Fuel mix ratio and carb tuning

[GCJenks204]

M-Tronic FTW!!!

 

[Keith Gandy]

In doing some reading, and thinking outloud, I would just like some feedback on this. Not sure if anyone else ever wondered about the same thing.

Does one need to retune a carb based on fuel mix ratio? For example, If I had a carb tuned for 40:1, then I decided to try 32:1, do I have to retune the carb for the new mix ratio? Or will it run ok based on the previous tune?

Yes it will be lean

 

[RIDE-RED 350r]

I agree with everything said here thus far. I would also add that if you are one who keeps even a stock saw tuned to the ragged edge, you may need to re-tune with a ratio change.

Smokey brings up a very good point about long cuts and heat soak. Exhaust temp is directly related to whether the piston survives or not. The piston will start to melt on the exhaust side of the edge of the crown first at a given temp. What may seem like a good tune initially after a quick warm-up and a couple quick cuts can be danger zone when noodling 36" hard maple.

It really boils down to paying attention to your saw as you work it and it comes fully up to temp and making an adjustment when needed. Just last week I had to tweek my ported 357 after making a few cuts due to the drastically different weather conditions from the last time I ran it before.

 

[RI Chevy]

Thanks guys. I usually most always tune on the heavy side anyway.
And I most always check from tank to tank as well. Sometimes on bigger trees, cut to cut. Just wanted to reaffirm that I was on the right track.
I usually run a fatter mix as well. Probably overkill, as I had a Stihl 029 for over 20 years that was given heavy usage and piston and cylinder looked new when I sold it a few months back. [emoji106]

 

[huskihl]

Thanks guys. I usually most always tune on the heavy side anyway.
And I most always check from tank to tank as well. Sometimes on bigger trees, cut to cut. Just wanted to reaffirm that I was on the right track.
I usually run a fatter mix as well. Probably overkill, as I had a Stihl 029 for over 20 years that was given heavy usage and piston and cylinder looked new when I sold it a few months back. [emoji106]

If we checked our saws wfo for a second every 5 minutes of cut time listening for a 4 stroke we'd never burn up a 2 cycle engine. If it has oil and it's 4 stroking, I don't think they'd ever burn up

 

[RI Chevy]

That is my motto. If its fat, even too fat, it will never blow. Lol

 

[Greenerpastures]

Nice topic,
I think sometimes I need to hide my orange screwdriver and just run it.
My normal tune is a touch fat usually.
I have wondered the same thing about mix ratio. The idea being if I run more and better oil can I lean a saw out more and keep it protected?


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No, the more oil in the mix, the less fuel, so you are already running lean,
and to further lean it will cook it.
Adding more oil to the mix to lubricate the saw is not as good as running
the saw on 50:1 and have her rich, I read somewhere that this was tested
and saws wear less and run cooler on a slightly rich mixture as opposed
to saws that ran with more oil in the mix.

If you study it, heat is the real killer in a saw engine, and do all you can to
keep it cool especially under heavy loads, sharp chain, take a little more time
and plenty of fuel to run her cool.

A saw tuned for high altitude will also run too lean when brought to a lower
altitude, and this alone will cook your saw too.
Its about knowing what needs doing for a given situation.

 

[Greenerpastures]

If we checked our saws wfo for a second every 5 minutes of cut time listening for a 4 stroke we'd never burn up a 2 cycle engine. If it has oil and it's 4 stroking, I don't think they'd ever burn up

I tested 46:1 and 50:1 in the same Echo CS -501, and the 46:1 stopped her fourstroking.
Some saws are pernikity, the saw did not heat up because of this, as she is a little twister
and is hard to get to fourstroke, she will almost stop it at 10,000 rpm, unless the tune is
spot on on the L and H, thats why even going from 50:1 to 46:1 stopped her fourstroking.
I used the same fuel and oil other than the ratio being different.
I can now get her to fourstroke up to 11,000, where I stop, pernikity wee thing.

 

[RI Chevy]

I would think 40:1 would always run cooler than 50:1 due to more oil. Taking that one step further, I would think 32:1 would run a bit cooler than 40:1. Oil doesn't burn as hot as fuel, thus keeping engine a bit cooler.
Am I wrong in my theory?

 

[huskihl]

No, the more oil in the mix, the less fuel, so you are already running lean,
and to further lean it will cook it.
Adding more oil to the mix to lubricate the saw is not as good as running
the saw on 50:1 and have her rich, I read somewner that this was tested
and saws wear less and run cooler on a slightly rich mixture as opposed
to saws that ran with more oil in the mix.

If you study it, heat is the real killer in a saw engine, and do all you can to
keep it cool especially under heavy loads, sharp chain, take a little more time
and plenty of fuel to run her cool.

A saw tuned for high altitude will also run too lean when brought to a lower
altitude, and this alone will cook your saw too.
Its about knowing what needs doing for a given situation.

If you were tuned properly, and did not retune after switching mix ratios, your statement may be correct. But once retuned, the theory of 50:1 being richer than 40:1 goes out the window.
At 32:1, and tuned properly, everything inside the saw has more lubrication and will last longer. Ive seen 2 year old strato saws with bad bearings already from being run on 50:1. Never again

 

[huskihl]

I would think 40:1 would always run cooler than 50:1 due to more oil. Taking that one step further, I would think 32:1 would run a bit cooler than 40:1. Oil doesn't burn as hot as fuel, thus keeping engine a bit cooler.
Am I wrong in my theory?

