Two-Stroke or Four-Stroke - Part Two

When choosing a suitable outboard for your trailerable fishing boat, the main decisions are what percentage of your fishing will be on fresh or saltwater, whether or not this will include extended trolling periods and what percentage of your fishing will be at sea level...

TWO-STROKES
If you plan to fish only on saltwater, don't intend trolling for extended periods and will use your motor for only about 50 hours a year, then a premix two-stroke is the most cost-effective way of getting to your favourite fishing spot.

Premix two-strokes have a number of advantages over oil-injected motors, such as less to go wrong, lower servicing costs and better protection of bearings and piston rings during extended periods of disuse due to the additional residual oil.

Of course, there has always been the problem of premix fuel going stale after a few months, but since the advent of Isopropyl Alcohol this has been eliminated. I've tested OMC/Bombardier 2+4 Conditioner and Yamaha Fuel Conditioner and Stabiliser in a range of outboards for the past eight years, and even after periods of disuse spanning one year, the test fuel/oil batches remained as potent as the day they were mixed!

If you intend using your motor more frequently and want less oil smoke at low speeds, then an oil-injected motor is the answer. Mercury Marine and Tohatsu motors have fuel/oil ratios varying from 50:1 at Wide Open Throttle (WOT) down to 100:1 and 120:1 respectively at Dead Slow Troll (DST), while Yamahas operate at 50:1 to 200:1.

Carburetted Mariner/Mercury and Tohatsu motors have singlepoint oil-injection, where oil is mixed with fuel near the fuel pump and the fuel/oil mix passes through the carbies. This system provides lubrication of the carbie throttle butterfly valves, eliminating verdigris build-up. But unless the carbies are drained before extended disuse, Isopropyl Alcohol must be mixed in the fuel to prevent the fuel/oil mix remaining in the carbie bowls going stale.

Yamahas have multipoint oil injection where the carbies run straight petrol and oil is injected via individual injectors at the reed valves. This eliminates the need to drain the carbies, but no lubrication is provided to the butterfly shafts, and the relatively small diameter oil delivery lines may be blocked due to 'gelling' should different brands of oil be mixed together.

Although the EFI two-strokes made by Mercury Marine and Yamaha provide only about 5% better fuel efficiency at WOT than carburetted competitors, they provide turnkey starting, just like an EFI car engine. They also run better in all climatic conditions because the air/fuel ratios are constantly altered according to barometric pressure and load. And if you pick up a tank of fuel diluted with cleaning fluid or ethanol, the EFI's anti-knock sensor will automatically retard the ignition timing to compensate, preventing 'pinking' or pre-detonation.

Mercury Marine's EFI models inject fuel at the reed valves and operate on fuel/oil ratios of 50:1 to 100:1, while Yamaha's models inject fuel at the same location and have fuel/oil ratios of 50:1 to 200:1.

If you fish freshwater as well as salt, then a DFI two-stroke is the answer. Because DFI motors such as FICHT, Optimax and Tohatsu TLDI operate on a mix of stratified and homogenous combustion, the fuel savings are enormous compared to even EFI motors.

For example, EFI two-strokes such as those made by Mercury and Yamaha operate on air/fuel ratios of 14-15:1. But FICHT, Optimax and TLDI motors have air/fuel ratios of about 40:1 below 2000rpm and 25:1 above. At DST the fuel consumption is about one third that of an EFI motor, just over half that of a comparable four-stroke and about 20% less at WOT than an EFI two-stroke.

Also, FICHT, Optimax and TLDI motors have fuel/oil ratios of 60:1 - 300:1, 44:1 - 400:1 and 50:1 - 450:1 respectively and use a lot less oil than EFI two-strokes. DFI two-strokes will troll at lower revs than comparable EFI four-strokes and maintain peak tune regardless of the altitude.

FOUR-STROKES
If you fish mostly at sea level, then a carburetted four-stroke is the answer because it can to be tuned by a carbie specialist to troll much slower than originally designed to. For example, a carburetted Mariner/Mercury F40 will troll down to 620rpm, whereas the EFI version won't troll below 750rpm unless the microprocessor is re-mapped.

However, should you fish alpine lakes such as Eucumbene then the carbies may need re-jetting to prevent the motor running rich, whereas an EFI system automatically adjusts the air/fuel ratios as the altitude increases.

Until the advent of EFI, four-strokes usually had lacklustre performance compared to equivalent two-strokes. If you wanted a low pollution motor for angling offshore, then a DFI two-stroke was the only choice. But tested against a FICHT 115 on identical 5.5m fibreglass hulls and using similar prop pitch/gear ratio combinations, a Mercury F115 was 7% faster at WOT and returned 12% better fuel efficiency. However, it was not as throttle-responsive across the rev range.

Honda's EFI BF225 returns 10% better fuel efficiency than an Optimax 225 both at DST and WOT and provides comparable throttle-response, but only because of its variable-flow air intake system and VTEC.

WHICH TYPE IS BETTER?
For 50hp and under I recommend a four-stroke if you DST extensively and want to get closer to schools of fish, as the exhaust note is less threatening. But be prepared to use the motor more frequently as four-strokes don't suffer extended periods of disuse well. And if you want true portability, remember that a four-stroke 15 weighs as much as a two-stroke 25.

For 50hp and above I recommend a DFI two-stroke providing you can afford one, as only at 90hp and above do DFI motors have similar pricing to comparable four-strokes. But remember that, due to their recirculating lubrication systems, watercooled four-strokes will always run cleaner than even DFI two-strokes.

Finally, don't forget servicing costs. According to Hamilton's Marine of Wickham NSW, excluding parts, servicing costs for a two-stroke Merc 90 are $396 for the first year then $132 for each year thereafter, compared to $528 and $352 for a carburetted four-stroke Merc F90.