- they produce good power to weight ratio for their capacity
- they are inherently simple in design; and subsequently
- economical and easy to fix
Due to their sometimes quirky and temperamental personalities (and yes anyone who has owned one will know what I mean by this) two strokes (2T) are up against stiff opposition with the modern four stroke (4T) design which has come along in leaps and bounds in the last 5-10 years.
The biggest concern with 2Ts and where a lot of their “multiple personalties” are derived from is due to the quality of the oil which is used. 2T engines use their oils differently from a 4T in that the oil is mixed with the fuel and is burnt as part of the combustion process. This is called a “total loss system” due to the fact once it is burnt it is gone. Alternatively 4Ts circulate the same oil throughout the engine continuously which is then (or should be) routinely changed.
“Why the then do we need a good quality oil if it just gets burnt?”
Like the oil in your car, motorcycle or any 4T engine, the 2T oil is used as a lubricant as it passes through the major engine components on its way to the cylinder and piston before being burnt and discharged, commonly known as the “Blue Smoke” seen blowing out your exhaust.
The "blue smoke" syndrome. Photo owned by nualabugeye@flicker.
In the old days the oil was mixed at a set ratio with your fuel and then put into the main fuel tank and metered through the carburettor. This method was messy and fraught with danger in that if you got the ratio wrong major engine damage could result if to lean (too little oil) or a succession of fouled spark plugs if to rich (too much oil). These days however all 2T machines run an oil injection pump which meters and injects the oil at the correct ratio relative to engine speed and throttle position. This system however is not perfect and like the old premix engines has its share of anomalies as well.
Getting back to how the oil quality affects your engine; again in the old days 2T engines were big old “agricultural” things - big lump of slow revving piston, thick piston rings, open exhaust port with a basic or no inlet reed valve. What this meant was the engines were under stressed and could afford to run a bit of variance in oil quality and the aforementioned fuel/oil premix ratio and still survive.
“CARBON!!”
The nemesis of all 2T engines, it is an unavoidable by-product of 2T combustion and like ash in the bottom of a wood barbeque is what is left of your oil once it is burned. Old school 2Ts (eg Vespa, Lambretta, old European motocross bikes, etc) are not overly affected other than clogging up the exhaust pipe or expansion chamber (Invite your mates over to use that aforementioned BBQ, throw it on after the steaks were cooked and ‘problem solved!’).
The modern high performance 2T (and yes that does include the humble 50cc scooter) are a totally different animal! They now rev very high for sustained periods. This is achieved through much lighter reciprocal masses ie- pistons, con rods, crank shafts etc. The pistons run a very thin piston ring(s) which are prone to jamming in the piston ring grove as carbon builds up. This in turn causes the engine to create a double edged sword with poor primary and secondary compressions. Primary compression being under the piston in the crankcase, which is then transferred to the secondary compression area being above the piston in the main cylinder area - As suggested when one is gone it also affects the other.
Most manufacturers suggest a 10,000klm de-coke service; the cylinder and piston are removed, the rings taken off the piston, and all carbon being physically removed via a mechanical method - usually a sharp scraper and an apprentice.
Like most service industries a technicians labour time is expensive and it has now become a more practical option (on a fifty at least) to just replace the piston and rings, take steps to either avoid it or at least delay it as long as possible. This is where the modern full synthetic 2T oils become so important. If from new these oils are used the 10k de-coke service can now be done at 20,000klms. Do the math it works out as follows.;
10k De-coke service (just T/E not routine items) $350.00
If new piston and rings required $450.00
Alternatively;
Synthetic 2T oil average per litre $ 20.00
Mineral (or lower grade) $13-$18 $15.00
If the average scooter uses 1 litre per 1000klms synthetic oil costs $200.00 per 10K. Mineral or lower grade costs $150.00 for the same distance travelled. The price difference over 20,000klms in oil is $100.00 however the same de-coke servicing costs are $450.00 less due to the increased mileage gained by using a better grade of oil.
