Here's my 2 cents.
Carb models are the "old" technology and least fuel efficient of the bunch (according to conventional wisdom).
Everything (pretty much) on a carb model powerhead is sychronized through manual adjustment which can obviously lead to errors or changes through use/wear.
A carb motor can be the toughest to start out of all options, under certain conditions (cold weather as an example).
But on a positive note, most parts are inexpensive in comparison to "higher tech" stuff and just about anyone with even a inkling of mechanical ability can work on these in their driveway with nothing more than a repair manual and a few basic tools.
Additionally, carb models (whether or not they have oil injection) can use regular "dinosaur" oil - the lowest cost 2 stroke oil on the market.
The EFI models are the "upgraded" version of the carb models.
In many cases they are simply a carb motor that had it's carb removed and replaced with an intake (of various formats) and injectors. An engine computer and an array of sensors (some more than others) figure out fuel/air ratio for you, and vary the timing etc as needed to make the engine run properly.
They start easier than a carb motor in almost all circumstances but can be significantly more expensive to repair if a major component fails.
They (claim) to be more fuel efficient than a carb model, and mathematically are, but for any given horsepower the "average" boater would not really notice a big difference.
Many components can still be tested and repaired in your driveway, but sometimes it must go to a dealer to hook to a diagnostic terminal to determine what is wrong.
Depending on how advanced the system is, you often can not simply disconnect a sensor to see if it's the problem because the engine computer will "pretend" that it's still there and feed itself the data that "it thinks it would get" if the sensor was functioning.
Most EFI outboards can be put in the same "technology" range as the average car in terms of engine/system complexity.
EFI must at least run semi-synthetic oil (and fully sythetic is better) to prevent clogging the injectors. So the minimal gas savings is more than gobbled up by the higher cost for premium oil.
The Opti's are a 2 stroke, but nothing even resembling the previous two.
These use a direct injection technology based on the Orbit (or Orbital) design.
Yes, every stroke is a powerstroke (which makes them a 2 stroke) but that's where the similarity ends.
The oil and gas never mixes on an Opti until it gets into the cylinder.
Each cylinder is (ultimately) fed by two injectors.
The gas from the tank is sucked in by two fuel pumps - one conventional and one electric. It travels through filters and a vapour separator. It is then injected into a combination air/fuel rail by a standard injector - left over gas at the rail is liquid cooled and returned to the electric pump for recirculation.
A compressor (single cylinder piston pump) and a tracker valve keep the air and the fuel pressure balanced to the engines requirements.
When the totally computerized, sensor driven system decides that cylinder #1 needs gas, a "direct injector" is opened to allow the "gas only" to be atomized in the cylinder head when the piston has already closed off the intake and exhaust ports.
The oil system only mixes the oil with air and it follows the typical 2 stroke "path", through the intake, past the reeds, into the lower crankcase and then into the cylinder through the intake port - so the air and the oil is waiting in the cylinder for the gas to get there via the direct injector.
(this means that you can NEVER disable the oil system on this motor and run "mixed" gas)
The process results in a quieter and more fuel efficient motor than a more conventional 2 stroke - and in terms of torque, top end speed and even fuel efficiencey at full throttle, it will kick the snot out of any 4 stroke currently on the market.
On the down side - you need a diagnostic computer to do any work on these things more complicated than changing the spark plugs.
On that note, the Orbit DFI system's one drawback is - it eats sparkplugs. If you get 100 hours out of a plug you have done well - 50 to 75 hours before replacement is common and currently each plug retails for about 15 bucks (expensive on a 6 cylinder that is used often).
It does save you gas, but again there is the oil to consider. You MUST use specifically formulated DFI oil. Unlike regular 2 strokes, you can not count on the gas carrying the oil through the system for you. So the special oil is required since it is only ever mixed with air.
But on the upside, the Opti's use the equivalent of mixing at between 100:1 and 300:1, so even though the oil can be upto 3 times more expensive than "regular" oil, the average boater would only use between 1/3 to 1/2 as much as a regular 2 stroke - which evens out the cost.
My personal picks - If buying brand new, go with the Opti.
If buying used, and you have some mechanical experience, find a nice mid 80's to mid 90's carb model and use all the money you save (compared to new) to pay for your gas, oil and maintenance for the next 10 years
