2-stroke piston window, rear boost port.

[Daryl Jones]

My understanding is a optimum fuel to air ratio is around the 14:1

AFR 12:1 makes the best power on a 2T not that AFR is a reliable guide on a two stroke

Lots of factors, inc. fuel quality, temperature, humidity, air pressure, combustion time, engine load & throttle opening affect the best AFR, in any situation.
Carby's do an amazing job, (Fuel Injection is a topic for another thread)

Best power AFR is when you have 'enough' fuel to use up every available O2 molecule in the charge. (A Bit Rich)

Best economy is when you have 'enough' air (O2) to burn up every fuel molecule in the charge. (A Bit Lean)

The stoichiometric ratio for complete combustion of petrol is 14.7 parts of air to one part of fuel,
but it's really hard to manage because of all the variable conditions listed above. 

Best to be A Bit Rich.  A Bit Lean is really hard on engines & if your looking for economy you're in the wrong forum.


Cheers, DJ

[sleepy]

I know it has been done to death a million times on here.. but I can't for the life of me, with modern synth oils , work out why anyone would want to run 28:1.. even on an old banger aircooled engine..40:1 is fine..

The difference between 28 and 40:1.
28 will put a bit of black goup out the pipe.
28 will make more power.
28 will run cooler.
28 will make bigends and pistons last longer.
40 will cost less per tank but more in rebuilds.

I recommend 25:1 for air cooled MX motors. Kawasaki KX125 2000 model, factory recommend 25:1 with modern oils.

 

[Daryl Jones]

The energy density of oil (mineral or vegetable) is greater than that of petrol.

If your burning Most of it, you will be making Power from it.

A Bit Rich (oil) is better than A Bit lean (oil) even if you get a goupy pipe.

Cheers, DJ

[Lozza]

I know it has been done to death a million times on here.. but I can't for the life of me, with modern synth oils , work out why anyone would want to run 28:1.. even on an old banger aircooled engine..40:1 is fine..

If your running 40:1 in an air cooled engine it's more about the load and the tune of the engine. An air cooled 100cc kart engine would last a matter of laps with that ratio. Spooge out the pipe is jetting and ignition rather than oil ratios.

[FourstrokeForever]

I know it has been done to death a million times on here.. but I can't for the life of me, with modern synth oils , work out why anyone would want to run 28:1.. even on an old banger aircooled engine..40:1 is fine..

If your running 40:1 in an air cooled engine it's more about the load and the tune of the engine. An air cooled 100cc kart engine would last a matter of laps with that ratio. Spooge out the pipe is jetting and ignition rather than oil ratios.

I can remember when 2 stroke lawn mowers n whipp snippers etc used to run a recommended ratio of 15:1. Now with better metallurgy and oil quality it's 50:1. 2 stroke MXers were 20:1 ratios and then the research about best fuel/air/oil ratio took on another logic and came to the conclusion that too much oil does infact cause a lean situation because the oil isn't being burnt efficiently. I'm with Watto here. 40:1 is fine for my 2 strokers. Even after an entire season on an Elsinore, 6rounds x 3 races x 3 laps (54 laps) with the odd ten lapper thrown in, there was no evidence of piston or bore wear, very minimal carbon build up (wiped off with a rag and petrol) and I only changed the rings because I thought I should while I had the top end off. That's all the proof I need.

[sleepy]

I know it has been done to death a million times on here.. but I can't for the life of me, with modern synth oils , work out why anyone would want to run 28:1.. even on an old banger aircooled engine..40:1 is fine..

If your running 40:1 in an air cooled engine it's more about the load and the tune of the engine. An air cooled 100cc kart engine would last a matter of laps with that ratio. Spooge out the pipe is jetting and ignition rather than oil ratios.

