Fuel Oil Ratio

[Richard B]

Is the 20:1 fuel/oil ratio still recommended? Or can the oil content be reduced based on more modern oils?

[paul.darnell]

I use 33:1 with synthetic oil with no problems.

[bh.sprint]

Ariel eventually recommended a leaner oil ratio of 32 to 1 in the later workshop manuals. Some use 40 to 1 with synthetic oil. Brian H

[nevhunter]

Allowing it to warm up before revving the crap out of it helps the oil to separate from the Mix. Once the motor is hot it could run with less oil.Running fast with hardly any throttle. (conrod straight at Bathurst) some premix bikes needed an extra shot of oil.( Bultaco's Iron sleeves).Nev

[FrancisD]

Dear All,
In 1966 I ran my Golden Arrow at 16:1 using Castrol R40, come 2024 the newly acquired Golden Arrow is on a ratio of 50:1 using Millers Motorsport KR2T, an oil and ratio I've used for several years in the Husqvarna chain saws I own and maintain for others.
The advances in oil quality over the near 60 year gap has been phenomenal although the R40 I reckon is still up there if you can put up with the petrol/ oil separation and ash residue.

[ian williams]

easiest to remember is 2 1/2 tank caps full of 2t oil to one gallon [5 litres] of petrol.

[Ivor Collins]

Hi Francis.

Your experience is an interesting insight into what’s possible with modern oils. I have never been brave enough to run with such a lean oil mix but maybe, armed with your knowledge, I should now try.
All the oil I’m using is diluting the petrol the engine is getting and robbing it of much needed power. I also believe that additional oil actually increases engine running temperature rather than decreasing it?

I run Stihl chainsaws which operate at up to 14,000rpm and, as you say, 50:1 is the petroil mix though maybe they also have a better developed oil delivery or bearing design to aid longevity.
Of course, the oil ratio specified for current petroil engines is dictated by emission regulations, with the engine being expressly developed to operate on minimal oil ratios.
I think I may be talking myself out of going too far with the idea but your success is encouraging

Thanks for sharing
Ivor

[Bob.Murphy]

I have a 1975, 5hp Yamaha (air cooled) outboard motor that has had a hard life on 100:1 oil mix without problems.

Bob.

[FrancisD]

Aye Ivor,
This discussion on petroil ratios brings to mind my racing days with the two strokes, the Greeves Silverstone was 16:1 using Castrol R40 but graduating to Yamaha's TZs then 30:1 using the semi synthetic Castrol A747. Some lads made the mistake of increasing the oil ratio to lengthen component service life only to be faced with partial seizures, they'd failed to jet up to compensate for the greater petroil viscosity resulting in overly lean running.
For best power and cleaner internals run as lean as possible regarding ratios then jetting accordingly hence the 50:1 with Millers fully synthetic KR2T.

Power saws are abused and worked really hard, only time I've had to do an engine rebuild is when the user has filled up with straight fuel, not used a fine mesh funnel to filter out water or leaning the "H" screw too much thus over revving. Currently awaiting the delivery of a barrel / piston kit for an autotune Husqvarna 545 whose owner opened up the wrong jerry can, the daftie !
No doubt you'll have similar experiences with the Stihls.

[Mick D]

Hi

Just thinking 'out aloud', how much of this conversation is comparing apples with apples? ratios in the sixties were based on the technologies and oils available at the time as were later ones. Agreed, oils have moved on since then but the designs have not.

Regards Mick