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Old 12-29-2007, 03:38 PM
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Pablo Pablo is offline
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Join Date: Dec 2007
Location: Duvall, WA PNW
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Default Motorized Bicycle Engine Oil - 2 cycle Pre Mix

There is so much info and misinformation on the web about 2-T (2 stroke, 2 cycle) fuel/oil premix, it often leaves my head spinning.

Please feel free to ask, challenge, present data, record your experiences and anecdotes. Some folks have a tough time swallowing some of the concepts presented here....please don't hesitate to ask - it could very well be that my writing is unclear!

Fuel/Oil Mix Ratio

What is THE correct ratio? It depends. Next subject. OK, just kidding 16:1, 20:1, 25:1, 30:1, 32:1, 40:1, 50:1, 64:1, 70:1, 80:1, 100:1 – what do these all have in common, beside the obvious? They all are ratios I have recommend in the last year or so. So which ratio is correct? The one that meets your needs. If you don't know your needs, you can find a logical starting point, this leads us to a good place to start the discussion. Where to go for the right ratio?

Engine manufacturer vs. Oil manufacturer

The engine manufacturer can be the best starting point. Engine builder says 30:1. End of conversation? Oil company says 50:1. Stuck already......I guess I'll go with 40:1! Now I have three choices.....head spinning....:confused:

This is usually discussion battle royal, no holds barred....with the winner usually being the engine builder. When you read a 2 stroke owner's manual, they normally do not say "follow the instructions of the oil manufacturer" at least none of mine say that. The engine folks give you specific oil ratios to use and warn that deviating from these might result in damage. No matter what the oil, mix it at this ratio – but the main problem with this, oil brands vary a lot. Some can be run more dilute than others, because they have a heavier base oil, totally different additive packages. Not all oils should be diluted the same. So is the oil company always correct? 100:1 for everything using this oil? Of course NOT. That's exactly why some oil manufacturers make different kind of oils for different engines. On the other hand, the oil manufacturer does not know WHAT brand oil is going into the engine. (I won't bog this down with 2 stroke oil standards, such as TC-W3, or loosely followed and often outdated standards such as ISO-L-EGD, JASO FC and FD, API TC that's a whole other can of worms)

The problem with both these above approaches, is that they leave out many important variables. The most important beside engine design and oil type, in my opinion is engine tuning and jetting, but right behind that is are several other very key parameters. Engine size, rpm, temperature, load.....just to name a few.

Proper “Jetting” (and tuning) (oh crap)

Forget about fuel/oil mix for a second. (say what?) Think about an old fashioned carburettor and setting air:fuel mixture. This needs to be dialed in just right. It's the point about midway between 14 and 16 to 1 that is the point to get the most energy. 14.7 to 1 (air:fuel) is ideal for normal gasoline engines, called stoichiometric . Lean of stoichiometric can be hot (and damaging and lacking power, of course super lean won't fire and that might be cool), too rich of stoichiometric is cold (and wet rich and lacking power). Stoichiometric is energetic!

A wild example of why I think this is so important. If you are running a 50:1(fuel/oil) ratio and the bike is close to being lean air:fuel jetted, simply going to more oil, like a 20:1 (fuel/oil) ratio, will probably result in a leaned out holed piston.
But, what the heck - why? I put MORE oil in - you may ask...well, even though you added more oil, you have effectively robbed some of the volume used for fuel and air. You have leaned the bike out even further by adding more oil! If the bike has been jetted up correctly for use with a 50:1 fuel/oil ratio, going to a 20:1 ratio could leave the bike dangerously close to piston meltdown, as lean conditions run extremely hot.

Contrary to popular belief very little oil reaches the combustion chamber, IF the engine is metering the fuel mix - properly metering of the fuel mixture must be so that most of the oil falls out of suspension. This condensed oil then migrates to lubricate the friction surfaces of the crank bearings, as well as some of it being pulled into combustion chamber for lubricating and sealing the rings.

Some bikes do carry a very oil rich and a fuel rich mixture recommendation, and I suspect it to be a protective thing as mentioned earlier. The engine manufacturer knows that the correct ratio is dependent on engine and use specifics, and that proper metering of the air/fuel/oil mixture is dependent on many variables such as quality of fuel, air temperature, relative humidity, and elevation, etc. The engine manufacturer simply cannot provide a catch-all metering scenario, as his equipment is sold to areas with a wide array of conditions. The jetting chosen for a factory bike is basically an arbitrary median average. It may be fuel rich for one condition, and fuel lean for another.

