1981 Merc 25 random stumble at idle?

[redrum]

Can you guys help me diagnose the problem i'm having with my engine? It is a 1981 Merc 25 short shaft tiller pull start only. When at idle it will stumble at random times and go dead. I can up the idle and it won't go dead, it will just stumble and try to creep back up to the same idle RPM but if it stumbles multiple times it will eventually go dead.

I'm by no means an outboard expert, but I'm trying to get this thing going so I can get back on the river before fall is here.

I've ran about 12 gallons of fuel through it with two pints of seafoam. I've replaced the plugs with champion plugs gapped at .30. The plug wires seem to be ok. I've even swapped them with another engine and no change. Could it be the coil packs or the power pack going bad? I'm not sure how to test a powerpack.

Oh and compression is 120 psi on both cylinders.

Thoughts?

 

[redrum]

Could those who have an older Mercury 25 take a pictures down the throat of the carb and post it? I've notice that the carb from the engine I've having trouble with seems to have a tube missing from the inside of the carb.

I looked at another Mercury 25 and on the top of the throat of the carb is a brass tube about an inch long that protrudes down into the center of the throat. My carb does not have this tube. It looks like it may have broken off.

I'm not sure if that tube is for the idle circuit. I'd love to see if this varies from year to year, even on the same style carb.

 

[Natedog57]

If you have the model/serial number you can pull a diagram here:
https://www.boats.net/parts/search/Merc2/Mercury/parts.html

You didn't say so I'll ask, did you pull/disassemble/clean the carb? This sounds like a classic case of dirty carb.

My 98' Merc had an idle screw and adjusting that back (or very close) to the factory recommended setting helped smooth out the idle tremendously.

If the engine has run great for you in the past, and recently started acting up I'd think the carb needs to be cleaned and would do this before adjusting the idle screw.