OK guys I see a lot of you want to make your 18 or 20 merc go like a 25. First of all, you can not simply change the carb. The 18/20 merc has a smaller exhaust tube and you will not only loose power but GPM too boot. The 18,20,25 has the same power head. It is ported in the exaust tube and carbs are jetted and ported to match. A 25 lower housing exhaust tube and a 25 carb. will make your 18 or 20 to a 25. plus the timing may need a little bump. best wishes. Also if you have a mercosil block. DO NOT take the thermostat out of it. This will cause a cold scuff of the number two cylinder. The piston heats up before the block. This is why the mercosil blows up.I have ran a mercosil block for years with no problem. I spoke to a mercury rep.testing facility in Panama city FL that told me himself the reason the mercosil blows the number two cyl. is because the thermostat was removed. Service it as you change the H2O pump annually.
