All About 2 stroke Oil Ratios. Interesting find

[4x4van]

I’m constantly seeing things posted, here and on other websites, about 2-stroke premix ratios that are flat out wrong. Many 2-strokes that I see unnecessarily smoke like crazy, and drip oil out of the head pipe and tailpipe. Worse, engine damage could easily occur because readers are being given information and theories that are completely opposite of the truth. So I’d like to weigh in and try to clarify some things, if I may. Now to be honest, I am not a master mechanic, but I have been riding motorcycles and ATVs for 42 years (almost exclusively 2-strokes), spent 6 years racing professionally in the SoCal deserts in the late 70’s/early 80s, and have owned no fewer than 17 motorcylces/ATVs. I currently own two 1985 ATC250Rs (one of which I purchased brand new), a 1985 Yamaha BW200, a 2007 TRX400EX, and a completely restored vintage 1972 Yamaha AT2 125 Enduro.

Because it all ties together, we need to start with Carb jetting: An internal combustion engine burns a mixture of air & gas, which is provided by the carb (or fuel injection). The ideal ratio of air to gas is around 14:1 (don’t remember the exact number, but that’s not important here). At any given throttle position, there is a fixed volume of air flowing through the carb (past the slide) mixed with a fixed volume of fuel (flowing through the various jetting circuits: pilot jet, main jet, needle, nozzle). Ideally, at every throttle position, the jet circuits involved at that throttle position will flow the correct amount of gas to mix with the air resulting in that 14:1 ratio.

Now a 2-stroke makes maximum power when the carb jetting is as lean as possible without going too far and overheating. A performance bike with a good rider will typically be jetted at or near that line, as the rider is attuned to his machine and knows when he can push it and when he needs to back off a bit and let the machine rest. A recreational rider on the other hand is better off jetted slightly rich, which is more forgiving over a wider range of conditions, even though the performance will not be quite as good.

Now let’s look at Premix: Despite the fact that Honda puts a “20:1 premix ratio” sticker on the frame of the 250R, that is NOT the ideal premix ratio to run in that or any other 2-stroke. They place that sticker because they cannot control the quality (or lack thereof) of oil that many users will use. Some of you will go down to the local hardware store and buy the cheapest 2-stroke oil you can find (ironic, considering that you are putting it into probably the most technologically advanced 3-wheeler ever mass-produced). The ideal premix ratio is determined, not by the machine, but by the quality of the oil you run. Read the label on the bottle; it will typically tell you what the oil manufacturer recommends THEIR oil be mixed at. And that ratio will work in ANY 2-stroke. In all the years that I have ridden/raced/wrenched on 2-strokes, I have always run Golden Spectro brand oil at 53:1. That’s right, 53:1, which is only 12 oz to 5 gallons (exactly what the GS oil bottle says to mix it at). My bikes run good, run hard, never blubber, don’t smoke, don’t drip oil (from headpipe nor tailpipe), and in 42 years have never had a oil/fuel-related breakdown. I also own a Yamaha FX1 standup jetski that is built, pumping out 85+ HP. In it, I run either Yamalube or Kawasaki 2-stroke watercraft oil at 49:1 (13 oz in 5 gallons, which again is what the oil bottle recommends). Again, runs strong with no smoke. (BTW, I also run the same 53:1 Golden Spectro in my weedeater!)

Here is the important part about the relationship between premix ratios and carb jetting: As I’ve said, the ideal air-to-gas ratio is around 14:1, but it’s extremely important to remember that oil is NOT a part of that combustion equation. The oil is there only to lubricate the bottom end bearings and cylinder walls below the rings, and is not “burned” as part of the 14:1 mix. However, that oil DOES take up space in the fuel (gas/oil mix), and since there is only a fixed volume of fuel that is allowed through the carb jets, more oil equals less gas in that "fixed volume". Less gas mixed with the same amount of air = leaner fuel/air ratio. In simple terms; increasing the amount of oil in your premix reduces the amount of gas and that results in leaner carb jetting. You can actually seize an engine by running premix too rich due to the resulting overly lean fuel/air ratio (yes, I’ve seen it happen).

