Hitchcocks Forum

Any advice for the 'best' fuel to oil ratio for these pre-mix type two strokes when using modern two stroke oil? I guess the old advice about 20:1, 25:1 and 30:1 has been out-dated, and much lower ratios are OK with semi-synthetic and synthetic oils.
Thanks

Hi, RICHARD R }----- until recently I was actively involved with the National Autocycle & Cyclemotor Club ( NACC)-----check-out their site, good stuff to be had there. I ran a couple of 1970's Mobylettes and a Puch Maxi or two ----all 50cc, pre-mix 2-Stroke stinkwheelers !! I ran 1/4 pint oil to 1 gallon petrol which, when you do the maths, = 32:1. I completed their Coast-to-Coast 3 times on these machines with ABSOLUTELY NO, repeat NO-- TROUBLE at all !! Does exactly what it says on the tin. Have a chat with the Bantam lads as well---I think you'll find they are in that same area. OH---one additional benefit }---- NO WHISKERING either.

Thanks Alan.
Thing is, I've heard that with fully synthetics you can safely use pre-mix ratios of around 60:1, 80:1 or even 100:1, whatever age, size or type of engine.

I know that a performance stroker will work with a ratio of about 25:1, though they're pump delivery, of course you're talking tighter tolerances, higher revs greater heat build up etc.
Personally I wouldn't exceed 30 - 35:1, if it's for cost saving then looking logically, at it, even at 60:1 the fully synth stuff is so pricey it'll still be dearer, with the older 2 stroke motors any old bog basic mineral will be fine too, the only real downside is the need to clean the plug and decoke the head more frequently but back when I rode 2 strokes I could decoke and have the thing back together in 2 hours, if you have a solid copper head gasket you can re-anneal and re-use at least once also.

Two strokes with pump type oiling use a fuel/oil ratio of something in the range of 100:1 - 200:1 when the engine is not under load. And rarely [if ever] will a two stroke 'autolube' type pump deliever anything approaching 25:1. This is due in part to the far higher efficiency of pump delivered oiling and that the settings for the pumps are regulated with high quality modern oils in mind.
Competition engines (e.g motocross) using fully synthetic pre-mix these days work on a ratio of in some cases as low as 100:1.
I was interested to know if any users have used fully synthetic oil in these older engines, and if so have you found that far lower ratios have benefits? Its not about saving money, but better, cleaner engine protection and performance.

100:1 I find hard to believe at anything above tickover/idle, less so 200:1.
Pump systems deliver a much lower ratio at idle as the final ratio is an optimum based on maximum revs maintained over a period.
At consistent revs a performance machine will be oiled at somewhere like the above ratio of 25:1 very unlikely any lower than 35:1
The mention of 25:1 was based on a review of three magazine testers taking three performance strokers to the last 2 stroke 250 road races couple of years ago, they were quite assured of the fact they were using 1L of synthetic 2 stroke to every 25L of fuel.
I think you'd be taking a big risk of scoring/seizure if you tried running at such a great ratio.

To clarify, the bigger ratio at idle is simply because the greater quantity isn't needed to maintain adequate lubrication at very low revs whereas greater lubrication is required to prevent seizure at higher OR extended revs.

Even the humble MZ's and Jawas use 50:1 and achieve decent engine life. I would say 32: 1 is as high a ratio that you would need to go. Whether a two stroke uses a pre-mix or pumped (Autolube)oil system, the MIXTURE ratio remains the SAME in relation to throttle opening position. So if it is 50:1 at wide open throttle it is also 50:1 every where else in the throttle range. What does vary massively is the amount of oil delivered in relation to the engine revs. If you are running at high revs and shut the throttle and go onto the overrun, the lubrication of both types of system reduces the oil delivery to same as if the engine was at tickover. The actual fuel to oil ratio has not changed,(still 50:1) but the amount of oil quantity per rev has dropped enormously, this sounds extremely worrying but fortunately the engine load has reduced considerably too, even so, using engine braking on a two stroke is a bad method of slowing down.....With modern electronics, that problem could be addressed with an engine management system that metered oil delivery in relation to throttle position and also took account of the engine revs too..... The 250 MZ’s, by the way, completely stopped any delivery of oil when the clutch was pulled in as the oil pump drive was driven after the clutch (which is on the crankshaft end) so when the clutch was operated the pump stopped pumping!

CLARIFICATION: I SAID: "but the amount of oil quantity per rev has dropped enormously"....I SHOULD HAVE SAID: "but the amount of oil quantity for the high engine revs has dropped enormously"

Ahh NACC man eh Alan. I have a couple of Berini cycle motors (Eggs) that go on a couple of mid 50's 28" English roadsters one Hercules and one a BSA. I have had no luck locating piston rings for the 23cc motors though and have decided I'll have to make them. I'd love someone to interrupt me with a "don't bother I have a good source" type comment though.
s:)