Cutting with saw using less than full throttle

Mingara · Oct 10, 2005

Hello all

I have a Dolmar 120si and I have read on occasion in these threads that you should cut with full throttle at all times. What is likely to happen if you cut logs on the ground without full throttle? I have a pretty powerful and loud saw so I normally don't apply full throttle while cutting, particularly when the timber is only about 5 or 6 inches in diameter. I've had no problem so far so am I doing the wrong thing?

MacDaddy · Oct 10, 2005

nothing wrong with that

Stumper · Oct 10, 2005

Ah, Dolmar 120si, my favorite saw. If you are afraid to wind her up I'll swap you for a lighter saw.

Seriously, part throttle does no harm so long as the clutch is fully engaged/not slipping under the load. -Most operators feather the throttle when cutting the last bit on stuff resting on the ground or light pieces that want to go flying when a fast chain makes contact. Generally speaking-"rev 'er up" is the way to go

Diesel JD · Oct 10, 2005

I cut that way all the time....but it may be another bad habit. I think it's a safety thing to do with kickback reduction...not schooled enough in saws or physics to know why...?

jokers · Oct 10, 2005

Well actually depending on what carb your saw has, there could be harm to your engine running for extended periods at partial throttle. Many, if not most diaphragm carbs have no mid range fuel circuit. They have a lo speed and a high speed circuit that begins to supply fuel at a higher vacuum. The area between these two ranges is not precisely metered.

It`s my belief that this is why so many carvers and log home builders who "sweep" with their saws have such a high failure rate.

On the other hand, bumping knots with the throttle pinned is also bad.

Russ

jokers · Oct 10, 2005

Diesel JD said:
I cut that way all the time....but it may be another bad habit. I think it's a safety thing to do with kickback reduction...not schooled enough in saws or physics to know why...?

You have a greater risk of kickback at partial throttle although the energy will be less.

Russ

Mange · Oct 10, 2005

I agree with both Russ's posts.

The saws are designed to run differently depending on what saw you have.
Not one is designed to run at partial throttle, as far as I know.
If you don't use the saw for what it is meant to, I think it can do harm.
The oilers give less oil at 5000 rpm than 11000 rpm.
The biggest reason for the high chainspeeds today is of course cutting faster, but it is safer to run the chain faster and load the saw less instead.
If the engin run on full rpm and is not under load during long periods, is as bad.
To get as much life and work out of the saw as possible think it is best run the way it was desinged to.

bugfart · Oct 10, 2005

I love it when he talks about carburetors.
Any carburetor talk is good carburetor talk. Gotta learn more about those single adjusts.

Just for clarification, I have two single adjust carbs and a nice set of driil bits (In this situation drool bits) I suspect these carbs a potential trap for muffler mods.

Lakeside53 · Oct 10, 2005

bugfart said:
I love it when he talks about carburetors.
Any carburetor talk is good carburetor talk. Gotta learn more about those single adjusts.

Just for clarification, I have two single adjust carbs and a nice set of driil bits (In this situation drool bits) I suspect these carbs a potential trap for muffler mods.

Generally not.... If you want to try changing the jets in your carb, you can buy bigger or smaller jets and outlet valves for just a few $ in .01mm for many carbs, but the overall balance of the carb, the venturi size and other factors will limit you quickly. If you have a stihl, there is a good tech note on changing the jets for diffferent altitudes. It's about "quantity of the correct mixture", not just more or less gas. I have seem some very good threads about this on AS in the past... My opinion, just dump the single adjust screws and get a two screw carb, preferably one with partial power jet. I have 10 brand new two screw 026 carbs just for this perpose

Stihl has gone back to the two screw carbs on their pro saws. These are "semi-fixed jet" and not quite the same as the old carbs, but at least you get to adjust the H within a reasonable range. A lot of 20T carbs get sold for MS200t's that had the single screw.

Chris J. · Oct 10, 2005

I replaced the carb on my 025 with a FAC. I read a thread here on adding a jet, and knew that I didn't have the mechtech skills or experience for it. Now maybe if I had a couple hundred carbs that I didn't mind ruining...lol.

rbtree · Oct 10, 2005

Mange said:
I agree with both Russ's posts.

The biggest reason for the high chainspeeds today is of course cutting faster, but it is safer to run the chain faster and load the saw less instead.
If the engin run on full rpm and is not under load during long periods, is as bad.

Mange, I'm not sure if I'm understanding your meaning in the first sentence, but I agree with the second.

I always instruct my workers, not to allow the saw to run at full rpm, but to dog it into the wood. When this is hard to achieve, as in limbing with a big saw, I want the throttle feathered. Right or wrong for the carb, the saw will last longer.

pinus · Oct 10, 2005

Mange said:
I agree with both Russ's posts.

