Saw Tuning

Nicholas · Nov 9, 2005

Was is the best way to tune a saw?

Is it better to tune the saw by ear, listening for proper idle, rapid acceleration, and a slight 4 cycle @w.o.t. no load?

Is is better to use a tachometer, to set idle rpm, and w.o.t. no load rpm according to manufacturer specs?

Is is better to tune a saw by listening for proper idle, rapid acceleration, and tuning maximum power under load or in a cut?

I tune my saws by:

1.) Warming the saw up by cutting a few pieces of wood.
2.) Listen and tune for rapid acceration.
3.) Listen and tune for proper idle.
4.) Cut more wood to locate power band.
5.) Tune carburator in such a way that the peak of the power band is at w.o.t.
6.) Verify idle setting and acceleration.

I do own a tachometer, however, I find I have to tune for a richer setting after using tach. I often find the power band is not at w.o.t. when using a tach.

Anyways, I am just looking for more input on saw tuning. I checked out Madsens section on saw tuning, and it helped but not all that much.

Thanks,

Nick Gajewski

Lakeside53 · Nov 9, 2005

I think you have it all figured out! All three work. If you're trying for the best possible performance, you're doing the right things.

A Tach is very useful to make sure you haven't exceeded the Max allowable RPM or, if you do, by how much. I always use a tach to set customers saws after repair, but "tweak" a lot for the Pro customers.

For constant load devices, like a blower, the tach is the only way to get it set as the "sound" tells you nothing until it's either too rich or too lean.

bouden · Nov 10, 2005

And always check to make sure the throttle linkage is opening the throttle plate fully, prior to tuning. There's a severe danger of lean adjustment trying to come up to rated rpms without WOT. No power, but no fuel mix either.

J_Ashley · Nov 10, 2005

Quick question. Why should it be 4-stroking at WOT no load?

Justin

eric_271 · Nov 11, 2005

J_Ashley said:
Quick question. Why should it be 4-stroking at WOT no load?

Justin

Justin the 4 stroking sound tells you that your not to lean on your H needle. That should be in the saws power band when in the wood or with in a few hundred rpm.

J_Ashley · Nov 11, 2005

I always tuned the H jet to get the peak RPMs, and then richen the mixture some to drop the RPMs down by a few hundred. Am I doing this wrong?

Justin

Lakeside53 · Nov 11, 2005

yes, it's wrong; that's how you set a blower, not a saw, because the blower is running at full HP. If I do that on say a 200T or even an 026, It will be running 2000 rpm too high.... which means VERY lean and lower power. To use you technique, you'd need to be buried in the cut or on a dyno.

eric_271 · Nov 11, 2005

J_Ashley said:
I always tuned the H jet to get the peak RPMs, and then richen the mixture some to drop the RPMs down by a few hundred. Am I doing this wrong?

Justin

Way wrong. From where its at now richen it till you hear the 4 stroke tone. It takes some getting used to but after you learn the sound it gets real fast and easy to know if your carb is set right or not. If your that far in with the H needle you may have to adjust the L also.

J_Ashley · Nov 12, 2005

Ok here's what I don't understand, so bare with me please. When testing a string trimmer, you let the string out till it hits the razor and run the trimmer free-standing (not cutitng anything). With the trimmer at WOT, it revs until it runs out of steam and can't spin the string/head any faster due to the aero-dynamic drag induced by the string. Once the string wears down from use, it will rev higher, to the point it hits the rev-limiter and starts missing.

On a chainsaw, with the bar/chain mounted, running the saw free standing (not cutting wood) should have a similar effect right? At WOT, the saw should rev until it can't spin the chain any faster, correct?

I don't understand when you saw that the blower is running at full power, meaning that the chainsaw is not? Yes, a blower is always under full LOAD. Although the chainsaw is not under full load, even when it is reved at WOT when not cutting, it's still making full power. There is no governer to close the throttle with these. So WOT means full power.

I must be missing something???

Justin

16gauge · Nov 12, 2005

J_Ashley said:
I don't understand when you saw that the blower is running at full power, meaning that the chainsaw is not? Yes, a blower is always under full LOAD. Although the chainsaw is not under full load, even when it is reved at WOT when not cutting, it's still making full power. There is no governer to close the throttle with these. So WOT means full power.

