Yamaha F100A

Sometimes it takes a while to get around to testing a particular motor. I've tested plenty of Yamaha EFI F115A models but so far only three F100A models, which is based on the 1.6lt Ford Sierra car engine.

Trailer Boat readers probably realise by now that I prefer EFI four-strokes to carburetted engines, particularly where the motors will be used on high altitude dams such as Lake Eucumbene. The microcomputer-control of EFI systems provides the optimum air/fuel ratio regardless of barometric pressure, so the motor will run as well on alpine lakes as it does at sea level.

But for anglers who mainly fish at sea level, carburetted four-strokes still have plenty to offer. If the engine suffers "phase separation" when using fuels with a high Ethanol content, carbies are easier to drain and clean than EFI systems and are generally cheaper to repair down the track.

The F100A is the largest of Yamaha's carburetted four-strokes and is substantially more powerful than its de-rated F80A counterpart. It's also the lightest four-stroke of any in the 90-100hp range and every motor I've tested has run as well as Yamaha's F115A, although at 800rpm the idle revs were 200rpm higher, reducing the motor's appeal for extended Dead Slow Troll operation.

Well equipped for a carburetted four-stroke, the F100A has electronic ignition timing advance with a cold start fuel primer and automatic reduction of revs should the engine over-spin, overheat or lose oil pressure, plus there's a voltage-regulated alternator which produces 12 amps at only 1000rpm.

Mounted on a Southwind SD500 Classic centre-console and spinning a 19in pitch stainless steel prop, the demo F100A easily pushed the total weight of 1050kg including two adults and fishing tackle. It started instantly hot or cold and never blew any oil smoke. Providing the anti-ventilation plate was kept immersed, power astern was good, no cooling water starvation occurred and at all times the pilot water discharge was clearly visible.

Trolling at 800rpm, the demo motor averaged 4.5kmh consuming 1.3lt/h and at 2000rpm averaged 10.6kmh. A clean plane was achieved at 26.0kmh on 3200rpm and cruising at 4000rpm the average was 39.7kmh and fuel consumption 13lt/h. Through tight figure-eight turns there was no prop ventilation.

The Wide Open Throttle average across a 30cm chop was 66.0kmh on 5850rpm using 31lt/h, and though noisier than the F115A due to carbie induction roar, we could still conduct a normal conversation at the helm.

Once the strong upper cowl (as we found out when the Yamaha dealer accidentally slammed the motor into another boat alongside the jetty) has been removed, powerhead access is good. Although the balancing linkages for the four carbies are a little finicky, once correctly set up the carbies should hold their tune and engine components such as the spin-on oil filter are easily reached. Yamaha recommends servicing the F100A every 50 hours or six months with waterpump impeller replacement every 100 hours or once a year. The camshaft timing belt should also be checked at this time for any tears.

As I've stated many times in Trailer Boat, never use an automotive-type SAE20W50 oil in four-strokes such as the F100A as the viscosity index improvers used in these oils won't stand up to the high loads marine four-strokes are subjected to. Yamaha's own SAE10W40 oil should be used in temperatures from -20° to 40° unless the motor is operated in colder climates such as Tasmania, where an SAE10W30 oil will suffice.

If you operate the motor at extended DST speeds, always change the oil and filter every six months regardless of how few hours are clocked up in this period. Remember, as with all four-strokes, to warm up the motor from cold at idle speed for at least five minutes before use and idle for a similar period after a long run at high revs.

The F100A complies with US EPA 2006 exhaust emission regulations and California Air Resources Board 2004 requirements.