If someone says they got higher HP numbers on a dyno by increasing the amount of oil in the gas, it is only because they in essence leaned out the fuel mixture by doing so. That means that either the bike was jetted too rich to begin with, or it would have shortly blown up if run for any length of time using that richer premix/leaner jetting. Either one makes any of their subsequent dyno results or conclusions highly suspect and potentially damaging. Anyone who states that more oil in the gas equals more horsepower is smoking as much as their bike will smoke.

Bottom line, ignore the sticker on the frame. Use a good quality MC/ATV 2-stroke oil mixed at the Oil Manufacturer’s recommended ratio. And be consistent with that oil. Every time you change the amount of oil you run in your fuel, you are inadvertently changing the results of your carb jetting as well.

[4x4van]

I ...run Blendzall green label at 32:1. Thats one whole 16 oz to 5 gal.

Actually that's 40:1.

Blendzall is pretty good oil, but it runs dirty, with alot of carbon build-up. Next time you pull your top end apart for a hone or ring job, you'll likely see alot of carbon build-up in the exhaust port. Been there, done that. I tried Blendzall for awhile on my '82 ATC250R. After seeing the amount of build up in the exhaust ports, I went back to Golden Spectro. It burns so clean I've never had any carbon build up at all, on any of my bikes, in 42 years.

[3Razors]

4x4 Van there is more keys to the equation about more oil giving more power. Yes running richer oil mixture will lean the jetting and give more power. However also with heavier mixes of oil the added oil gives a tighter ring seal for more compression, also the added oil content will give higher btu's to contribute to the combustion process. But you have to run you bike hard enough to burn the heavier mix to see the added benefits. Also this mainly holds true with castor or syn blend oils, certain full synthetics will not burn off at high oil-ratios.

I have seen numerous dyno charts with backing up those facts with the same bike jetted spot on for 20-32:1 compared to 50:1.

[TimSr]

Here is the important part about the relationship between premix ratios and carb jetting: As I’ve said, the ideal air-to-gas ratio is around 14:1, but it’s extremely important to remember that oil is NOT a part of that equation. The oil is there only to lubricate the bottom end bearings and cylinder walls below the rings, and is not “burned” as part of the 14:1 mix. However, that oil DOES take up space in the fuel (gas/oil mix), and since there is only a fixed volume of fuel that is allowed through the carb jets, more oil equals less gas in that "fixed volume". Less gas mixed with the same amount of air = leaner jetting. In simple terms; increasing the amount of oil in your premix reduces the amount of gas and that results in leaner carb jetting. You can actually seize an engine by running premix too rich due to the resulting overly lean jetting (yes, I’ve seen it happen).

I was with you until I got to this paragraph.
First, the terminology - "Jetting" is the physical size of your jet and is fixed, with a numbered value. Your fuel mixture may require a change in jetting, but cannot "change your jetting". That is physically fixed. To explain it as such is very confusing. Your fuel mix can cause a lean or rich condition, but cannot cause richer or leaner jetting. Only physically changing a jet can change your jetting.

I've heard this "oil makes it leaner" theory repeated many times, and its totally without scientific merit.

Fuel = gas + oil. All of your compressed fuel has between the time the plug fires and the time the piston reaches the exhaust port to burn. Anything left seeps into the crank or gets blown out the exhaust. An excess of unburned fuel is a rich condition. An absence of unburned fuel is a lean condition.

The higher the oil content, the slower the rate at which fuel burns. The more oil you add, the more oil you see on your plug, builds up in the crank, or blows through the exhaust. This is a richer condition.

The purpose of air is to accelerate burning. (only the oxygen in air has purpose). If you slow the rate your fuel burns with oil, you need to offset it by accelerating burn with more air.


Yes, burned oil produces a higher Brittish Thermal Unit rating than gasoline. Unburned oil blown out the exhaust produces nothing and reduces the amount of gasoline being burned by slowing its burn rate so that more of it blows out the exhaust with the excess oil. You will only get a higher BTU output from running more oil if you actually burn it, and to do that you must increase the amount of air to accelerate the burn rate enough to offset the burn retardation fom the oil.

