Mysterious two stroke engine failure

[thehollowmen]

Right.
A lot of people on two strokers have been complaining they've been on a long ride and wham, everything has locked up.

Now, I was reading the other day (yes I read) in an old 1980's text and I found a little snippet about two strokers and seizure. I'll scan it later.

Now, a portion of lubrication in a two stroker comes from the oil mixed in with fuel in the tank. When the throttle is in a steady position for too long, or if the throttle closes for too long the lubrication is under the amount the engine needs causeing wear.

This means you're gonna need to blip the throttle repeately in long straights, or when slowing down.

Now this is out of line with running it economically, but saving a few bucks in petrol is nothing compared to saving the engine.

Run it how it is ment to be run people.

[ZorsT]

To stop from having the throttle in the same place too much, maybe you should pass all the cagers in the way... one by one... in the best interests of the engine of course...

[nsrpaul]

only true for premix bikes , the vast majority of road going two strokes have a oil injector pump , so oil supply is independent of fuel supply or if you like keeps lubricating when the throtle is shut

two strokes is all I need

[Ixion]

Hm. I remember reading something about this years ago, when there were more two strokes round.

Even those with injection pumps often just inject the oil into the inlet ports, so the result is the same as mixing it with the fuel, just simpler.

And , IIRC (but it was a lot of years ago, so correction is welcome), once the oil is mixed with the fuel, what causes it to come out of the mixture and actually start lubricating (as opposed to being whizzed up into the cyclinder and all being burnt - as of course at least part of it is), is the fact that at some point in the cycle the crankcase is under a partial vaccuum. If it were not the fuel could not be drawn in. And for reasons that I do not now recall, the change in pressure causes the oil to drop out of the mix and be available as lubricant.

But if you hold a long period of same throttle/same revs (ie same airflow) you dont get the variation in pressure that is needed to get the oil to come out of the mixture. The fuel/oil mix just whizzes right on through. So you need to vary the throttle opening and or revs a bit not keep them both contant.

Scott owners used to discourse learnedly about this. (They would!)

[nsrpaul]

Hm. I remember reading something about this years ago, when there were more two strokes round.

Even those with injection pumps often just inject the oil into the inlet ports, so the result is the same as mixing it with the fuel, just simpler.

And , IIRC (but it was a lot of years ago, so correction is welcome), once the oil is mixed with the fuel, what causes it to come out of the mixture and actually start lubricating (as opposed to being whizzed up into the cyclinder and all being burnt - as of course at least part of it is), is the fact that at some point in the cycle the crankcase is under a partial vaccuum. If it were not the fuel could not be drawn in. And for reasons that I do not now recall, the change in pressure causes the oil to drop out of the mix and be available as lubricant.

But if you hold a long period of same throttle/same revs (ie same airflow) you dont get the variation in pressure that is needed to get the oil to come out of the mixture. The fuel/oil mix just whizzes right on through. So you need to vary the throttle opening and or revs a bit not keep them both contant.

Scott owners used to discourse learnedly about this. (They would!)

this is true the vacum in the crank case does separate the oil from the fuel , but there is still plenty of vacum on constant throtle the bike would not run without it , and variable vacum at that , cause its not under vacum all the way thru the stroke , for half of it , it is under pressure

[Ixion]

this is true the vacum in the crank case does separate the oil from the fuel , but there is still plenty of vacum on constant throtle the bike would not run without it , and variable vacum at that , cause its not under vacum all the way thru the stroke , for half of it , it is under pressure

That is true. But, the theory about constant speed/throttle referred to be Mr thehollowmen is an old one, so maybe there is something in it. I doubt anyone quite understands everything that happens in a two stroke, tis part of their charm.That and the lovely ringa ding dinga noise.

[DingDong]

I thought this was common knowledge? I learnt this at the age of 7 on a TS185.
However holding the throttle open for long periods will fry the piston before the engine jams... as I understand it

[Motu]

Hm. I remember reading something about this years ago, when there were more two strokes round.

Even those with injection pumps often just inject the oil into the inlet ports, so the result is the same as mixing it with the fuel, just simpler.

And , IIRC (but it was a lot of years ago, so correction is welcome), once the oil is mixed with the fuel, what causes it to come out of the mixture and actually start lubricating (as opposed to being whizzed up into the cyclinder and all being burnt - as of course at least part of it is), is the fact that at some point in the cycle the crankcase is under a partial vaccuum. If it were not the fuel could not be drawn in. And for reasons that I do not now recall, the change in pressure causes the oil to drop out of the mix and be available as lubricant.

But if you hold a long period of same throttle/same revs (ie same airflow) you dont get the variation in pressure that is needed to get the oil to come out of the mixture. The fuel/oil mix just whizzes right on through. So you need to vary the throttle opening and or revs a bit not keep them both contant.

Scott owners used to discourse learnedly about this. (They would!)

