John, I had a 1979 25hp Johnson and the OMC rule of thumb
is to bottom the stem out firmly but gently, then back up
1 and 1/2 turns and fine tune it from there after the motor warms up.
have you checked the needle point to ensure it is not just worn out?
are you running 50:1 gas/oil mix or something else.
Do you have a manual for that motor ??
I found the parts manual and shop service manual on ebay for about $10 each.
I am FAR from being any kind of mechanic, but the manuals sure do help.
https://www.marineengine.com/parts/johnson-evinrude-parts.php?year=1977&hp=25&model=25E77S&manufacturer=Johnson§ion=Carburetor
part numbers 29, 35 and 33 are usually the culprit.
very easy to replace and not so expensive.
This is a post from iboats:
Re: 1976 johnson 25HP low speed idle carb adjustment
Initial setting is: Slow speed = seat gently, then open 1-1/4 turns.
Start engine and set the rpms to where it just stays running. In segments of 1/8 turns,
start to turn the S/S needle valve in. Wait a few seconds for the engine to respond.
As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.
Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire).
At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.
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