Synthetic oils tend to burn much slower than conventional oils, which is why they usually recommend mixing less of it in your gas. The gas burns faster than the oil in it, so most of the unburned residue is oil.

I disput your assertion that too much oil can cause an engine seizure. Quite the opposite. If your theory was correct, running straight gas with no oil should cause you to foul plugs and run rich. When your fuel burns completely and too fast, you have a lean condition, and you leave a portion of your piston travel unlubricated and dry with poor heat transfer, which is why you start running hot and melting pistons.

I've only been riding about 40 years, but of all those times where somebody runs out of their special 50:1 synthetic mix, and borrrow somebody else's 20:1 chainsaw can, the only negative result was more fouled plugs, more smoke, and oily exhausts, none of which would be considered a leaner condition.

[4x4van]

I was with you until I got to this paragraph.
First, the terminology - "Jetting" is the physical size of your jet and is fixed, with a numbered value. Your fuel mixture may require a change in jetting, but cannot "change your jetting". That is physically fixed. To explain it as such is very confusing. Your fuel mix can cause a lean or rich condition, but cannot cause richer or leaner jetting. Only physically changing a jet can change your jetting.

Okay, you are correct in the "terminology"; when I say that your oil mix changes your jetting, I mean that it changes the resulting gas:air ratio, not the physical jet size.

I've heard this "oil makes it leaner" theory repeated many times, and its totally without scientific merit.

Then I guess we will have to agree to disagree. And it seems that pretty much everyone in the know agrees with me. Try Googling it;

http://www.off-road.com/dirtbike/tec...ios-20502.html
http://classicmotorcycles.about.com/...anMixtures.htm
http://www.macdizzy.com/oil2gas.htm
http://www.angelfire.com/zine2/boost...or_tuning.html
http://www.thumpertalk.com/topic/347...ich-read-here/
http://www.rotaxjetting.com/PDFs/Doc...t%20Tuning.pdf
http://articles.superhunky.com/4/194
http://justkdx.dirtrider.net/printcarbtuning.html

I've witnessed it myself, with a bike that was jetted a bit on the rich side (verified with both running symptoms and plug chop). Because we had no additional jets with us at the time, the issue was solved by increasing the amount of oil for the weekend. Ran 14 oz. of oil per 5 gallons rather than 12 oz. The result? Crisp clean running, perfect plug color.

I disput your assertion that too much oil can cause an engine seizure.Quite the opposite. If your theory was correct, running straight gas with no oil should cause you to foul plugs and run rich.

Now you are comparing apples to oranges. Obviously, running straight gas will cause a seizure; there is no oil for lubrication, regardless of the gas/air ratio. But that type of seizure is NOT caused by a lean jetting condition, it is caused by no lubrication. If YOUR "quite the opposite" theory was correct, then running straight oil with no gas would result in the best power, right? Obviously not. So you see, taking EITHER theory to the end extreme is illogical and non-applicable.

I've only been riding about 40 years, but of all those times where somebody runs out of their special 50:1 synthetic mix, and borrrow somebody else's 20:1 chainsaw can, the only negative result was more fouled plugs, more smoke, and oily exhausts, none of which would be considered a leaner condition.

Again, you are mixing up two different things. Personally, I don't think that the gas/oil mixture should ever be referred to using the terms rich or lean, because that is what leads to the confusion. In your example the fouled plugs, more smoke, and oily exhausts are not from a rich "jetting" condition, but rather a super-rich "gas/oil" condition. As for myself, I would never "borrow" someone's 20:1 mix to run in my bike that is set up and jetted for a 53:1 mixture; that would be like playing russian roulette with my motor. Again, gas/air ratio is different than gas/oil ratio, and each results in different effects. As an example; A lean jetting condition (gas/air) will run like an F-16 for a short period before it overheats and melts the piston; a lean fuel condition (gas/oil) on the other hand will not run much differently power-wise than a correct mixture, but will seize from lack of lubrication. Two entirely different root causes of seizure, with the same basic end result (an expensive doorstop and a ruined weekend!).

