Which is Idle Screw on 71 Mercury outboard

[Dan Salvin]

I have a 71 40 HP Mercury Outboard. I am new to this and am waiting for the shop manual and Seloc manual that i ordered to arrive. I'm trying to figure out which of the three screws on the motor is the idle screw. It has two screws facing away from the boat (on marked "throttle" and the other marked "spark") and one screw facing the boat which is not marked. Should I assume the screw facing the boat is the idle screw? How far should I turn the screw to try to deal with an engine that sometimes stalls when I idle it down to put it in gear? I will probably rebuild and clean the carb when I get the manuals but want to try to adjust idle screw in the meantime. Thanks.

 

[usskydvr]

Re: Which is Idle Screw on 71 Mercury outboard

Neither is your idle screw. The 'throttle' is used to adjust the butterfly to open fully. The 'spark' is for timing. The idle screw is on the carb. As you are looking at it from the front it is on the right side just above the center of the throat. It is about 1/4 in diameter. When making adjustments, only turn 1/8 between checks.

 

[Dan Salvin]

Re: Which is Idle Screw on 71 Mercury outboard

Ok. Thanks you very much. Do you know what the lone screw in the front facing the boat is? Are there any diagrams or images that you know of online that would should the exact location of the idle screw? As I said, i ordered the manual but it will not be here for a few days. Thanks again!

 

[blue90HP50]

Re: Which is Idle Screw on 71 Mercury outboard

Skydvr explained the idle screw location and adjustment, 1/8 of a turn at a time between trys. Also, our old motors need to be warmed up a bit using the 'fast idle lever" for a few minutes, jjuussstt above slow idle speed. Hope this helps. If fuel lines haven't been replaced since the "ethanol additives" came about, they need to be replaced. Remember to use fresh fuel.

Good Luck

Blue

 

[Dan Salvin]

Re: Which is Idle Screw on 71 Mercury outboard

Thanks. Should I be using an ethanol treatment (like Stabil) in the fuel?

 

[usskydvr]

Re: Which is Idle Screw on 71 Mercury outboard

It won't hurt, but I generally don't use any unless I expect my fuel to sit more than 3-4 months between refills.