How old are the sparkplugs?
You are correct - the portable Merc/Mariners are ultra-reliable, not alot breaks on them.
These are my "bread/butter" motors. Some of the most common issues I see with these are:
1) old sparkplugs - after a couple of years they need to be changed. Although these have Merc's Thunderbolt 4 ignition they use coils that produce less power than Merc's (standard) coils. Just my opinion, but (think) this results in a weaker spark/worn out plug quicker than is normally expected on an outboard.
2) cracked coils - OEM coils develop small hairline cracks after a number of years. Very difficult to see unless you remove the coils, wipe them down and really look hard. However, if it is bad coils this is the perfect opportunity to cure part of the first problem I note. Sierra makes an aftermarket coil for Merc's. To save money they do not differentiate between the high and lower power coils - they only build the higher power types and they will fit both style of mounts (and yes, the small mercs can run the high power coils - used to be about a 100 buck "retrofit" from Merc which was nothing more than a pair of coils and a "holder" for them - the portable fasten on with a bolt right through the middle of the coil - the sierra models also have the hole so no holder is required). The higher power coils significantly improve cold starting and smooth out the idle - and as an added bonus, they are about half the cost of a replacement OEM coil.
3) bad intake gasket - not the one at the carb, but the one back near the head (ok, not really a head, it's just a water jacket cover). Standing behind the motor, on the right hand side there is a long rectangular cover (about 4'ish inches long) held on by 3 bolts (on a remote control model the middle bolt also acts to hold the linkage to the powerhead). The little gasket under that will blow out over time effectively turning these motors into a "1 cylinder". The indication that this is the problem is often a slimy/goopy/oily mess in the lower pan on that side of the motor which is the left over oil after the gas has evaporated from the mix when it's blown out of the intake.
4) bad upper crank (oil) seal - there is a seal under the flywheel on the upper crank. It's a combination of a soft metal race, rubber and a round curly spring. The idea is the spring keeps the rubber cinched around the crank where it exits the powerhead, and the rubber is attached to the race which keeps it all in place. Of course, the rubber wears out, the spring breaks loose and you end up with your oil/gas/air mix (for the upper cylinder) escaping from under the flywheel. This is often detected by a heavy film on the powerhead - looks like it has been sprayed down with fogging oil.
So if your fuel is good and there are no blockages, and as suggested, the ignition tests out ok, one of these (common) issues may be your problem...