I can remember when 2 stroke lawn mowers n whipp snippers etc used to run a recommended ratio of 15:1. Now with better metallurgy and oil quality it's 50:1. 2 stroke MXers were 20:1 ratios and then the research about best fuel/air/oil ratio took on another logic and came to the conclusion that too much oil does infact cause a lean situation because the oil isn't being burnt efficiently. I'm with Watto here. 40:1 is fine for my 2 strokers. Even after an entire season on an Elsinore, 6rounds x 3 races x 3 laps (54 laps) with the odd ten lapper thrown in, there was no evidence of piston or bore wear, very minimal carbon build up (wiped off with a rag and petrol) and I only changed the rings because I thought I should while I had the top end off. That's all the proof I need.

You say that to much oil causes a lean situation. Almost all the air cooled 2 strokes in the 70's came from the factory jetted to suit 20:1 so to go to 40:1 would require richer jetting. I bought a new RM250A in 76 and raced it for a hole season which include practicing for an hour every weekend when not racing. It only ever had 20:1 R30 and at the end of the season still didn't need a ring. It did get decoked a couple of time though.

Could you post a link to the research that shows the benefits of using 40:1 over 25:1. Things like dyno comparisons or technical papers.

[John Orchard]

Less oil in the mix does make the jetting richer.

[Tim754]

VERY VERY relevant statement of FACT from Lozza    "Spooge out the pipe is jetting and ignition rather than oil ratios."
Learn and practice tuning and maintenance of your engine, follow manufacturers recommended ratios for the oil you wish to use. (ask them to find out...)

[sleepy]

Less oil in the mix does make the jetting richer.

I had a bit of brain fade there. It should be leaner jetting required.

[GMC]

Could you post a link to the research that shows the benefits of using 40:1 over 25:1. Things like dyno comparisons or technical papers.

I have no research papers or dyno reports to speak of but I don’t dispute the power claims of the more oil brigade.

My experience from early days was running mineral oils at 20:1 and constantly suffering fouled plugs if I let my bike idle too long or rode slowly too long (no I was talking about riding in tricky terrain)
Then I moved to Synthetic oils and started running at 40:1 and hardly ever fouled a plug again.

Most of the reports I see about running more oil are usually about road race bikes or Karts that are nearly always running at full throttle as they are when they are on a dyno, most off road stuff is far removed from that. I have no doubt that more oil would be beneficial if running flat out all day.
 
I will take a bit more notice when a dyno simulates riding single track for 20 minutes or dropping down some knarly valley with the throttle chopped most of the way.

Bulk horsepower isn’t always necessary for off road stuff which is why it appeals to me as you can use many other skills to overcome obstacles and other competitors.
To finish first, first you must finish

[Lozza]

A 15yr old on 125 will be flat knacker on what ever terrain is in front of them, likewise when Shayne Watts smashed everyone in a 4 day on a KTM125 on1 set of tyres.
The Gordon Jennings oil ratio test was completed on a PE250 with load applied to the dyno drum with the engine held at 5500rpm. Closed throttle is when you need the oil the most, reeds are not really opening and the rear wheel is driving the engine.

http://www.bridgestonemotorcycle.com/documents/oilpremix6.pdf

[sleepy]

Could you post a link to the research that shows the benefits of using 40:1 over 25:1. Things like dyno comparisons or technical papers.

I have no research papers or dyno reports to speak of but I don’t dispute the power claims of the more oil brigade.

My experience from early days was running mineral oils at 20:1 and constantly suffering fouled plugs if I let my bike idle too long or rode slowly too long (no I was talking about riding in tricky terrain)
Then I moved to Synthetic oils and started running at 40:1 and hardly ever fouled a plug again.

Most of the reports I see about running more oil are usually about road race bikes or Karts that are nearly always running at full throttle as they are when they are on a dyno, most off road stuff is far removed from that. I have no doubt that more oil would be beneficial if running flat out all day.
 
I will take a bit more notice when a dyno simulates riding single track for 20 minutes or dropping down some knarly valley with the throttle chopped most of the way.

