Mooney mod is a modification to a 10-3878 carb to change it to a 10-3878-M. It makes the carb slightly richer and also helps a little bit with fuel distribution. Other O-360 carbs 10-4164-1(exact same thing as a 10-3878M) and 10-5193 already incorporate the mod. So if you have a 10-3878 it is worth doing if you have an intermittent miss. You will have a slightly richer fuel burn at full rich after it is done but the intermittent miss should be eliminated. One of the classic symptoms of needing the mod on a 10-3878 carb is an intermittent miss in cruise that is worse when hot and is also sometimes but not always caused by starting to lean or enrichen the engine or by starting to activate or turn off the carb heat.
If you have one of the other part number crabs, the mod is already incorporated.
Good Luck,
Mahlon
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