I also have a problem with a Tohatsu 18hp outboard motor and would appreciate it if you could help with the diagnoses.
The motor will sometimes run well for up to 1 hour and then would cut out on the bottom cylinder and then run up again on two cylinders for approx 30 seconds up to a few minutes and then cut out again on the bottom cylinder and run just on one cylinder for a minute or so and then go again on two cylinders for a short while again. it would continuously do this, once it started doing it. It did this on 3 days and on the 4th day it did it right from the start. It made some popping type noises now and then while it was missing and sputtering.
I have cleaned the Carby,and fitted another one just to be sure and fitted another good HV Coil, cleaned the tank and Mixed fresh fuel, with no difference, so I am now picking it to be the lower oil seal on the crankshaft. I have struck this similar thing on another make of motor, where it would run for a while and then start missing and mucking around. In that case it was the lower crankshaft seal.
The Motor is a Tohatsu M18C, 350A
I have since fitted a new lower crankshaft seal and absolutely no difference. I have checked the exhaust ports which are clean and checked the inlet reed valves which all appear to be OK. There is another motor the same here on Niue Island that has similar symptoms. I have had these 12 - 18 hp Tohatsu's for 25 years and have never struck this problem before.
Anybody out there with some insight into what the problem might be I am all ears. Thanks