I must be missing something???

Justin

Justin, Listen to these guys when it comes to chainsaws...they know of which they speak. Or write...whatever.

J_Ashley · Nov 12, 2005

Ok i believe them, but I'm not completely understanding of something.

When any engine is held at WOT throttle, it will rev freely and produce maximum power until one of three things happens. 1- A governer closes the throttle, reducing power production and therefore keeping RPM's at a manageable level. 2- An electronic rev-limiter skips ignition fires in order to keep the RPM's at a manageable level. 3- A load is applied to the motor, which will therefore keep the RPMs at a manageable level.

Justin

Lakeside53 · Nov 12, 2005

The saw does not develop "maximum power" when held open at WOT. It just develops maximum RPM. Quite different, Max power is many 1000's of rpm less, and if you "tune" for max rpm, then you will NEVER develop "max power" as your mixture will be too lean. It takes almost no "power" to drive an engine to max rpm.

case 1: a governor, if the saw has one and most do not, only limits RPM, not Power. You can have badly tuned carb, like way too lean, and the governor will limit, but you'll never be able to pull full power.

case 2: as case 1

case 3: sure, load up the motor and the rpm will drop, but this has nothing to do with power developed.

Other can give you a much more succinct and scientific explanation on the relationship between RPM, torque and power.

Lakeside53 · Nov 12, 2005

J_Ashley said:
Ok here's what I don't understand, so bare with me please. When testing a string trimmer, you let the string out till it hits the razor and run the trimmer free-standing (not cutting anything). With the trimmer at WOT, it revs until it runs out of steam and can't spin the string/head any faster due to the aero-dynamic drag induced by the string. Once the string wears down from use, it will rev higher, to the point it hits the rev-limiter and starts missing.

On a chainsaw, with the bar/chain mounted, running the saw free standing (not cutting wood) should have a similar effect right? At WOT, the saw should rev until it can't spin the chain any faster, correct?

I don't understand when you saw that the blower is running at full power, meaning that the chainsaw is not? Yes, a blower is always under full LOAD. Although the chainsaw is not under full load, even when it is reved at WOT when not cutting, it's still making full power. There is no governer to close the throttle with these. So WOT means full power.

I must be missing something???

Justin

yes, you're missing something : WOT does NOT mean full power... RPM is not Power. Power is only developed by demand of a load.

You trimmer is like a blower and runs at full power with the right length string. As the string gets shorter, the engine revs higher as the load is less and it doesn't take as much power to run the string (more string, more power, within limits) and the rpms can rise. At NO string, it's like reving without a chain in the wood - the only power required is to overcome the frictional forces of the engine and chain, and that's very little.

J_Ashley · Nov 12, 2005

Lakeside, I understand (partially) what you're saying. I understand that power (work) is a demand of load etc.

I guess what I really need is someone to sit down and show me how to do this correctly. I appreciate the advice from you all, but it's not the same as someone giving you the saw and a screw-driver.

Thanks for the patience and advice guys. I don't know if I've said it before, but I'm fairly new with the whole 2-Stroke game. I've got only several months experience with them, and I'm continuing to learn more everyday. Thanks for bearing with me.

Justin

Lakeside53 · Nov 12, 2005

Assuming you saw does not have limiter caps, I'd strongly suggest that you set your carb for "standard" settings, which is most likely 1 turn out from gently seating thr H+L screws. Leave it there until you can learn a little more. This way the saw will start and run pretty well, and you won't be too lean. This is how they leave the factory.

J_Ashley · Nov 12, 2005

Most everytime i adjust a carb, I screw them in all the way, and back them out to the recomended setting. Sometimes I'll tweak them a little afterwords if it requires, but I almost always try to put more fuel in...

Justin

buck futter · Nov 20, 2005

If you are at the manufacturers rec'd setting, run it there for a while and pull the plug out. you will understand how it runs and sounds and the color is fairly staight forward, white, tan, black/sooty. Lets say you put a couple tanks through and the plug is black, maybe give th H screw a tweak in and repeat. maybe this isn't the best method but it gives the novice more information to make tuning decisions with.

Lucky

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