25 hp Merc surging

[BladeRunner]

Lookin for possible causes. My 25 hp Merc runs great when you first leave the ramp, can go a mile or more with no problems full trottle smoothe running. Stop and fish for an hour or so then you crank back up, the motor will run smoothely to about 1/3 throttle. Any more and it begins to surg and will not smoothe out. Replaced tank, fuel line, bulb, added inline filter, new gas, stabil, spark plugs, etc., but still same problem.

Anyone got an idea what might be causing this type of problem?

[olduckhunter]

air leak or clean he carb

[RCS]

Bad cdi. When it gets warm from running, it expands and creates the surge. This is very common on all outboards. Usually a $ 100 to 125.

[BladeRunner]

Thanks EarlWaynetom, I think your likely right on target about the CDI power pak. Is this just a match up the year, model and replace or are there other adjustments that have to be made when adding a new power pak?

[Barnacle Bill]

Mercury calls them a switchbox.

[BigRiverMarine]

Why doesn't the switch box get warm and cause problems the first time it's run full throttle? Only after an hour of not running, it starts this problem, before it gets warm. I would clean the carb and choke first. Sucking trash in the carb. then starving for gas. If carb cleaning doesn't work, then go electrical.