Two Stroke Oils/Fuel

[Nathan]

So… I bought a brush cutter from ebay for the grand sum of $90! The manual says to use a two stroke mix of 25:1.

My question is, I have 50:1 oil (unmixed) that I use for my Stihl chainsaw - is this oil different to 25:1 oil or is it OK to just put 50:1 oil in at 25:1 mix - ie. double what I use for my chainsaw?

Thanks.

[steveG]

I've never heard of a specific 50:1 oil. Everything I've used is just 2 stroke oil and you mix it to whatever ratio you need.

If it was me I'd just be mixing it at 25:1 as you suggest.

Steve

[Nathan]

Yeah, I think the bottle of Stihl branded oil that came with my chainsaw is just labelled at 50:1 to make it idiot proof.

[jakeslouw]

Actually a 50:1 ratio is LESS oil per liter of petrol, so LESS lubrication from the oil.

25:1 is idiot proof.

[Nathan]

I have a mixer bottle that you just fill up to the desired ratio so I wasn't thinking about it that hard!

But... this website says 25:1 is more oil than 50:1

Hints & Tips (http://www.thomsonsmowercentre.com.au/hints_&_tips.htm)

[jakeslouw]

Ja that's what I said mate ! :p

"Actually a 50:1 ratio is LESS oil per liter of petrol, so LESS lubrication from the oil."

Ergo and therefore a 25:1 mix is MORE oil per liter, and also implies LESS performance for the motor as the 2SO doesn't contribute to the combustion process and your fuel:air ratio is less because of the extra oil.....

50:1 is pretty radical and calls for a hi spec 2SO. Mix as per the bottle instructions, but with a 25:1 2SO you can safely mix at 30:1.

[Nathan]

That's what I was saying too! 25:1 is more oil per litre!

When I said about being idiot proof I was referring to the label, not the mix!:D

[Blknight.aus]

caltex used to sell a 25:1 and 50:1 bottle of oil.

one was twice the size of the other and the idea was that you simply turn up, buy (or supply)the 5l tin put the mix in add the fuel and you were good to go.

[roverv8]

25:1
easy terms = per 25 litres petrol & 1 litre oil
half that = 12.5 litres petrol & 500mm oil
half again = 6.25 litres petrol & 250mm oil
safe to say you can mix 3 litres petrol & 125mm oil

[uninformed]

good god, I thought I escaped the dreaded 2t oil threads on the chainsaw forum. :D

50-1 is not extreme at all....there are some oils that will run at 100-1 (not that I would....)

Most Brand names (stihl, husky, dolmar etc) will specifiy to run THEIR OWN brand oil at 50-1, but if using ANYTHING else, 25-1.

Problem is, the motor is not tuned for both.....

A good example is my neighbour bough a blower from bunnings (not a cheap cheap one) he read the manual and followed the advise of using 25-1 when not using manufactures brand. After 6 months the blower was running poor and lacked power. A simple muffler clean (sootted up) and fresh fuel at 50-1 made all the difference......simply the motor wasnt tuned to 25-1

Any good quality oil made for AIR COOLED engine will be fine at 50-1 (tuned correctly of coarse)

no problem running at 25-1 either (if engine is tuned there)

I personally run my work stuff (blower, concrete cut off saw) at 40-1....any of my saws that are modded get 32-1

I only use BP ultimate 98 octane (NO ethonal...very important)

Fuel stored over 4 weeks gets used for cleaner.....any tool going to sit for longer than 2 weeks, gets drained and then started and run at idle to clear fuel lines.

and btw it has been proven that a lower ratio (ie MORE oil) produces more power....to a point, somewhere about 12-1 it stops making more)

[Blknight.aus]

and btw it has been proven that a lower ratio (ie MORE oil) produces more power....to a point, somewhere about 12-1 it stops making more)


yeah but you've got to get it up to temp first and it can make starting frustrating. I thought the magic point was about 17:1.


interesting side note the cooler your ambient running conditions the thinner you can run the mix.

[Basil135]

And who remembers the days when you just took your can to BP, and filled it with Zoom25... :eek:

[rick130]

yeah but you've got to get it up to temp first and it can make starting frustrating. I thought the magic point was about 17:1.



Nope, richer than that.

Most race air cooled two strokes that rev above 15,000RPM under load use around 16:1.

[vnx205]

Maybe oils have changed and engine technology has changed, but there used to be a simple reason why two strokes produced more power with more oil.

Extra oil made the fuel a bit thicker so that it was harder for it to flow through the main jet. That leaned out the mixture. Two strokes depended on the air/fuel mixture for part of the cooling, so needed to run a richer than optimum mixture to avoid overheating.

So adding extra oil gave a leaner mixture and more power until the engine began to lose power through overheating which made the air/fuel mixture in the crankcase less dense; the opposite effect to running an intercooler.

If pushed hard enough with too much oil, the engine would seize from overheating.

If the carby was jetted correctly for the thicker fuel mix, it would happily run the oily fuel, but if it was jetted for a mix with very little oil, adding more oil could make it seize.

[vnx205]

Nope, richer than that.

Most race air cooled two strokes that rev above 15,000RPM under load use around 16:1.

Years ago, I had a moment of brain fade as I was filling up my Hodaka 100. I realised when I noticed vehicles getting lost in the smoke cloud behind me at the traffic lights, that I had added double the required amount of oil.

Since it usually ran 20:1 and the tank was almost empty before the fill, it must have been close to 10:1.

It seemed to be running quite happily apart from the embarrassing cloud of smoke.

[rick130]

Maybe oils have changed and engine technology has changed, but there used to be a simple reason why two strokes produced more power with more oil.

Extra oil made the fuel a bit thicker so that it was harder for it to flow through the main jet. That leaned out the mixture. Two strokes depended on the air/fuel mixture for part of the cooling, so needed to run a richer than optimum mixture to avoid overheating.

So adding extra oil gave a leaner mixture and more power until the engine began to lose power through overheating which made the air/fuel mixture in the crankcase less dense; the opposite effect to running an intercooler.

If pushed hard enough with too much oil, the engine would seize from overheating.

If the carby was jetted correctly for the thicker fuel mix, it would happily run the oily fuel, but if it was jetted for a mix with very little oil, adding more oil could make it seize.

The main reason a richer mix works is better ring seal.

Air cooled engines have looser clearances to allow for expansion so this is where the richer mix helps (as it also did with karts spinning to 21,000RPM. Skirt seizure and little end failure was a real danger)

Water cooled engines with their tighter clearances just don't need the richer mix.

All the two strokes i've run (karts and saws) use simple adjustable carbies which makes tuning a snap (if you know what you're doing)

[vnx205]

Like so many things, there are probably a number of factors involved.

If better ring sealing was the only effect, that would not explain why engines could seize with too much oil in the fuel.

My experience with two strokes was in the 1970s and the scramblers and short circuit bikes I was familiar with did about half the revs of the engines you are describing. :)

[rick130]

Like so many things, there are probably a number of factors involved.

If better ring sealing was the only effect, that would not explain why engines could seize with too much oil in the fuel.

My experience with two strokes was in the 1970s and the scramblers and short circuit bikes I was familiar with did about half the revs of the engines you are describing. :)

You're right when you say that the richer oil/fuel mix effectively leans the engine out without retuning.

That's why I mentioned karts, as unlike a Mikuni bike carb they use taper needle adjustable jets, so it's dead simple to get a light four stroke and back it off a smidge at the end of a straight, or have a saw four stroking till it loads up in the cut.