Fuel mix ratio and carb tuning

[bwalker]

The differance between 32 and 40:1 is 6/10ths of a percent.

 

[bwalker]

I use a tachometer during every carb retune so I can be both accurate AND consistent. I don't leave for the woods without it.

Once i find qhere a saws sweat spot is i record the no load RPM and tune with a tach
to that number going forward.
Keep in mind that as load changes so does fuel requirements. So going from a 16 to a 24" bar would require more fuel as would going from skip to full comp chain or going from cutting dried oak to pine.
All of these factors are why autotune and Mtroni c are good things in the long term. Most users are too lazy or lack the ability to properly set a carb so that the engine runs right all the time.

 

[PogoInTheWoods]

Most users are too lazy or lack the ability to properly set a carb so that the engine runs right all the time.

And why most folks tune a little rich or maybe run a heavier mix ratio in order to run safer all the time.

I think this discussion has also co-mingled the term 'lean' a bit loosely to conveniently fit individual assertions relating to both the gas/oil ratio and the fuel/air ratio. Two different applications of the term having different (some would argue opposite) effects on a saw engine. A fatter fuel/oil ratio is not going to burn up a saw due to less fuel in the mix creating a 'lean' condition relative to the actual fuel/air mix. It just won't run as efficiently due to more oil in the mix. It won't run as hot either. On the other hand, using less oil in the mix but tuning a hair fatter for better performance will create more heat simply due to more efficient combustion by virtue of a leaner mix ratio even though the mix to air ratio was fattened. At least that's how I've come to understand all this. And what typically suffers in the case of the latter (no matter how good your ear is) are the bottom ends of higher performance saws running 50:1 mixes. A lot less room for error and dynamic variables running right at the edge. Then there are the ethanol and octane factors to consider as additional variables on either side of the discussion.

As for using a tach, a modded saw is only going to need one as a reference point relative to the gains realized by the mods. A stock saw can benefit by a tach if the operator isn't on top of tuning by ear and still wants to stay on the safe side of his stock specs. And all saws don't four-stroke as obviously as others, so there is a degree of interpretation involved that is learned over time. A tach certainly helps build ear tuning confidence during that learning curve. Then there are the limited coils that mimic four-stroking right until a saw burns up because it was tuned too lean by someone who didn't realize they were tuning against a limited coil in the first place. There is a place for a tach same as a just a good ear and a little orange screwdriver..., occasionally even at the same time.

On the AT/M-tronic being the silver bullet to all this..., I just pulled a crank from a MS661C-M with a complete meltdown of the big end bearing. Top end still looked fine with the exception of some minor debris damage from the bearing itself. Until the chips can measure bottom end temps and adjust the mix ratio accordingly, I advise anyone running a high performance saw to run 40:1 high quality synthetic oil religiously. The guy who owns this saw certainly will from now on.

But do you actually need to re-tune when changing your mix ratio? Seems like 'it depends' on any number of factors requiring consideration based on one's particular saw, cutting conditions, and level of experience making such subtle adjustments.

Sorry for the ramble. This thread has been a good read and the aspects above just seemed to remain sorta abstract despite everyone's excellent input. Now it all makes sense to me. LOL

 

[bwalker]

And why most folks tune a little rich or maybe run a heavier mix ratio in order to run safer all the time.

I think this discussion has also co-mingled the term 'lean' a bit loosely to conveniently fit individual assertions relating to both the gas/oil ratio and the fuel/air ratio. Two different applications of the term having different (some would argue opposite) effects on a saw engine. A fatter fuel/oil ratio is not going to burn up a saw due to less fuel in the mix creating a 'lean' condition relative to the actual fuel/air mix. It just won't run as efficiently due to more oil in the mix. It won't run as hot either. On the other hand, using less oil in the mix but tuning a hair fatter for better performance will create more heat simply due to more efficient combustion by virtue of a leaner mix ratio even though the mix to air ratio was fattened. At least that's how I've come to understand all this. And what typically suffers in the case of the latter (no matter how good your ear is) are the bottom ends of higher performance saws running 50:1 mixes. A lot less room for error and dynamic variables running right at the edge. Then there are the ethanol and octane factors to consider as additional variables on either side of the discussion.

As for using a tach, a modded saw is only going to need one as a reference point relative to the gains realized by the mods. A stock saw can benefit by a tach if the operator isn't on top of tuning by ear and still wants to stay on the safe side of his stock specs. And all saws don't four-stroke as obviously as others, so there is a degree of interpretation involved that is learned over time. A tach certainly helps build ear tuning confidence during that learning curve. Then there are the limited coils that mimic four-stroking right until a saw burns up because it was tuned too lean by someone who didn't realize they were tuning against a limited coil in the first place. There is a place for a tach same as a just a good ear and a little orange screwdriver..., occasionally even at the same time.

On the AT/M-tronic being the silver bullet to all this..., I just pulled a crank from a MS661C-M with a complete meltdown of the big end bearing. Top end still looked fine with the exception of some minor debris damage from the bearing itself. Until the chips can measure bottom end temps and adjust the mix ratio accordingly, I advise anyone running a high performance saw to run 40:1 high quality synthetic oil religiously. The guy who owns this saw certainly will from now on.

But do you actually need to re-tune when changing your mix ratio? Seems like 'it depends' on any number of factors requiring consideration based on one's particular saw, cutting conditions, and level of experience making such subtle adjustments.

Sorry for the ramble. This thread has been a good read and the aspects above just seemed to remain sorta abstract despite everyone's excellent input. Now it all makes sense to me. LOL

In my experience when using a good oil that doesnt inhibit combustion a richer oil concentrations make more power and thus runs slightly hotter.
The tach is only used as a referance point to enable one to return to a repeatable state consistantly. To find the set point you use the cut and feel method to determine where the saw makes the most torque. Tuning by ear is a crap shoot, especially with muffler modded ported saws. And as you mentioned rev limiters complicate tuning by tach or by ear. You have to be cognizant of what your dealing with as it pertains to rev limited saws. In fact I wouldnt deal with a rev limited saw again because IMO they are a real stupid idea.
Mtronic or autotune cant compensate for poor oil, not enough oil, air leaks etc. However its a huge step forward for the average Joe who cant tune a two cycle to save his life.