Bottom line, (and no fewer than 8 experts above agree): Increasing the amount of oil in your gas results in a leaner air/gas condition and will require richer jetting to compensate; Less oil results in a richer air/gas condition and requires leaner jetting to compensate.

[BOB MARLIN]

I once had it explaned to me in yet a different way.

The 2 stroke oil is designed to "Drop out" of the fuel air mixture when it gets to the crank case, forming little droplets of oil that cling to the inside of the case and crankshaft. When the piston comes down, it forces these little drops of oil up to the cylinder through the ports. If the oil mix is to "rich" the drops are to big to get transferred up to the cylinder. If the mix is to "lean" the drops are not big enough to lubricate . Both situations will cause a seized engine. Oil manufactures make their oils with different formulas so the optimal mixture will be different, so once you get it right- don't change brands.

I have no idea if this is true or not, but it sure made sence to me.

[4x4van]

I once had it explaned to me in yet a different way.

The 2 stroke oil is designed to "Drop out" of the fuel air mixture when it gets to the crank case, forming little droplets of oil that cling to the inside of the case and crankshaft. When the piston comes down, it forces these little drops of oil up to the cylinder through the ports. If the oil mix is to "rich" the drops are to big to get transferred up to the cylinder. If the mix is to "lean" the drops are not big enough to lubricate . Both situations will cause a seized engine. Oil manufactures make their oils with different formulas so the optimal mixture will be different, so once you get it right- don't change brands.

I have no idea if this is true or not, but it sure made sence to me.

That is a great way of looking at it, and also explains why the oil is NOT considered in the gas/air ratio that the carb provides for combustion. The fuel turns into a vapor, but the oil doesn't vaporize, and instead drops out to lubricate the engine. What's left for combustion is the gas/air vapor (without the oil). However, that oil does take up space in the gas/oil mix while it is being fed through the carb jetting circuits (before it gets to the vapor stage). More oil in the fuel means less gas in that fuel flowing through any given jet, and less gas mixed with the same volume of air means a leaner gas/air ratio.

[BOB MARLIN]

Wow I just realized how long ago this thread was started !.

[TimSr]

The fuel turns into a vapor, but the oil doesn't vaporize, and instead drops out to lubricate the engine. What's left for combustion is the gas/air vapor (without the oil).

Oil doesn't vaporize or burn in the fuel mix?

The oils primary purpose is to first lubricate the cylinder wall. The majority of oil burns with the fuel. The unburned and underburned vapors blow out the exhaust, and the condensed residual accummulates in the crank. Synthetic oils burn less easily, and leave more reisdue, which is why you use less of it. The gas burns faster than the oil, which is why the residual is mostly oil. A lean condition is precisely caused by insufficient residual from over burning. It is corrected by slowing the burn rate, either by reducing oxygen, or by increasing the amount of oil.

If your theory held true, and the oil did not vaporize and burn with the gas, your crank would fill to to the top with oil after 3 or 4 tanks of gas, depending on your mix. If oil does not vaporize and burn, please explain what all that extra smoke is when you double your oil to gas mix or lose a right side crank seal. If it does not burn, where does that 13-25 oz. of oil you put in your 5 gallon gas can go? It just keeps on filling the crank?

My explanation of burning properties is easy to test. Take 3 squares of TP. Wet one with straight gasoline, one with gas/oil 20:1 and one with gas/oil 50:1. Lay then on a screen, elevate it off the ground, light them all at the same time. When the 20:1 square is gone, tell me which of the other two looks "lean" and which square looks "rich"? Which is dry ash, and which is still burning with bubbling liquid residue on it? It will eventually burn up. If you blow on it and apply oxygen, it will happen faster.
Still not convinced oil burns? Add another sqaure, and drench it with pure oil and light it.

If all other factors stay the same, and you reduce the speed at which your fuel burns, more of it will be left unburned when you get to the exhaust stroke, which is a rich condition. Its pretty simple until people try to overthink it and make it complicated.

[4x4van]

Oil doesn't vaporize or burn in the fuel mix?