Bulk horsepower isn’t always necessary for off road stuff which is why it appeals to me as you can use many other skills to overcome obstacles and other competitors.
To finish first, first you must finish

If you fouled plugs it was the tuning not the mix ratio. My first trials bike was a 1976 bultaco sherpa 250 which would idle all day with a 20:1 mineral oil and never foul a plug also got a new PE175 in 78 and it ran 20:1 R30 without oiling up plugs either.

[80-85 husky]

Ive run 75 to 100 to 1 in my 010 kato 300 and my wr 400. I only use motorex as it has a deep base construction allowing it to maintain the required film strength at these ratios.

r30 is a real light base oil and needs to be at 20: 1 to lube. don't use  normal oils at leaner ratios  than 40:1 other than full synthetics that have a deep base.

the bikes have run fine, start very well and the kato has just had a piston kit thru it for the same reasons ol mate changed his rings..cos I had the top off for a look...

the husky has done 3 Vinduro seasons and other riding on the piston and rings it had in it when I bought it. it runs very nicely at 200 rpm when you fluff a hill or a tricky bit and pulls out of trouble cleanly. I have the needle jet just on the lean side and run the leaner fuel to compensate. works well.

also note being water cooled helps. I would not be keen on going past 75:1 on an aircooled bike due to heat dispersion wanting more oil (in my tiny mind)

a few mates went to a kato ride in wa with leisky and he told them they were running 100:1 when they were fanging across some salt lake in top gear. spooked em a bit but it started us all running those style ratios

At end of the day its a personal choice and if the big end seizes out from lack of oil, I will go back to higher ratios but so far so good

[FourstrokeForever]

I don't have any of the dyno reports or any of the real time test reports that I had saved on my last PC. It all vanished when the lap top went to computer heaven. I will say however, that attempting to compare what fuel/oil ratios we all used to run our 2 strokes on back in the late 70's and 80's and what we do today isn't really comparing apples with apples. Modern synthetic oils are much better and actually cling to parts even at extreme high temps. And the metallurgy that comes with our replacement pistons and rings these days is so much better than it was back then. I'm not trying to convince anybody that they need to run leaner oil ratios just because I do and have positive experiences from it. Run what you like. It's your bike. I'm only talking about my experience.

[Daryl Jones]

Here is some information from a Company that spent a lot of time & money (dyno & racing) researching this stuff.

Yamaha did as much as (or more than) anybody to develop the 2T engine technology that is relevant to our VMX era.

They determined that 2 stroke engines require different Fuel:Oil ratios at different engine speeds, loads & throttle openings.

This is the chart they released with the new Autolube system back in 1968.

From Digga's article link.

see the whole DTI thread here: http://forum.ozvmx.com/index.php?topic=37632.0

From this chart it is simple to see that 40:1 or even leaner, is more than sufficient for many riders.
Just locate your own "successfully tested" F:O ratio in the chart and then read off your average throttle setting & speed.

Of course We All Knew Better than Yamaha, and it was the first thing pulled off a DT or MX.

Before we hear the argument; that technology & manufacturing & metallurgy has improve greatly since then, of course it has.
Modern Go-Kart engines have the full benefit of all off that and more but as Lozza said. they still need a lot of oil, just to survive.
In 1968, Yamaha were first and second place in 125cc & 250 cc  GP championships, in 1970 they held the first 5 or 6 championship places in the 350cc.
Their technology was world leading then and was way beyond normal production standards for the era.

I might argue about lubrication improvements, as much of the 'progress' has been to designed to minimise environmental impacts (eg. reducing smoke levels and providing biodegradable emissions). From a 'pure lubrication' point of view we haven't bettered Castor Oil.
Better engineering has allowed oil manufacturers to get away with lesser grade base oils for their retail products.

No hard recommendations for any-one here, you are free to use what 'works', for you.
(But an honest look at the chart provides some clues).

Cheers, DJ