Changing oil brand, premix two-stroke?

[joshua]

hey guys.
I've bought a 91 RGV250 for the track, its had the oil system removed and so now is premix.
My question is - If I want to change the oil I use, do I need to drain all existing fuel with the previous oil in it from the tank before filling up?

when I sold my last RGV (which wasn't premix) the guy who bought it changed oils and it went all jelly-like and nasty in and around the filters and pumps.

Just wanna make sure that doesn't happen here.

Cheer cheer
Josh

[FilthyLuka]

your best bet is to drain, yeah. Some of the oils just don't get along

[Pedrostt500]

No you dont, Depending how quick you go through a tank of gas, will determin how fast the old mix will dilute itself out of the system, though if you want to change mix ratios as well then you could run the old mix into a seperate jerry can for latter use. Mixing on an as you need bassis can cut down the amount of premix you have around as well.

[Robert Taylor]

No you dont, Depending how quick you go through a tank of gas, will determin how fast the old mix will dilute itself out of the system, though if you want to change mix ratios as well then you could run the old mix into a seperate jerry can for latter use. Mixing on an as you need bassis can cut down the amount of premix you have around as well.

Bear in mind also that if you change premix ratio it changes the specific gravity of the fuel mix, how relatively ''heavy'' or ''light'' that fuel/oil mix is determines how relatively rich or lean the bike will run. If for example the jetting was very sharp with a particular ratio you were running and you change it to a new ratio with more oil the bike will now run leaner on the air fuel ratio. BE VERY CAREFUL.
Years ago we did some very subjective dyno tests with a very sharply jetted YZ125 that had erstwhile been running on Motul 8002T at 40:1. We changed to 32:1 for a test with no other changes, exact same jetting. We lost a sizable amount of peak and delivery horsepower because the air fuel ratio was now too lean.
Note also we ran Motul oil ( that we had to pay for ) decanted into the oil sponsors bottles as the sponsors oil caused big end bearing roller skid at elevated rpm and burned dirty. There are oils and then there are good oils!

[davereid]

Good point Robert. I like to add about 100:1 synthetic oil mix to scooters. They have oil injection, but if its allowed to get low, you can get severe oil starvation, as air bubbles enter the oil lines.

On the other hand, oil increases the SG of the oil. So you lean out the mixture a bit.

My decision, which I will cheerfully take advice on, is to add the extra oil, and take the risk of leaning out the mixture a bit.

This is because 100:1 is enough to protect the engine if you are using a good oil, even if the injection system fails.

But most small engines have very simple carbs. In most cases they don't make a terribly good job of managing the air/fuel mix anyway, and IMHO 100:1 oil is much less likely to disturb the engine than 0:1 oil.

But... advice cheerfully taken if anyone has better info.

Actually... I think most 2-strokes starve of oil on the over-run.. when rpm are high, but throttle shut down. Pip the throttle a bit if you can while going down long downhills, or anytime throttle is closed, but rpm are up a bit.

[Robert Taylor]

Good point Robert. I like to add about 100:1 synthetic oil mix to scooters. They have oil injection, but if its allowed to get low, you can get severe oil starvation, as air bubbles enter the oil lines.

On the other hand, oil increases the SG of the oil. So you lean out the mixture a bit.

My decision, which I will cheerfully take advice on, is to add the extra oil, and take the risk of leaning out the mixture a bit.

This is because 100:1 is enough to protect the engine if you are using a good oil, even if the injection system fails.

But most small engines have very simple carbs. In most cases they don't make a terribly good job of managing the air/fuel mix anyway, and IMHO 100:1 oil is much less likely to disturb the engine than 0:1 oil.

But... advice cheerfully taken if anyone has better info.

Actually... I think most 2-strokes starve of oil on the over-run.. when rpm are high, but throttle shut down. Pip the throttle a bit if you can while going down long downhills, or anytime throttle is closed, but rpm are up a bit.

Yes thats sensible and many of these scooters are almost ''jetted for ice racing in Canada'' so some careful leaning off can often pay dividends. Its actually quite disgraceful how badly jetted many production bikes are, many small bore 4 strokes that arrive here are ( conversely to 2 strokes )''jetted for ( the thin air of ) Mexico city''.
On over-run of course the only fuel and oil mix is predominantly off the pilot jet, unless the throttle is blipped. Also, consider that the ignition curve is actually advancing as the engine decelerates and any exhaust powervalves are closing off and lowering the ports, in doing so raising the secondary compression ratio. Most racing seizures will occur on over-run after a high speed run and protracted braking distance.

[joshua]

Awesome, thanks for your input guys - a few points I hadn't thought of...

Is there typically much of a difference in viscosity between high grade 2stroke oils? My thinking is that, even if I retain the current mix ratio, the specific gravity may be altered anyway if the new oil is heavier or lighter, and I'll inadvertantly be changing the A/F ratio.

I imagine the mix ratio used by the previous owner was arrived at rather arbitrarily.. how do I find the best ratio given the oil and the current carb/jet setup?

What do most people run? my engine is standard, but I have home-made race pipes fitted...

Thanks in advance you guys.

[Robert Taylor]

Awesome, thanks for your input guys - a few points I hadn't thought of...

Is there typically much of a difference in viscosity between high grade 2stroke oils? My thinking is that, even if I retain the current mix ratio, the specific gravity may be altered anyway if the new oil is heavier or lighter, and I'll inadvertantly be changing the A/F ratio.

I imagine the mix ratio used by the previous owner was arrived at rather arbitrarily.. how do I find the best ratio given the oil and the current carb/jet setup?

What do most people run? my engine is standard, but I have home-made race pipes fitted...

Thanks in advance you guys.

Frankly there is no easy answer as different oil manufacturers will reccommend different ratios and the sg ratings will vary slightly. You just have to do the hard yards!

[GSVR]

Note also we ran Motul oil ( that we had to pay for ) decanted into the oil sponsors bottles as the sponsors oil caused big end bearing roller skid at elevated rpm and burned dirty. There are oils and then there are good oils!

You really shouldn't say things like this Robert or at least not implicate yourself in such behaviour. I'm sure you probably meant "they" and not "we".

Someone once told me (years ago) that the really expensive 3n1 oil ( http://www.3inone.com/ ) in little bottles was just Tellus pnematic oil and the company selling it was making a huge markup.

Wonder where Ohlins gets their oil from? Swedish Refinery/Oilfield. Or was it specailly developed by Shell.