Motor with 35 psi?

[Dirtjunkie]

Ok gurus, I need some help. I have a 91 Yamaha Pro V 150 on a 1999 Pro Gator bass boat. I recently bought this boat from a "friend of a friend". I have been around race engines most of my life so this is on me for not checking FIRST. Anyhow, the motor cranks very easily, runs great with fresh plugs but kills them pretty quick. So I do a compression check, I'm getting between 35 and 40 on ALL cylinders. I used 2 gauges, one a new Snap-On. I talked to a guy at A local repair shop and said he doubted the readings, said it should be a pain to start and not run worth a darn, that that much compression shouldn't even fire the fuel, I tend to agree but don't know what would cause the false readings with 2 gauges to be so consistent. He said the plug fouling could be from a pin hole in a fuel pump diaphragm spraying fuel directly into the cylinder killing the plugs, which does make since to me. He said it would run between 5-6500$ for a rebuild but I'm not putting that much into this old of a boat. Any ideas or suggestions?

[99yam40]

Stuck rings maybe, but strange it is all cylinders.

might check a different motor to see if gauges are functioning correctly

[boscoe99]

How is the compression check being done?

Should be: Hot engine, throttle wide open, all plugs removed, fully charged battery, crank until the compression value peaks.

Does the motor itself run well, other than the plug problem?

[99yam40]

I was told before that the 2 strokes do not need the throttle open to check compression like the 4 stokes need it.

They can get enough air from the exhaust port to get a good reading. I did not believe it so I tried it with throttle open and closed, and sure enough I got the same reading

[Dirtjunkie]

It was done with cold motor, throttle closed, as I was under the impression that a 2 stroke didn't matter. Battery "might" have been a lil low, but turned over fine. All plugs out. With fresh plugs runs and cranks fine, maybe just a bit down on power as will not turn over 5k with 21Powertech prop on 180V(18 foot) Pro Gator bass boat. But "touches" 50 mph per gps, so I'm happy with that.

[boscoe99]

I believe that it does matter.

Air is drawn through the throttle body(s), across the reed valves and then into the crankcase. The piston going downward pressurizes the air somewhat and when the intake port is uncovered the air is forced into the combustion chamber. The exhaust port does not do this. Air is trying to go out the combustion chamber via the exhaust port, not into the motor as it would do so via the intake port.

Shouldn't a test be performed so as to simulate the way that the motor operates?

Here is what Yamaha tune up spec book has to say:

"Make sure the engine is at operating temperature.
Always use full throttle with ignition disabled when testing compression. This lets as much air as possible enter the cylinder so the cylinder condition, not the throttle restriction, may be tested."

Here is what a Yamaha Marine Technical Guide has to say:



Why not retest the motor with it hot, and the throttles wide open, to see for yourself what, if any, difference that it makes?

[Dirtjunkie]

Thank you for the info, I don't know much about 2 strokes, all my knowledge is from being around 4 stroke car race engines most of my life. I will make sure to warm the motor too and I will make sure the battery is fully charged and will get someone to help me so I can hold the throttle wide open(hot foot) Even if it doubles it is still what is considered low. I am mystified because of the way it starts and runs(with fresh plugs). Thought about trying a leak-down but have been told they aren't really accurate on a 2 stroke? Am going to at least change out the diaphragms and see if this doesn't help with the plug fouling problem, because they never really look oily, just wet and dead.

[99yam40]

like I said before try the gauges on a different motor to make sure they read correctly

[99yam40]

I believe that it does matter.

Air is drawn through the throttle body(s), across the reed valves and then into the crankcase. The piston going downward pressurizes the air somewhat and when the intake port is uncovered the air is forced into the combustion chamber. The exhaust port does not do this. Air is trying to go out the combustion chamber via the exhaust port, not into the motor as it would do so via the intake port.

Shouldn't a test be performed so as to simulate the way that the motor operates?

Here is what Yamaha tune up spec book has to say:

"Make sure the engine is at operating temperature.
Always use full throttle with ignition disabled when testing compression. This lets as much air as possible enter the cylinder so the cylinder condition, not the throttle restriction, may be tested."

Here is what a Yamaha Marine Technical Guide has to say:



Why not retest the motor with it hot, and the throttles wide open, to see for yourself what, if any, difference that it makes?

That is what I thought too, but then I tried it on my motor just for kicks and it did not make any difference if open or closed.

Motor is not turning fast enough to make a difference I guess and it grabs enough air from exhaust port.
Next time you run a test on one try it yourself