When the petrol is under lower pressure it vapourises - like LPG y'know,under pressure it's a liquid,at atmospheric it's a gas...what we call petrol is a liquid at atmospheric and a gas at say 8 in/hg - but the oil doesn't vapourise at that pressure and drops out.

It's really only long downhills that are a problem,normal riding gives enough variation - but as a born again 2 stroke rider I'm kinda nervous until I get more familia with the bike.I haven't seized many 2 strokes,maybe just lucky...

[Pixie]

It's really only long downhills that are a problem,normal riding gives enough variation - but as a born again 2 stroke rider I'm kinda nervous until I get more familia with the bike.I haven't seized many 2 strokes,maybe just lucky...

Maybe it's just because on long downhills the throttle is only open slightly and the oil pump,which has it's metering controlled by the throttle,is not pumping the amount of oil that the engine speed requires.This would apply to premix as well.
Also having too much oil is a seizure risk because it leans out the total fuel/air mixture,thus making the engine run hot.

[Ixion]

When the petrol is under lower pressure it vapourises - like LPG y'know,under pressure it's a liquid,at atmospheric it's a gas...what we call petrol is a liquid at atmospheric and a gas at say 8 in/hg - but the oil doesn't vapourise at that pressure and drops out.

It's really only long downhills that are a problem,normal riding gives enough variation - but as a born again 2 stroke rider I'm kinda nervous until I get more familia with the bike.I haven't seized many 2 strokes,maybe just lucky...

That seems logical. But surely the fuel is already vapourised as it passes through the inlet port ? I have a suspicion , devoid of the slightest trace of evidence, that it is related to the cycle itself, the positive pressure (in the crankcase) and then the negative pressure. I am trying to remember if my notorious experiments with increasing BSA Bantam crankcase compression (the ordeals those poor bikes suffered !), were accompanied by higher seizure rates.

[GSVR]

Even those with injection pumps often just inject the oil into the inlet ports, so the result is the same as mixing it with the fuel, just simpler.

This is my understanding

The less oil you put in two stroke premix the better combustion/performance you will get but the greater the chance of seizure.

The more oil you put in the more smoke and plug fouling.

The good thing about oil injection is that rather than having a set ratio of say 30 to 1 it varies depending on the engine revs and throttle.

Also the bike runs really great just before the back whell locking up at high speed.

[Jackrat]

Maybe Suzuki's were less prone to this but none of mine ever locked up.
This was most of the T series and a number of the GT's.
I did pop a neat hole in the piston of an A50 that I rode from AK to Hamilton,but I think that had more to do with long term abuse than anything else.
I really loved them old two strokes

[Ixion]

Maybe Suzuki's were less prone to this but none of mine ever locked up.
This was most of the T series and a number of the GT's.
I did pop a neat hole in the piston of an A50 that I rode from AK to Hamilton,but I think that had more to do with long term abuse than anything else.
I really loved them old two strokes

I've never locked the T500. But I think that is because Suzukis of that era didn't just inject the fuel into the inlet port, they actually piped it from the pump direct to the bearings, (and I think a jet onto the cylinder wall, not sure about that). So the bearings had oil come what may, just like a four stroke.

I always thought that was a really good lube system for a two smoker, was disappointed when they dropped it. Cost I suppose.

[Motu]

The beauty of the oil pump is that it is supplying some oil to the engine even with the throttle closed,and at a rate dependant on engine speed.I've had the pump cable break with no engine siezure - mind you that was on a trials bike so no test really.

The fuel is atomised at the carb,and vapourised when it gets to heat/vacuum - the pressure variations in a 2 stroke crankcase must play havoc with the fuel state,something I hadn't thought about before - isn't this internet thing a great learning tool?

About fuel/oil ratio,we all know more oil means less fuel and lean mixture,but more smoke,potential siezure,but with more oil for protection...more paradoxes.But some years ago I read a 2 stroke tuning book and whoever it was didn't like low oil ratios which were just coming in then with synthetics....we used to run 50:1 no worries.He was from the old school of Yamaha racing and swore by 16:1 as the only way to go.So he did a dyno test on an engine with different ratios - and he proved his point - he started at 50:1 and worked his way down to 16:1,with power increases all the way.He got the most power out of his engine on 16:1 - but if I premix I go 40 or 50:1 depending on the bike.

[geoffm]

Maybe it's just because on long downhills the throttle is only open slightly and the oil pump,which has it's metering controlled by the throttle,is not pumping the amount of oil that the engine speed requires.This would apply to premix as well.
Also having too much oil is a seizure risk because it leans out the total fuel/air mixture,thus making the engine run hot.

The oil pumps (or at least anything made in the last 30 years) is motor driven, the oil metering is a function of engine RPM and thrrottle opening. As either one increases, so does the oil flow.
I have had more problems with my 4 strokes than any of the 2 strokes. Run good synthetic oil and they can take a mighy thrashing.
Geoff