Re: Starting issues
Sounds like you did all the normal things. On sitting up one first thinks of varnished fuel clogging tiny passages, or stuck rings which will not allow you to develop full compression. Then there is moisture intrusion into electronics, especially the high voltage pulse packs that power the plugs, or cracked hoses.
A quick check of a pulse pack is to remove one of the plug caps (leaving that plug in the engine) and see if it makes a difference in performance/starting. If no change, then something is wrong with that cylinder.
Merc moved numbers around a lot and I don't know what your engine configuration is. You said "carb" singular grammar which suggests that you have a 2 cylinder with 1 carb...if you had a 3 cyl you would have mentioned "carbs" meaning 3 or if the old 4 cyl you would again have mentioned plural carbs as they had 2. Ignition is via a central control module of some sort with separate pulse packs to generate the high voltage pulses for the plugs. Have no problem history with the control module, but have had to replace pulse packs on more than one engine and they have funny ways of failing.
So ignition could be the problem if one cylinder doesn't affect operation.....carb problems and compression affect both. A compression test will solve any compression issues and I would look for around 100 psig spinning the starter with the other plug removed.....I don't have the exact range on compression, but if you can get at least 100 the engine will run. If you spray the cylinder with light oil and do it again, an increase of 20 to 30# would indicate stuck or worn rings. You did the carb so, unless you missed a needle sized port, that should be good to go. Don't know what the threaded hole at the top of the carb is.
One last thought, check all hoses in the engine for signs of deterioration or cracks. Sucking air from the wrong place can have a big impact upon performance.
You might get a service manual.....
HTH,
Mark
