Luc Vinet
06-03-2003, 08:56 AM
I just got a mercury 25 hp serial OA743255. The previous owner did not ad the operation manual. Is there anyone that can explain how the adjustment on the carb as to be done. It is a little erratic at slow speed. How to verify the timing? What is the exact fuel and oil mix? Where can i get an operation manual? What is the recommended spark plug to put on it?(just whant to make sure i have the good ones).

Thank you in advance


06-04-2003, 11:00 AM

That is as 1989 engine. The spark plug should be Champion L78YC ( or its equivalent), gap .040in. Use a 50-1 gas/oil ratio.

You can adjust the idle mixture. The screw on the front of the carb, just above the bore is the mixture screw. Turn it clockwise until it is lightly seated. Then back it out 1 1/2 turns. From there you might find that it runs better if you lean it a bit, by turning the screw clockwise ( maybe as much as a half turn). If you go too far it will begin to sputter.

The engine should idle with the throttle plate closed. Set the timing so that it idles at a comfortable rpm. I like to keep the idle as low as is practical.

You should be able to order a service manual right here.


Luc Vinet
06-04-2003, 12:14 PM
" Thank you very munch Tony for your respond,

But how do you set the timing so that it will idles at a comfortable rpm?
The other thing is that the spark plugs that are on it right now are NGK BUH-W2. These spark plugs can not be gapped because they are the flat type.
Do you think that these are wrong?
How much $$ for the manual with delivery? I live in Montreal, Canada.


Luc "