Extract more hp from Merc 30hp 2cyl 2 stroke

[FMP]

I have a 25 merc that's been a 1-2 pull motor since day one. Almost as good as my Chryslers, a good stern stare at the flywheel gets them running.

[NZ Sidewinder]

These motors very in compression.
I've got one with a standard bore that got a light lapping of the head and its 150psi.
Another with plus .020 bore and very light lap of the head, again 150psi.
I've been told they range between 125 and 150psi new

[NZ Sidewinder]

the talk of 180 to 200 psi compression on the tohatsu 25/30hp straight out the box is total rubbish.
around 125 psi is fine on the tohatsu/mercury 25/30. head skimming can take the compression to around 150 to 160.
chris carson for a set of sport or race reeds.

but you need to find out the rules on the mods before making any changes.

I have 4 of these 30hp Tohatsu/ Merc motors.
1 of them has +.020 bore, two of them are std with fresh hone and rings, one is used, std, untouched.

They all have between 140 and 150 psi compression.
The 3 that have been rebuilt have had the heads lapped flat.
You can tell how much has come off the head by looking at the anode recess.
The takeaway is that 150psi is typical for these motors.
I once ran one with a bad coil for about 400m and it bogged down and limped home.
The extra oil in the bores took the psi to 170 but once a new coil was fitted and the engine run again, compression was back to 150psi.


One observation has been that there is a large range of port to casting mis-match.
Of the 4 engines, a 2003 motor has almost perfect match up, a 2015 one has shocking match up and the other one is run of the mill ok, one is unkwown