The saws are designed to run differently depending on what saw you have.
Not one is designed to run at partial throttle, as far as I know.
If you don't use the saw for what it is meant to, I think it can do harm.
The oilers give less oil at 5000 rpm than 11000 rpm.
The biggest reason for the high chainspeeds today is of course cutting faster, but it is safer to run the chain faster and load the saw less instead.
If the engin run on full rpm and is not under load during long periods, is as bad.
To get as much life and work out of the saw as possible think it is best run the way it was desinged to.

That is only opinion of

My opinion is that if you keep rpms at max torque rpms or higher it is not "crime" to run somewhat less than at full throtle

There is another thing that can happen, if from time to time not to "clean" chain runing saw few seconds at max rpms. That is jamming of bar and chain with dust, especially if to cut some trees like dead aspen, which wood is very fibrous
v.

Mingara · Oct 13, 2005

Gentelmen

thanks for your input and I believe that in the future, I will be using my saw at full power when I am cutting a decent size piece of timber. I think that using it at full throttle for small diameter logs and while I am cutting small amounts is an over kill, with appears to be supported in this thread. So I will conntinue running the saw mid range and above for these occasions. However for the bigger stuff I will be using it at full throttle as suggested also.

I don't appear to be having a problem with the bar oil amount whatever range I use.

I am always aware of kickback incidents so I take your point about kick back being more likely with half throttle regardless of whether it is less severe. I believe I will take even more care when cutting small timber.

thanks to all for the advice.

Jim

SawTroll · Oct 13, 2005

pinus said:
My opinion is that if you keep rpms at max torque rpms or higher it is not "crime" to run somewhat less than at full throtle

Maybe not, but max torque isn't happening unless you run at full trottle!

neverenough · Oct 13, 2005

Mingara said:
Hello all

I have a Dolmar 120si and I have read on occasion in these threads that you should cut with full throttle at all times. What is likely to happen if you cut logs on the ground without full throttle? I have a pretty powerful and loud saw so I normally don't apply full throttle while cutting, particularly when the timber is only about 5 or 6 inches in diameter. I've had no problem so far so am I doing the wrong thing?

This is why I have limbing saw(s) too. And also why I have multiple bar sizes for my saws. I run at a minimum of 3/4 throttle, but most of the time I run WOT. I run my limbing saw for, up to 5-8 inch cuts, depending on the wood, and I run the bigger saw for the larger cuts.

As far as the "doing anything wrong" part, run the saw at the lower speed for a little bit of cutting, then while the saw is at that speed, turn it off. Check the color of the spark plug and see if the electrode is white, tan, brown, or black. If it's white or light tan, your close to saw meltdown, if it's tan or brown, you should be ok, if it's black, it's running rich, and you might, depending on the carb, be able to lean it out in that operating range. With most saw carbs, it's not likely. Checking piston wash sometimes helps too, but it might not be a good indicator on a saw piston. I'm not sure on that, does anyone check piston wash on saw pistons???

pinus · Oct 13, 2005

SawTroll said:
Maybe not, but max torque isn't happening unless you run at full trottle!

You are right here, but I dont need the sawing at max torque all the time.

I should say that reading last days these threads about throtling :dizzy:

I´m now in the forest wery nervous and stressed. All the time something is bubling under the hat, is the mix normal or lean :blob2:

jokers · Oct 13, 2005

Mingara said:
I think that using it at full throttle for small diameter logs and while I am cutting small amounts is an over kill, with appears to be supported in this thread. So I will conntinue running the saw mid range and above for these occasions.
Jim

Supported by who? So far as I can tell, only a couple of guys who can`t explain how it is justifiable to run the engine with an improperly metered fuel/air ratio.

To each their own. No it`s not a crime but it`s not smart either.

Russ

pinus · Oct 13, 2005

jokers said:
Supported by who? So far as I can tell, only a couple of guys who can`t explain how it is justifiable to run the engine with an improperly metered fuel/air ratio.

To each their own. No it`s not a crime but it`s not smart either.

Russ

Russ, .....

what is the fuel/air ratio at full throtle and at half throtle?
Give us a chanche

jokers · Oct 13, 2005

pinus said:
Russ, .....

what is the fuel/air ratio at full throtle and at half throtle?
Give us a chanche

The stoichiometric ratio should be roughly 15:1. Can you say with confidence that you are over coming the anti-bleedback valve in the high speed circuit with the engine vacuum produced at half throttle?

No? Didn`t think so. You are however free to do as you`d like with your own saw, even in Estonia. :laugh:

Russ

pinus · Oct 13, 2005

jokers said:
The stoichiometric ratio should be roughly 15:1. Can you say with confidence that you are over coming the anti-bleedback valve in the high speed circuit with the engine vacuum produced at half throttle?

No? Didn`t think so. You are however free to do as you`d like with your own saw, even in Estonia.

Russ

:Eye:
Give the values

Dont forget, that bore is not max too at half throtle, so air speed and vacuum can be the same

Cutting with saw using less than full throttle

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