tach carb tuning question

[Shade]

I've read all the info I can find on tuning the carb on the top end using a tach, and me being 2-stroke illiterate, I have a question if anyone can help me.

I understand how the air to fuel ratio affects power, and I've seen a few numbers for top RPM to tune to (13000 no bar/chain for an 066, and the manual says 12000rpm with a bar and tensioned chain). I'm confused to how it all relates, and I'm trying to get it all straight.

Leaning the mixture out makes it run faster, how do you insure that you don't lean it out too much because of extra drag? For example, making it run at 13000 rpm with the chain installed would be over reving, and running lean. or for some reason does it all just work out regardless of loads? or are additional loads negligable for tuning purposes?

 

[glens]

Hi Shade.

It seems like if it took so much throttle to get so much speed with no load at all, that it would take more throttle to get the same speed <i>with</i> a load (and the bar/chain itself is certainly some load), so the mixture would have to get changed when it's already WOT.

I agree with your assessment that if 13k is used spinning a chain regardless the bar/chain setup, trouble won't be far away.

I don't use a tach, but rather listen for the burble at wide open no-load throttle which <i>just</i> disappears under load (cutting).&nbsp; If a tach is used without checking it by ear, well, that's flirting with trouble I think.&nbsp; And if the ear is the final say, why not just use it in the first place?

A tach can be valuable for a second opinion and/or for troubleshooting but shouldn't be the primary method in my opinion.&nbsp; I won't be offended in the least if others disagree.

Some ideas are available at <a href="http://www.madsens1.com/sub8.htm">Madsens's maintenance & service menu</a> page.

Glen

 

[stihltech]

tach

The tach is neede to make sure the saw is not overreving after a proper adjustment. Stihl, and I am sure most other manufacturers, publish these top rpm numbers, HOWEVER, length of bar will make a difference as to how close to this number you will be. Application will also affect it. An 066 with a 16 inch bar with be much closer to the top than with a 24 inch bar, but I would be more concered with how lean the saw ran in the cut. A saw on a mill would be much happier with a bit more fuel.

he ear is the best adjusting tool, most of us do not use a tach all that often

Oh god, I can here them typing now.

 

[tony marks]

a goodpiece of advice. assuming u have an shop that u think is competant.ask the tree men around your area they can probably steer u to a good local shop. or send it to stihl tech or one of the other competant mechanics on this site.jmo

 

[glens]

Good points, Brian.&nbsp; I know I didn't make the correlation with the other thread until you brought it up.&nbsp; It looks like an older (pre-EPA?) saw, and there shouldn't have been any reason to open up the "magnum" muffler of that vintage.

Seems I recall hearing some warnings against extended WOT un-loaded operation...

Glen

 

[JimL]

Re: tach

Originally posted by stihltech
The tach is neede to make sure the saw is not overreving after a proper adjustment. Stihl, and I am sure most other manufacturers, publish these top rpm numbers, HOWEVER, length of bar will make a difference as to how close to this number you will be. Application will also affect it. An 066 with a 16 inch bar with be much closer to the top than with a 24 inch bar, but I would be more concered with how lean the saw ran in the cut. A saw on a mill would be much happier with a bit more fuel.

he ear is the best adjusting tool, most of us do not use a tach all that often

Oh god, I can here them typing now.

I don't use my tach very often but when I do its on the older stuff it seems like. The ones im not sure how fast they should be going..

 

[Shade]

Thanks guys, I'm just trying to get a basic understanding of how it all meshes together.

The tach as a backup to make sure you aren't over reving makes sense.

On a 4 stroke you can use vacuum readings to learn a lot about what's going on inside of the engine and get a good reading for how the carb is doing along with reading plugs, but I haven't seen anything referencing vacuum readings and carb adjustments with a 2-stroke.

Makes sense to not hold anything WOT with no load, it was pumping that oil out pretty fast even with the pump turned down.

I'm getting the hang of this stuff, i just don't want to lean out the saw and lock it up.

 

[tony marks]

if u do use a tach to tune it yourself. tune it in the wood for strongest cut settings.