Exhause port scratches

[Chickaloon]

I saw a post recently that claimed their piston scratched and I believe seized due to too much carbon build-up on the exhaust port.
This due to an oil rich mixture below 40:1.

Can anyone back this up ?

My first impression would be that the carbon doesn't readily build up to something that damaging....?

 

[timberwolf]

Another oil thread, good :censored: luck to ya...

BS, run saws all the time at 40:1 or more oil, no problems. Something else is wrong poor carb adjustment, not enough oil, bad fuel, old fuel or lots of ethanol with oil dropping out of mix, air leaks, saw run too hard, cooling fins plugged up, plugged muffler screen...

 

[Mad Professor]

I saw a post recently that claimed their piston scratched and I believe seized due to too much carbon build-up on the exhaust port.
This due to an oil rich mixture below 40:1.

Can anyone back this up ?

My first impression would be that the carbon doesn't readily build up to something that damaging....?

See page two of the attachment, Jonserd factory seems to think it happens. The picture speaks for itself.

 

[ray benson]

That Jonsered document says to mix 33/1 or 25/1 if not Jonsered oil. Just reread Timerwolf's post.

 

[timberwolf]

Good link, they note possible causes as wrong oil and wrong carb adjustment as 2 of the 3 possible reasons for the problem. I'd add a fourth, being bad fuel, old fuel that is not burring well is also likely to contribute to build up.

Just experience I have not seen excessive buildup running a properly tuned saw on lots of oil, even milling at 32:1 no trouble. But combine lots of the wrong type of oil at the wrong air fuel mixture and then it will carbon up.

Too much oil on it's own is not the problem in my experience, that why I say there is something else wrong as well.

 

[Mad Professor]

That Jonsered document says to mix 33/1 or 25/1 if not Jonsered oil. Just reread Timerwolf's post.

So what causes this "carbon build up"?

Too rich oil/fuel or fuel/air. Either way too much unburnt oil that carbonizes and builds up.

 

[timberwolf]

Very rich oil mixes 20:1 often result in lower carbon build up on the piston. There are several documented experiments on this topic. If there is enough oil to keep surfaces wetted and quenched with oil the oil tends to clean rather than deposit and bake on. Just like a well oil frying pan resists things sticking to it.

On a well adjusted saw the worst carbon buildup I have seen is running at 50:1 with factory reccomended mix. Ironically 32:1 left the piston crown significantly cleaner.

 

[Mad Professor]

Good link, they note possible causes as wrong oil and wrong carb adjustment as 2 of the 3 possible reasons for the problem. I'd add a fourth, being bad fuel, old fuel that is not burring well is also likely to contribute to build up.

Just experience I have not seen excessive buildup running a properly tuned saw on lots of oil, even milling at 32:1 no trouble. But combine lots of the wrong type of oil at the wrong air fuel mixture and then it will carbon up.

Too much oil on it's own is not the problem in my experience, that why I say there is something else wrong as well.

O.K. I'll agree there is an interplay between mix ratio and oil ratio. If you carb is set for 50:1 then you mix 32:1, without readjusting the carb, you'll be too rich and carbon will result.

 

[timberwolf]

That is not what I was talking about.

properly tuned at 32:1 and properly tuned at 50:1

I have observed 32:1 running cleaner, by that i mean pulling a saw down after 10 or 20 cords of wood and still being able to see aluminium in places on the piston crown vs a fully black piston crown.

 

[brages]

O.K. I'll agree there is an interplay between mix ratio and oil ratio. If you carb is set for 50:1 then you mix 32:1, without readjusting the carb, you'll be too rich and carbon will result.

Woundn't you be too lean, rather than too rich? I dunno...

 

[Mad Professor]

That is not what I was talking about.

properly tuned at 32:1 and properly tuned at 50:1

I have observed 32:1 running cleaner, by that i mean pulling a saw down after 10 or 20 cords of wood and still being able to see aluminium in places on the piston crown vs a fully black piston crown.

That's what I'm talking about. People think more oil is better, their saw is tuned to run at 50:1, they figure MORE oil is better and mix at 32: 1 and get carbon buildup.

Don't tell me the carb settings will be the same, with the same fuel and oil, mixed at 50:1 vs 32:1

 

[scotclayshooter]

O.K. I'll agree there is an interplay between mix ratio and oil ratio. If you carb is set for 50:1 then you mix 32:1, without readjusting the carb, you'll be too rich and carbon will result.

Shoud that not be leaner as theres less petrol and more oil?

 

[Mad Professor]

Shoud that not be leaner as theres less petrol and more oil?

The air/fuel mixture is THE SAME, but it has more oil.

Yes, when mixed the oil is part of the fuel, and the fuel has more oil............

As my late father said when arguing with my mother: "I'm done banging my head against/talking to the wall, I'm going to the bar/pub..."

 

[Lakeside53]

Don't tell me the carb settings will be the same, with the same fuel and oil, mixed at 50:1 vs 32:1

In theory you are right. In practice, little meaniful adjustement will be made... Very few tune the carb to absolute perfection.. and if they did, it's only valid for a very short time.


"Scratched" pistons - Blocked exhaust screens and blocked up airfilter - not "a days cutting", but the years of packed fine black crap that won't let light go though. Guys that use a lot of partial power - I see it on weedeaters and blowers - run it hard now and then...

 

[timberwolf]

Here are some calcs and a graph.

All based off the factory reccomended 50:1 oil and the approximate ideal air fuel ratio of a two stroke engine at full throttle normal load.

AF ratios change much more rapidly with oil mixtures richer than ~35:1. Very little difference between 50:1 and 40:1. Like lakeside mentioned real world this is hardly precievable, however if a saw is tuned and tached at 50:1 and some 40:1 or 32:1 is dumped in it will gain enough RPM to show up on the tach.

 

[scotclayshooter]

Here are some calcs and a graph.

All based off the factory reccomended 50:1 oil and the approximate ideal air fuel ratio of a two stroke engine at full throttle normal load.

AF ratios change much more rapidly with oil mixtures richer than ~35:1. Very little difference between 50:1 and 40:1. Like lakeside mentioned real world this is hardly precievable, however if a saw is tuned and tached at 50:1 and some 40:1 or 32:1 is dumped in it will gain enough RPM to show up on the tach.

I was confused for a few seconds thinking 50:1 was 2% but i forgot we were talking about 5050ML total.

I put exactly 5L petrol in my cans but the oil could be out by a little and i doubt most people measure accurately enough to say its a 50:1 or whatever ratio.

 

[Chickaloon]

oil mix

Wait a minute...did I read that right ?

More air in the richer mixed fuel...means it runs leaner....means more RPMs !!!!!!!!!! ?????????????????????