What should the compression be on 2003 Tohatsu 9.8 2 stroke?

[sparky4453]

This is an '03 M9.8B.
I have a Seloc service manual that says compression for this year and model should be at 57 psi.
I believe this to be incorrect as both cylinders read 109 psi each, when using a compression gauge.
A tachometer is on my to-do list, but the motor runs great and it sounds like it's within it's operating range.
Am I performing this compression test incorrectly? Or is the manual wrong?

 

[millacd]

Re: What should the compression be on 2003 Tohatsu 9.8 2 stroke?

This is an '03 M9.8B.
I have a Seloc service manual that says compression for this year and model should be at 57 psi.
I believe this to be incorrect as both cylinders read 109 psi each, when using a compression gauge.
A tachometer is on my to-do list, but the motor runs great and it sounds like it's within it's operating range.
Am I performing this compression test incorrectly? Or is the manual wrong?

well, i've never heard of any engine that runs with compression that low (57), but i've been wrong before.

describe how you are performing the test

 

[Sea Rider]

Re: What should the compression be on 2003 Tohatsu 9.8 2 stroke?

Procedure :

-Operate engine to normal operating temperature, stop.
-Disconnect magneto leads.
-Remove spark plug(s) and install thread-type compression tester in spark plug hole.
-Place throttle in fully open position and crank engine through with at least four compression strokes.

Take reading for each cylinder and verify specified pressure is obtained. 57 psi is correct for 2 strokes 8 & 9.8 models.

Happy Boating

 

[sparky4453]

Re: What should the compression be on 2003 Tohatsu 9.8 2 stroke?

Procedure :

-Operate engine to normal operating temperature, stop.
-Disconnect magneto leads.
-Remove spark plug(s) and install thread-type compression tester in spark plug hole.
-Place throttle in fully open position and crank engine through with at least four compression strokes.

Take reading for each cylinder and verify specified pressure is obtained. 57 psi is correct for 2 strokes 8 & 9.8 models.

Happy Boating

This is the exact procedure I have performed, except for the fact that the motor was cold, and I may or may not have opened up the throttle fully.
I will repeat the procedure (correctly) and report back with the findings.
Thank you.

 

[sparky4453]

Re: What should the compression be on 2003 Tohatsu 9.8 2 stroke?

I have performed the test again with the motor warm and throttle opened all the way. The test was done twice, with and without one spark plug installed in each instance.
This time the cylinders were at an even 110 psi each.
The tach shows correct rpms at idle and in gear, 900 and 750 respectively, when testing in a tank. After some minor fine tuning of the pilot mixture adjustment and idle stop screws, that is.
The compression gauge is a standard automotive issue. The same gauge was used in a 3 cylinder personal watercraft, and verified proper pressure according to the pwc's service manual.
I think I'll chalk this up to the outboard service manual presenting incorrect information.

 

[millacd]

Re: What should the compression be on 2003 Tohatsu 9.8 2 stroke?

I have performed the test again with the motor warm and throttle opened all the way. The test was done twice, with and without one spark plug installed in each instance.
This time the cylinders were at an even 110 psi each.
The tach shows correct rpms at idle and in gear, 900 and 750 respectively, when testing in a tank. After some minor fine tuning of the pilot mixture adjustment and idle stop screws, that is.
The compression gauge is a standard automotive issue. The same gauge was used in a 3 cylinder personal watercraft, and verified proper pressure according to the pwc's service manual.
I think I'll chalk this up to the outboard service manual presenting incorrect information.



View attachment 203388

good work detective sparky.

I would have been amazed to see an internal combustion engine run with a compression test of 57 psi (except for kerosene engines, i believe they are very low usually under 70 psi). i'm not saying it's impossible for 57 to be the proper number, but it sounds very very unlikely.

 

[sparky4453]

Re: What should the compression be on 2003 Tohatsu 9.8 2 stroke?

good work detective sparky.

I would have been amazed to see an internal combustion engine run with a compression test of 57 psi (except for kerosene engines, i believe they are very low usually under 70 psi). i'm not saying it's impossible for 57 to be the proper number, but it sounds very very unlikely.

My thoughts precisely. Just skimming through the manual, and comparing the specifications to the actual, measured compression values, has made me question the manual's validity as a whole. Oh well, I could always use more TP

I did find a source that lists an another set of measured values on an outboard they had for sale, this time at 120 psi.

8hp Tohatsu Outboard

This seems more like it.

 

[Sea Rider]

Re: What should the compression be on 2003 Tohatsu 9.8 2 stroke?

Definitely seems a wrong data that's not been corrected. It's too low compared to other 2 strokes same brand engines.

Happy Boating

 

[TOHATSU GURU]

Re: What should the compression be on 2003 Tohatsu 9.8 2 stroke?

Never worry about the compression number itself...You just look for even compression between the cylinders.