I never said it doesn't burn. I said it is not burned as part of the ideal gas:air ratio that every internal combustion engine is looking for. Of course it "burns", just as bits of tissue paper introduced into the combustion chamber would "burn". But it does not contribute to the gas:air ratio, any more than the tissue paper would.

The oils primary purpose is to first lubricate the cylinder wall. The majority of oil burns with the fuel. The unburned and underburned vapors blow out the exhaust, and the condensed residual accummulates in the crank.

So does it lubricate the cylinder wall? Or does it burn with the fuel? Or does it blow out the exhaust? Or does it accumulate in the crank? Actually, a little of all, but you are still ignoring the point of my original post; that increasing the amount of oil in fuel will result in a leaner gas:air mixture for combustion. That is fact, backed up by just about every single 2-stroke expert around (go back and click on the multiple links I've already provided). It is also simple physics. A jet (any jet) is a fixed size; only a specific volume of liquid will flow through it. So the more oil in that volume of liquid, the less gas in that volume of liquid. Gas and air is what is combined for combustion, not the oil. The oil, after doing it's job of lubricating the bottom & top end, is burned as a RESULT of the combustion, not as a PART of it.

[lincolnolli]

I can only add this,from my 1970/80s experience:
They had this stupid law here,that teenagers where only allowed to ride 50cc,actuall 49,9cc engines.But for mayn years there was no limit to engine power or speed!Obvious that this started a little horsepower race between the brands(Zündapp,Kreidler,KTM,Sachs,Puch).
They ended up with over 7,sometimes 8 to 9 horsepower and easy 100km/H topspeed stock.But,you guessed,nobody had their stuff stock...lol
So we all went 120km/H with little bolt nons,reving around 12000rpm all day.
To get to the point:All,i mean all manufactorers told you to use 1:50 mix,but use a quality oil.We did just that and never had a failure.But hi octane was the trick,as i felt with the awesome sound of a full throttle 2 stroke you'll never here the death ping.Show might be over,real fast!

The 1:25 mix was recommended on the oldish 3hp mopeds,but each and every manufactorer went away from this in the 70s and 80s.
Some adds said thats possible because of the nicalsil cylinders and much improved cooling now.
I must add,they where all aircooled!So the aluminium cylinders ended up looking as if you had a 250cc or something,but that really helped.Some called it the "thermotop" cylinders.
Compared to the old fart 3hp 50cc,they smoked very little and once fully warmed up didnt even stink or so.

And i fully agree,that when you looked inside a crankcase there was a lot of good looking oil smearing around.I never had the feeling as that wouldnt be enough.
So if i do the math we reached 10hp on 50cc aircooled,maybe one can compare?
I have no idea,but thats my 2 cents to the story...

[lincolnolli]

Sorry,found that only in german.But the pictures might speak for themselves:
https://de.wikipedia.org/wiki/Kreidler_Florett_RS
"..in 1977 Kreidler changed there recommended mix to 1:50"

[lincolnolli]

Nearly forgot:
And then the Japanese came up with there 2 strokes.They had a seperate oil tank and a oil pump.The pump would deliver a very small amount of oil right in the intake manifold.I must say they ran much better,not smearing the oil through the carb and tiny jets therein..

[Jinlong]

I have book recomended for oil ratio maybe it can helped much, A. Graham bell two stroke performance tuner you can download it at
https://www.pdfdrive.com/two-stroke-...159144784.html

About oil ratio chapter 8, page 166

[Hedbngr18]

I've always used MC-1 or H1-R (based on availability) and I am primarily a sea-level dune rider. I ran a stock pipe and jetting, FMF silencer, and a 1mm over top end. I've inspected a >100 top end in the past and there was minimal piston/ring wear and the cylinder hashing was still mostly visible. I'm positive that I am in the minority, but I'll just consider myself lucky. I have now gone 1.5mm over, done some pretty decent porting and switched to a full FMF setup. I'm going to start out with my old method, but I am not too stuck in my ways to make a change. That's what a Ratio-Rite is for anyways. If it smokes the right amount, runs properly and (most importantly) smells good, I'll run it. Whatever is best for my particular setup and condition, I'm game.