Premix Ratios

[anotherfive]

Now am really confused.For years I always thought that less oil in the mix meant a leaner running engine. Reading an off road site and talking to a colleague at work it seems the more oil the leaner the mix because the oil takes the place of some of the petrol. I can understand the theory but thought I'd ask for comments and also to make others aware. Have been running with 63ml in 5 litres.That is 80:1 I will now try a bit more oil in mix as plug still looks dark

[the addict]

Lean/rich usually refers to the fuel going through the engine, not how much oil is in the mix, 4 strokes dont have oil for example. I would try adding abit more oil into the mix ie; 70-75ml to 5 litres but lean the engine via the pilot or main jets or both. Good 2 stroke oil can actually improve power output, running very small amounts of it are likely to cause more harm and wear on the engine without any benefits at all.

The bit about the mix being leaner or richer due to replacement of or more fuel in the mix although technically correct is a load of b*****ks

Edited May 18, 2007 by Betarev3

[scorpa3]

Traditionally the term lean/rich refers to the ratio of fuel to air. The general starting point for internal combustion engines is based on 15:1, that is 15 parts air to one part fuel.

For ratios which use less fuel than this it is called lean, for more fuel- rich.

On engines with a higher state of tune, using a smaller main jet can, in some circumstances might give better power depending on many factors (heat, humidity, height above sea level etc) but you always run the risk of overheating the engine. It's a trade off. Naturally, there are times when a bigger jet is better.

Top engine tuners can get the best out of an engine and jetting is one of the tools they use. For the rank and file trials rider, there is probably very little to gain in real terms providing your engine is running well. There again, many riders love to eek out the smallest gain.

[kevin j]

I've heard the terms used both ways: lean/rich applied to carbureation, rich is obviosuly more fuel.

Lean/rich applied to premix, rich being more oil in the mix.

Yes, technically, replacing part of the fuel with oil means richer oil mix yield slightly leaner gas mix, but unless it is research level measurement it is meaningless. The amount of change is fractions, maybe second decimal fractions, of a percent. And both are indeed petroleum, there is some heat output in burning the oil. It's probably a slight viscosity effect more than the fuel value of the liguid volume. I've seen magazine articles that correct the jetting for this in tests, I think it is ego, and the accuracy of their tests was way less than this variable. Someone trying to impress with how technical the test is maybe....

There is much published evidence that more oil is better. Common sense of course, thrashing metal things benefit from more lubrication. Also, better heat transfer, better ring sealing, less friction, more power, etc.

'My mix is leaner than yours' can be an ego thing also: 'I run, 80:1-Oh, yeah? I run 100:1, I have super oil, run 120:1,......blah blah.

In trials, we don't need the lubircation for full power continuous operation. What we run 80:1 would seize in minutes in a road race engine.

There is a lot of oil on the inside crankcase surfaces, and SAE radioactive fuel testing showed the new mix shows up in the exhaust in a few seconds, but after running an engine a long time to coat the inisde and reach steady state, when the mix runs lean traces of oil show up in exhaust for up to 30 minutes. So we have sort of an 'average' oiling system that carries through the bursts of high loads.

The down side to too much oil is loss of crispness and smoke and exhaust gunk, especially when loaded and unloaded. It heats, melts, moves, solidifies, etc.

From people much wiser than me, I've settled in on 80:1 for the moderns, 60:1 on the aircooled. Both will full synthetic oils. Seems to give clean burn, and minimal deposits, and minimal wear when inspecting a teardown.

And, technically also, 1 ounce of oil in an 80 ounce jug, then filled with fuel, could be called 1 part in 80, but is not 80:1 gas to oil. It is 79:1. Only 79 spaces left for fuel once the one is filled with oil. No one cares, we don't measure that accurately anyway, and if we did, the choice of 75 or 80 or 85 to 1 doesn't matter. Just a fine point for us engineers.....

kcj

Edited May 18, 2007 by kevin j

[timleo]

Hi while on the subject what mix would you recommend for an 07 Beta 125? We run 66ml to 5 litres but still get a black goo running from the exhaust and really have to rev to clear it.Thanks

[copemech]

It looks dark? Good, keep up your 80:1 and do not rely on those plug readings!