This does not include larger ticket items like replacement crankshafts (burn one of these out on the average late model 50 and the repair cost = throwaway)
The last but not least problem with modern 2Ts is that the exhaust pipes now all have catalytic converters in them. I often use an analogy where catalytic converters in 2Ts are “a bit like trying to spit a mouthful of porridge out the kitchen window with a fly screen on it!” This is because in an effort to make 2T engines compliant with current emissions laws they must have one.
A catalytic converter or “cat” is a rolled up mesh of wire which essentially allows the noxious gasses to pass through where they react with properties within which render the gases less harmful to the atmosphere. This is great in a working 4T where the engine is not burning oil as part of the process. However our old enemy the “Carbon” has a nasty habit of clogging up the fine galleries which are the “Cat”. Using synthetic oils (like the deposits on piston rings) will reduce the rate at which this happens.
Yet again falling back to the economics of it, the average OEM pipes (factory original) usually start at $600.00 dollars to replace, with some up around $1200-$1400.00. Unlike the old school BBQ cook up you can not burn out the carbon deposits as you will destroy the somewhat sensitive Catalytic mesh material or at the very least melting it into an outlet clogging blob.
To finish up: a life story of the average 50cc 2T scooter
In an effort to keep the ever increasing costs of commuting to work “Customer X” will buy a new shiny scooter. The Commodore or 4WD can now be parked up and used on wet days or weekends. It is an exciting time and Customer X listens intently while the importance of the modern 2T oils virtues are extolled. With hand on heart Customer X promises to get it serviced by the book and never use any oil other than that recommended by the staff at point of sale/delivery. Customer X has done their homework they have bought a good quality scooter, which while under its warranty period is receiving the recommended servicing and same oil. This goes on for the first 18 months to 2 years.
If the scooter bug bites hard the love will continue and more than likely a bike licence is obtained and a larger capacity scooter is then a natural progression. A new love is on the scene and the old love (50cc 2T) is sold to help finance this “new excitement”. Customer X loves scooters so much now the next door neighbour/friend/work acquaintance has picked up on this and becomes the next proud owner. Customer X does not “extol the virtues” of the modern synthetic oil quite as strongly as the original sales person. Besides the scooter is now outside its warranty and Customer Y’s dad/grandad/personnel trainer used to have this Magic Mineral 2T Oil that he used to use in his Victa/Mercury outboard etc and its only $9.95 a litre so in it goes.
Three to four months later the low oil light starts to flash on the way home but its been a long day so Customer Y gets home goes inside and relaxes. A couple of days later the oil light is now on solid (that is Danger Zone), Grandad is busy out grey nomading around the countryside and is not easily contactable regarding the Magic Oil so something has to be done. “Oh look there is a “BP servo!” Sure enough Zoom outboard oil is available so in it all goes and its only $11.95 a litre.
Another three months go by but this time the available servo is a Caltex. They have 2T oil but it’s for mowers and whipper snippers so in it all goes. Because Customer Y remembers Customer X’s advice that was something about “synthetic oils” this one will be okay because it is Semi Synthetic and besides it is $14.95 a litre so it must be good.
Another three months go by the scooter is pretty much just a tool now and all Customer Y’s mates have borrowed it here and there. It’s getting a little worse for wear also getting hard to start. “That must be because it needs a general service!” so the repairer/service agent is called and the routine service is booked albeit 4-5000klms overdue but hey it’s a 2T! It’ll be right - it still runs.
What I have described here is what I call the “Oil Cocktail” and like those of the Molotov variety are a deadly brew. In the oil world Mineral, Semi Synthetic and Full Synthetics DO NOT MIX. The reason for this is because the base elements the oil starts life as are not compatible – not even within the same brands (ie Shell, Castrol, Motul etc). Worst case scenario they can “gel up” creating blockages within the injection system similar to hardening of the arteries. At the very least mixing oils will in fact “promote” carbonisation of the critical components. Outboard oils have additives which are designed to minimise carbon in lower operating temps (in an outboard the whole sea is your radiator). It also has additives to neutralise water should it find its way into the premixed fuel or injection reservoir. Whipper snipper/mower oils have different additives particular to their application.
If it is caught early enough the oil mixture can be drained or extracted and the correct quality and application of oil put back in the damage will be minimised, avoiding the need for a top end rebuild.
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