I don't know about which burns hotter, but I agree with your statement as long as your screwdriver has compensated for less fuel getting to the engine

 

[Dub11]

That is my motto. If its fat, even too fat, it will never blow. Lol

Spark plugs are cheap!

 

[RI Chevy]

Spark plugs are cheap!

Lol. I buy them by the dozen.

 

[huskyboy]

4 stroke out of cut, 2 stroke in cut.

 

[Greenerpastures]

If you were tuned properly, and did not retune after switching mix ratios, your statement may be correct. But once retuned, the theory of 50:1 being richer than 40:1 goes out the window.
At 32:1, and tuned properly, everything inside the saw has more lubrication and will last longer. Ive seen 2 year old strato saws with bad bearings already from being run on 50:1. Never again

Yes, the only thing changed was the ratio of oil in the mix, no retuning took place,
I discovered the pernickity nature of the saw when I took out the plug after it came from
the Echo dealer, it was too lean, and I tried to get it fourstroking.
After it was set up forustroking on 50:1, I tried it on 46:1, and no fourstroking.

I am now using 50:1 in all my saws, except the 7900, which is eqally happy with a bit more oil
in the mix, it seem to handle oil better, but I only use the 46:1 in her.
She too was lean when I got her, I commented on here that I thought there was just a little
too much heat being generated for my liking, and so testing and tuning began, and I can safely
see, that it was the rich tune that cooled her down, not more oil, I had her on 37:1, she was hot,
had her on 40:1 and she was cooler, then 46:1 and coller again, this was all with the same tune,
so she was getting richer as she progressed.
I tuned her at 46:1 and was pleased I had only a weep of oil at strart up, which when wiped off,
did not appear again after saw was hot, I then put 50: in her, and did not see the need to tune her,
she is running with the flocked filter, and will lean hersef when I change that over, so depending on
conditions, I now have a good idea of what to do as regards keeping her in tune.

I don't think lubrication is like in a four stroke, the lube in a two stroke is in the fuel,
it is also only there for a fraction in time as it is replaced on every stroke, so if it is too
thick it will not have time to reach the places it has to, a 362 for esample has a very small
clearance between the piston and cylinder, diluted oil stands a much better
chance at getting in there, this is also the reason I let a saw warm up before reving them.
It would take longer for thicker oil to get into all the crevices, and time for thicker oil to get
to where it is needed is something a fast reving two strok does not have enough of.

Its a balance, some engines have more clearances, they will be better with more oil,
but very small clearances require diluted oil, the tolerances are calculated to allow
sufficient oil at a certain concentration to get in and do its job.

I tested my own saws, and know how they behave as regard to different mixtures,
Sindaiwa 490, does not seem to care, between 40:1 and 50:1 and she stays clean and runs real cool, good engine design,
Echo 501, 40:1 to 50:1 and she stays cool, I blame good engine design for this, timing etc,
Makita 7900, cooler on 46:1 to 50:1, even without re tuning, will cut some on both mixes,
monitor temprature, and decide then, larger diamiter piston here seems to be more effected
by oil content, larger area retains more heat, so not planning on heating it.

So many variables, but for me, more oil has consistenty shown more heat in the saw,
you can tune out a good bit of it though and where clearances allow for it, the heavier
oil mixes will get to where they are needed.
Its a balance, that has to be tailored to the saws design.

 

[Greenerpastures]

I don't know about which burns hotter, but I agree with your statement as long as your screwdriver has compensated for less fuel getting to the engine

Oil has more calorific value and does burn hotter.
We all agree that if we don't put enough fuel in the engine
it will also run hotter, now combine the two, for that is in fact
what we do when we up the oil content.
Some engines are good with this, some don't take it well at all.

 

[RI Chevy]

We may have to agree to disagree on your post content this time.
Send the saws my way, I'll gett'em 4 stroking for you on 32:1 and 36:1. [emoji106] [emoji2]

 

[Greenerpastures]

We may have to agree to disagree on your post content this time.
Send the saws my way, I'll gett'em 4 stroking for you on 32:1 and 36:1. [emoji106] [emoji2]

Hi RI Chevy
No problem, am learning as I go allong, I test away on my own,
no mentor here to point me in the right direction, that Echo 501
is a great saw, and is the only one that seems difficult to get four
stroking.
But am getting a thermal temprature gun, and when I get the hang of
it, I will try tuning with it and double check with the Tach, am sure a
cool running engine will be much more bennefit to me than listening
for fourstroking which may tell a different tale on every engine, the
temperature however will paint another picture.

Regards, john

 

[RI Chevy]

I hear you John. No worries. That's what makes this site so good. Lots of different theories.
Most importantly, find out what works for you and keep doing it. [emoji106]

 

[huskihl]

Oil has more calorific value and does burn hotter.
We all agree that if we don't put enough fuel in the engine
it will also run hotter, now combine the two, for that is in fact
what we do when we up the oil content.
Some engines are good with this, some don't take it well at all.

Until you open the H screw to compensate.....