Temporary tach install

[summitlt]

I have a 1985 50hp merc on a 16' Duratech. It currently has 12p prop on it that I know is too small. It will do 30mph bit sounds like it's over revving. I have a tach for a car and one of those old dwell meter tachs. How can I hook one of these up to see what I'm turning to figure what size prop I need? I know being a 2 stroke it won't be right but I should be able to double the number to figure it out. Any ideas?

 

[Chris1956]

Those tachometers will likely not function at all. The tach is connected to the stator, where it counts AC pulses to determine RPM. You can try them, but don't expect too much.