warm garage!
You won’t remember all your fishin trips when you get up in age, but you WILL REMEMBER every single time you had motor problems when you finally found the time and money to go fishin! Good luck.
Great info guy’s.
smokin yammy do you still fix motors? where are you located. 
welcome to the site. 
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