1979 mercury 140 outboard

[rustyjcole]

Hello all, i have the above named motor and i just purchased it, i am having issues with it bogging down at WOT under load, it will not plane out and runs pretty crappy. this does not happen when its on muffs just when on load. i did 3 carbs rebuild kits and a fuel diaphragm kit, i re did all the fuel lines while in this process and added a clear inline fuel filter. the fuel diaphragm kit did not include instructions for reinstallation, maybe i reinstalled it incorrectly? still same issues after last test run. does anyone know where i could get a diagram of this? and also any suggestions on fixes? all the posts i have read say fuel starvation but after the rebuild unless i didnt get the diaphragm right then that was not the issue. i appreciate any advice.

 

[racerone]

Open slow speed mixture needles 1/8th turn at a time on the lake.----Test run.----They all rev up on muffs.----Means nothing.----It will rev up running on 3 of 6 cylinders !

 

[rustyjcole]

the idle mixture screw? im fairly new to adjusting carbs

 

[Chris1956]

The carbs only have a single adjustment screw. It is about 2PM on the face of the carb.

Gee, if you rebuilt the carbs, you should have seen it.

 

[rustyjcole]

i tried to adjust my mixture screw on the lake during myast test run, and still having the issue

 

[Chris1956]

You need to set the mixture screws to 1-1/2 turns open from lightly seated. The motor should run at this setting. Warm the engine up and run at idle in gear.

Usually that carb setting is too lean to allow acceleration. So try to accelerate. if the motor stalls or fails to rev up, open the top carb's idle mixture screw 1/8 turn and retry acceleration. if acceleration is not smooth, open mid carb screw 1/8 turn and try again. Repeat if necessary with the bottom carb.
Repeat whole sequence if necessary until you have a decent idle and smooth acceleration. Adjust idle stop screw and throttle cable until idle speed in neutral is 900 or so.

You might buy a service manual and check timing. That is important as well.

 

[rustyjcole]

thanks for the advice, when i was originally "adjusting my screws i set all between 1-1/4 to 1-1/2 turns, but when i adjusted i would adjust all 3 carbs every time i was under the impression that when i adjusted my needles i had to adjust all 3 carbs every time.

 

[Chris1956]

The top carb tends to run leaner than the mid carb. The bottom carb tends to run richer than the mid carb, in my experience.