How to take care of two-stroke outboards?
We prepared some basic tips to keep older two strokes running well.
Up until their demise in 2019 when they were banned from sale in Australia, carburetor two stroke outboards were ideal for recreational boating where owners didn't clock up big hours on their engines.
Sure, compared to DFI and four stroke outboards they were thirsty and smelly on cold starting, but their inherent simplicity ensured they would last a lifetime with only minimal maintenance. For example, my 1969 Johnson one and a half still runs well despite being 53 years old. Also, my 1993 Johnson 4 (for 31 years the world's smallest twin cylinder outboard) is perfectly balanced and almost vibration-free. No four stroke 4 can compare in this area.
To keep older two strokes running well I suggest some basic tips.
1. The right choice of fuel and oil.
Though most carbie two strokes will run well on standard unleaded (91 RON) petrol, I recommend using premium (95) but only in conjunction with synthetic blend oil. If a mineral-based oil is used with premium it won't stay in suspension with the petrol, resulting in ‘dry’ cylinder bores and rapid piston ring wear. For example, in my Johnsons I use Evinrude/Johnson XD50, while in my 2005 Tohatsu 8 I've found Quicksilver Premium Plus is the way to go. Using premium with synthetic blend oils helps with rapid cold starts due to the absence of spark plug fouling. Oil smoke emissions are reduced, too.
Before switching from one make of oil to another, I recommend draining the fuel system of the existing fuel/oil mix before mixing fresh fuel. At this time, I suggest removing and cleaning the spark plugs of any deposits and keeping a log of these deposits before re-fitting the plugs. Then you can do a comparison between the cleaner running (hopefully) of the new oil versus the old by removing the plugs again and checking the deposits after about ten running hours. I recommend fitting spark plugs yourself as I've found some dealers fit the wrong plugs for the engine.
Petrol is very expensive nowadays, and I suggest using fuel preservative such as Evinrude/Johnson 2 + 4 Fuel Conditioner, which mixed at three millilitres per litre (or 300:1) will keep fuel ‘fresh’ for up to one year. The oil will stay in suspension with the petrol and any moisture in the fuel is spread throughout the mix instead of separating to the bottom of the fuel tank where it can be drawn into the carbie(s) resulting in difficult starting.
2. Checking the gear case oil for water.
Number two is checking the gear case oil for water, a result of prop shaft seals damaged by stray fishing line around the prop shaft. I recommend removing the prop annually, clearing any stray line and greasing the prop shaft before re-fitting the prop. If not damaged on removal, the original drive (where fitted) and cotter pins can be re-used.
When checking the gear oil, unscrew the oil drain plug first and check for any water trickling out before the oil emerges. Because most gear oil is designed to absorb some water, the oil may be a milky colour, indicating possible prop shaft seal damage. If this is the case, I recommend taking the outboard to your nearest dealer for seal replacement.
If the oil is relatively clear, unscrew the oil fill plug and, when all the old oil has drained out, insert the nozzle of a tube of gear oil into the oil drain hole and squeeze until oil appears at the fill hole. When it runs out, screw in the fill hole plug, remove the tube nozzle and quickly screw in the drain plug. There may be some air bubbles in the oil so I would remove the fill plug and top up the oil until it flows from the hole, then screw in the fill plug. I also suggest using new plug washers on the fill and drain plugs just in case these are cracked from age.
Most carbie two strokes under 30 HP have single carbies which rarely go out of tune, so don’t play with the carbie jet screws unless the engine is running too rich when idling. However, more powerful outboards have multiple carbies which, over time, go out of synchronisation due to slop in the connecting linkages — this will cause erratic running and poor performance. If there is slop, I recommend taking the outboard to your local dealer to adjust the linkages, which will make a big difference to running qualities. A cantankerous engine can really dampen a day afloat and be dangerous if you need to rely on it to get back quickly should the weather deteriorate.