Many will want to add less oil to their 2 cycle engine when they see a dark, oily spooge coming from the tailpipe. It would seem logical that if there is oil coming from the tailpipe, that the mixture is carrying too much oil. I for one have used this as a guide when too lazy to pull the spark plug to at least check, or recommending an oil to someone who knows engines.

But that is again where conventional wisdom separates from facts. When an engine is producing this oily residue, it can be due to the air:fuel mixture not being metered properly by the carb, and as a result much of the fuel components get past complete combustion, and turns itself into an oily looking substance along with a small amount of the oil that didn't fall out of suspension prior to combustion.
Adding less oil will not always fix the spooging problem, but rather enhance it, since you have now added more fuel to the mix...and the metering devices (jets, needle, etc) are already allowing too much fuel into the chamber to begin with. The result is even more oily fuel spooge, and the engine a bit less powerful.

A properly set up engine, that is metering the fuel/oil/air mixture to optimum, will see most of the oil fall out of suspension prior to the combustion chamber. If the remaining fuel:air mixture results in about a 14.7:1 air:fuel ratio, then the resulting burn will be clean and powerful – energetic. But may be too hot still for long full throttle conditions. The goal of the tuner is to find these settings and keep then there. It's a moving target, so the exacting tuner has to be very sharp about these things, and know exactly what effects the changes make. This is why just playing with the fuel/oil ratio can be a bit deeper than first blush.

So how to best dial in air:fuel ratio? It depends - most engine manuals supply the basics. Some of the power mechanics here can probably help with your engine. I can't even pretend that I know many of your engines and carbs. My first advice is to contact someone that knows how. Learn where the main jet is, the air screw, the idle screw. Just as important, learn how to “read” the spark plug. Again there are pictures on the web of overly lean, overly rich and perfect mixture spark plugs. If it is white/glazed and looks new after a ride, you may be too lean, black/brown wet is too rich and very light deposits that are light chocolate brown (depending on fuel, fuel additives and oil brand) is about perfect.

Other factors, in general:

All these can effect your fuel/oil decision. BUT mind your air:fuel ratio.

Time at temperature (think wide open throttle for a few seconds vs. 30+ minutes)

Elevation also effects air:fuel ratio and can therefor effect fuel/oil mix ratios.

Temperature and climate (cooler temps and cool moist climate may need less oil)

Smaller engines need more oil – the old days used to be something like 20:1 for small engines, 32:1 for larger engines.

Higher RPM engines need more oil, simply put more piston movement per time period uses more lubrication. This one is sometimes overblown or is simply the same as “time at temperature”.

More oil for more load. Think of a 250 lb guy on a 50 cc bike.....everything in balance....

New engine break-in ratios – some engine manufacturers ask for a much more oil rich mixture for engine break in. This can be a topic by itself. I can see some reasons for this, but more is not better. Please by all means, follow the manual to keep the warranty intact, but please do this with knowledge.


Fresh mix – to the racers who have all the above dialed in, there is nothing more important than fresh gasoline freshly mixed with 2 cycle oil.

Tank and fuel line cleanliness – you would be amazed what people have found in their fuel tanks, and how fast some lousy fuel lines deteriorate on the inside.

Air Filtration – when your engine seizes up or your engine has some interesting wear problems – yet your plug looks OK, how fast are we still blaming the oil? This may not be a huge of a problem in a clean suburban environment, but plenty of bikes have less than optimal air filtration, which will mess with your carburettor and your engine.

Mixing – silly as it sounds, some folks don't do a great job of mixing their oil and fuel perfectly. People have their own techniques, but I always put some fuel (1/4-1/3 of total) in my (approved) container, add the correct amount of 2-stroke oil and mix well (put lid(s)) on and shake. Then add the remaining fuel and mix well again.

Storage and rust – not to get off track too far but many 2-stroke race oils don't contain anti-oxidants and and rust preventatives. Storing a bike over the winter with this fuel in the tank probably isn't the best anyway, plus there will be very little internal engine protection. Keep this in mind when selecting an oil, and as another aside, with a plastic fuel tank it's best to store it dry anyway.