Why doesn’t my 4-stroke smoke?!?!

Not as silly a question as it might first sound

Ok, so my mate runs go-karts, long story short – he had 7 litres of premix he wanted to get rid of - I took it off his hands and said I’d just stick it in my road bike …

Ok, so he runs a 16:1 mix in his Kart … which is normal for his Kart, but very oily compared to what I am used to … I put the 7L in a 20L jerrycan and topped it up with 13L of pump unsplendid. So that is a mix of about 45:1.

Now I run 50:1 in my 2-stroke minimoto, so 45:1 is still pretty oily …

Anyhoo, I topped my SV tank up with this concoction … I had a few litres of normal fuel in there still - so say ~50:1 all mixed up …

Now bearing in mind that is the same mix as I use in my 2-smoke minimoto and on startup that is particularly smoky … how come my 4-smoke doesn’t visibly smoke on startup or (overly) smell???

Is it something to do with the fact that it has valves shutting out the return from the exhaust? And/or that I get a cleaner burn??? Or perhaps a hotter burn?!

What do you reckon??

p.s. I so hope I don't end up oiling my plugs up and it not starting or it cuts out at the most inappropriate moment!!

You filled your tank with premix just to see what would happen?

You are indeed a strange breed.

Originally Posted by andyaikido

You filled your tank with premix just to see what would happen?

You are indeed a strange breed.

Yes I am strange but no, curiosity isn't the reason ... my mate had 7L of premix he wanted shot of ... and he offered it to me as his only other alternative was to chuck it in a hole in the garden or summin and set a match to it!! ... my bike doesn't have a 'cat' so it won't come to any harm running 2-smoke oil mix in it ...

I have run 2-smoke mix in my SV before as I get rid of my left over minimoto pre-mix in the same manner all the time (but that is gernerally ~1L so well watered down in a full SV tank of normal stuff ) ...

Ok, so I am unlikely to turn down ~£7 of 1 week old free fuel just coz it has 2-stoke in it ... would you?!?!

Bearing in mind I have never run a mix this oily in a 4-stoke before, I am just surprised that it doesn't apear to smoke ... the same mix in my 2-smoke does!!

I would think that a 4-stroke keeps the unburned fuel-air mix better separated from the exhaust gases - something that two-strokes are pretty bad at. in the high temperatures of proper combustion the oil probably burns reasonably well/cleanly, rather than a portion of it getting burned much less efficiently/cleanly by contact the hot - but comparatively much cooler - exhaust gases. aerobatic aircraft make their smoke trails by dumping diesel into their exhausts, which makes the thick white smoke

doesn't make it a clever thing to do still. if you hadn't spent all you money on shiny blue & yellow shocks you could afford proper petrol

Originally Posted by weazelz

I would think that a 4-stroke keeps the unburned fuel-air mix better separated from the exhaust gases - something that two-strokes are pretty bad at. in the high temperatures of proper combustion the oil probably burns reasonably well/cleanly, rather than a portion of it getting burned much less efficiently/cleanly by contact the hot - but comparatively much cooler - exhaust gases. aerobatic aircraft make their smoke trails by dumping diesel into their exhausts, which makes the thick white smoke

doesn't make it a clever thing to do still. if you hadn't spent all you money on shiny blue & yellow shocks you could afford proper petrol

That was one of my thinkings ... but still surprised/disappointed!! If nothing else I wanted the smell!!

Why 'not clever' its not like it will hurt anything ... if anything give the upper cylinder and valves a good lube?!?! ... and dude, FREE is FREE - never turn down FREE

Obviously very sensible, but I bet it drinks! - see we all have vices!

If it was fully synthetic I think I'm right in saying it doesn't smoke as much as mineral 2-stroke oil. Also, at that dilution rate it should be fine... probably good for it occasionally. An old friend used to add a drop of fully synth 2-stroke to a tankfull on his old 750 Honda, just to lube everything now and again. That's pre catalytic converter days, though.
Jon.

[Sid Squid]

Originally Posted by ujoni08

If it was fully synthetic I think I'm right in saying it doesn't smoke as much as mineral 2-stroke oil.

This would be a sizable part of my guess, modern two-stroke oils are a whole lot cleaner than they used to be.

Smoke is particulate matter, and the product of incomplete combustion, the gas dynamics of a modern four stroke - which needs to be pretty clean emissions wise - don't allow much in the way of fresh charge/exhaust gas mixing, so it's pretty certain that very little of the fresh gas, (containing said low volume of oil), will be making it out of the motor unburnt - good burn = minimised smoke.

Its fully synth ... but that is what I use in my smokey (when cold) minimoto ...

[Sid Squid]

Originally Posted by Blue_SV650S

Its fully synth ... but that is what I use in my smokey (when cold) minimoto ...

Exactly. When cold there isn't complete combustion, which produces more visible particles - smoke. And of course the reason that two strokes can't be made as clean as four strokes, (well, until someone sorts out direct injection properly anyhoo), is that they don't separate their gases as well, the transfer and the exhaust ports will always be open together, which means that gas mixing will always allow some fresh charge out of the exhaust*, the general low temperature of the engine and exhaust themselves at low temps won't be enough to cleanly burn away the oil residue, partial burns leading to - smoke!

*Another reason that two strokes are not as economical, if you arrange the gas flow of the motor so that the fresh charge doesn't partially chase the spent gas out, the power drops off as cylinder filling and volumetric efficiency drops away, VE not being something that stinkwheels are good at anyway...