Merc 2.5lt carb 2002 "loads up" at idle and dies?

[CircleHook]

Merc 2.5lt 2002, CDMs, carbs- Overall motor runs great, starts from cold reasonably quick, good throttle response thur power ranges.
After a 5-10 mile run when I come off plane and go to netural idle- it sounds good and idles smoothly. After a time of idling in netural- maybe 5 minutes- it begins to lose RPMs and runs progressily rougher- has a cough or two and eventually dies. Refires fine if throttle is advance- with throttle fully returned- start/die start/die. I go a little heavy on the oil- but not excessive.
Fuel pump and carbs and been built within last 10 hours- it did same before these were built.
Suggestions please !!

[Go Time]

Have you adjusted the idle as described in the service manual. The cough you describe may be lean pop. There are idle adjustment screws for each cylinder on the carbs and if memory serves, you start with them at 1 1/2 turns from lightly seated, warm up the engine to operating temp. then with the engine in gear idling (should be ~ 700 RPM), you adjust them in small increments until the cylinder stumbles then back it out 1/4 turn. Get the service manual and check this procedure, I may have something wrong....but you get the idea.

[CircleHook]

I figured other way around- too rich- loading up- I have a hard time detecting the "stumble" on 6 cylinder engines- good easy starting point for sure

[Go Time]

If it gets worse as the weather gets colder...it's a good indication that its on the lean side.

[Seanj85]

Thermal Air Valve may be stuck closed

[CircleHook]

It was an easy fix- turned all idle screws richer 1/4 turn- she will sit and putter all afternoon now- thanks

[Go Time]

There you go....