Yamaha 90 2003 2 stroke - Compression question

[Bubspeed1974]

Morning all,

Just bought a Key West 2004 Stealth 178 and had a compression test done on the motor. The motor looked pristine and started up and idled perfectly. However, the test came back as follows:

Good news:

All three cylinders were drawing the exact same reading

Not sure if good news:

The reading was 70 PSI...

The boat and motor has been sitting for a good 4-5 months at the dealership. The temperature at the time of the test was 30 degrees. The mechanic said that once it is run on a regular basis and gets a good carb clean then the PSI should improve to 80, 90, or even 100 psi. Is he pulling my leg and should I be concerned?

Please advise!

Bub

[BQUICK]

That is on the low side. I would fire it up and do the test warm. Should be 100+ warm. Also, could use some Ring Free or Seafoam to free up the rings.

[robert graham]

With all 3 readings exactly the same, I'd suspect the test equipment or method is faulty. Take the boat out and run it for an hour or so and then re-test it...

[rodbolt17]

a carb cleaning WILL NOT affect compression.
a decarb may.
however with all three reading the same I would suspect the test equipment or test method.

[Capt. Barry]

This is what I was taught many years ago. Works for me, hope it helps you.

1. Remove all spark plugs.
2. Ground all spark plug wires (use a length of wire with alligator clips on each end).
3. Install compression guage.
4. Move throttle to WOT.
5. Crank engine at least five revolutions.
6. Write down your readings and complete for all cylinders.
7. Squirt a bit of oil into each cylinder and repeat the test. Check to see how much of an increase in pressure you read.

For example, my little yamaha 2 stroke 70 read 130, 130, 125 before oil. On the second try with oil, 140, 140, 130. The last cylinder may have a stuck ring, so, time to drop Yamaha ring free in the tank.

[rodbolt17]

oil in a two stroke for compression testing is a waste of time and oil.
its an old school thing that should have died off with the dinosaurs but the myth persists.
in a 4 stroke the myth went that if compression increased it was rings if not it was valves.
I dont test compression much anymore on 4 strokes, a leakdown test is faster and more accurate.
but there you have the myth of oiling a cylinder for a compression test.

[Capt. Barry]

Rodbolt 17.

Sorry, I'm a old dinosaur.

Yes I agree about the four stroke test on whether a leak was rings or valves, however, we are not talking about much oil and I've found a large change in pressure MAY signal time for a rebuild in two strokes.

And, since outboards manufacturers are phasing out two strokes and are going to four stroke, newbies should know about the quick and easy test.

[rodbolt17]

leakdown testers are just getting to cheap.
they work on 2 and 4 strokes.
way more accurate than compression testing.
poor compression on a two stroke will indicate a head gasket or ring failure.
no need for oil cause either way the head is coming off.
4 strokes have ex and intake valves as well as head gaskets and rings.
a leakdown test injects a known air pressure in the (theoretically ) sealed cylinder and measures the amount of makeup air to maintain said pressure.
this is indicated as a percentage of leak rate.
4 strokes max is about 15% and two strokes typically at 10 percent.
once hooked up you can listen for air at the ex,intake or crankcase oil fill.

in this case with ALL even at 70 PSI I would suspect the meter or the test method.
odds of dead equal wear/damage on all 3 is slim to none.