idle mixture proublems

[jgsmokercraft]

I have a 1991 Mercury 90 HP motor with 3 carberators, I think it's running too lean, would like to richen it up. I know turning the idle mixture screw clockwise will do this. I also have a clymer Mercury outboard shop manual which doesn't tell me exactly where the idle mixture screw is at. Can anyone help?
Thanks,

 

[Chris]

What does the manual say?

Usually what they say is something like this...

Turn the screw all the way in until it seats itself, then back it out a certain number of turns and then make fine adjustments until the motor runs smoothly.

-Chris

 

[Fun Times]

I have a 1991 Mercury 90 HP motor with 3 carberators, I think it's running too lean, would like to richen it up. I know turning the idle mixture screw clockwise will do this. I also have a clymer Mercury outboard shop manual which doesn't tell me exactly where the idle mixture screw is at. Can anyone help?
Thanks,

I have found it in the mercury service manual for you.

The service manual part number for your engine from mercury is 90-136452, You should seriously consider ordering a mercury service manual, Either online someplace or through a mercury dealer in your area using that part number, The manual you have is not the greatest compared to the factory manual.


Service manual part# 90-136452 - 70/75/80/90 3 CYL (1987 - 1993) and 100/115 4 CYL (1988 - 1993)

Idle adjust screw 1-1/2 turns out from lightly seated (+ or - 1/4 turn) Set half way between lean and rich.

Also just as a FYI for you, The fuel pump pressure on the engine side at wide open throttle for your engine is 4-6 psi depending on RPM's.

Good luck, I hope this helps

 

[jgsmokercraft]

Chris. Thanks for the prompt reply. My manual shows the idle mixture screw on the side of the carberator but I'm not sure if it's right because it looks like a generic picture. Some carberators have it on the front. I understand how to lean it out and richen it. I'm just not sure which screw to adjust. Another gentleman responded to get the mercury manual instead of the Clymer's so I'm going to order it.

Thanks again.

 

[jgsmokercraft]

Thank you for your reply. Will order the manual from Mercury. Probably will take a little time to receive. In the meantime, can you see from your manual where the idle mixture adjusting screw is located. The side of all 3 carberators or the front of these carberators?

Thanks again.

 

[Chris]

Let us know how you ultimately make out.

-Chris

 

[Fun Times]

Thank you for your reply. Will order the manual from Mercury. Probably will take a little time to receive. In the meantime, can you see from your manual where the idle mixture adjusting screw is located. The side of all 3 carberators or the front of these carberators?

Thanks again.

It looks to be on the front side, in the center just under the fuel float bowl assembly, It's down low and it's screwed into the lower throttle body assembly.

The screw will have spring on it.

It looks like the adjustments will mainly be between 1 1/4 to 1 1/2 turns out. Start out with 1 1/4 turns out until engine is warm then do the final adjustment to help achieve a smoother idle speed.

Keep in mind that a good service shop would normally synchronize the carburetors together using a special tool to get the engine to run better.