Carby 2 stroke to EFI 4 stroke - what prop?

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A question I get asked a lot is from people who have taken a carby 2 stroke outboard off their boat and have upgraded to a new EFI 4 stroke outboard. They do their run in period, then the first time they open it up to wide open throttle.......OH NO, ITS REVVING TOO HIGH!!!!!

Usually, what has happened is they have gone from an over propped carby 2 stroke to the new EFI 4 stroke and seen their cruising RPM go up by 750rpm and their wide open throttle go up by 1000rpm or more.......that's bad, right?

So what they do is ask around at the pub and on multiple facebook pages and get a whole heap of advice from all the experts and then come to the conclusion that they need to change the prop. The dealer will tell them that they have the right prop, but the dealer is just trying to get out of changing the prop, right?

Well no. The dealer is usually right. Modern EFI 4 strokes are a quantum leap from old, inefficient carby 2 strokes. The have a completely different power curve and power delivery and make peak HP and torque much higher in the rev range than the old carby 2 stroke did.

Your old carby Yamaha or Mercury or Johnson had a big, fat peak power rev range which meant you could over prop them and reduce the RPM to get better fuel economy......carby motor fuel usage is RPM and cubic capacity dependent.....the bigger the motor or the harder you rev it, the more air that gets sucked in draws in more fuel with it......with a lot of the bigger, 50hp and up carby 2 strokes, you could prop them to rev 500-700rpm lower than peak HP and only drop a small amount of torque and power....

The EFI 4 stroke is different.....its computer controlled and relies on a bunch of sensors to tell it the RPM, intake temp, throttle opening, cam angle, airflow speed and more to work out how much fuel to squirt in to match the load......what this means is when you over prop a modern 4 stroke, the engine management system will sense its under much higher load and squirt more fuel in.....at best this makes the motor run rich and sluggish.....at worst it will literally fill the oil sump with unburnt fuel that gets past the rings.....modern 4 strokes need to be able to rev cleanly to the top of their recommended rev range to give the best performance and fuel economy.....over propping to save on fuel actually makes them thirstier!

The other argument is that the higher RPM wears the motor out faster......well yes, in theory, this is true, but with modern, properly maintained outboard motors now regularly seeing in excess of 15,000 operating hours without noticable wear........an extra 1000rpm at wide open throttle will probably mean it will last 39 years instead of 40..... ;)