Chainsaw Fast Idle Question

[peejay75]

(Finally getting around to tuning the carburetor on my chainsaw).

(Cold engine).

At fast idle, the chain is going to move, right? (Unless you have the chain brake applied).

How long should you leave the engine in the fast idle position before blipping the throttle?

From the manual:

"Allow the engine to run for approximately 30 seconds. Then, squeeze and release the throttle trigger to allow engine to return to idle speed."

Sound right? But the "return to idle speed" seems odd, it's not "returning" to idle, because before that it was at fast idle, right?

Thanks!

 

[2oolhound]

Are you confusing the 1/2 throttle speed with an idle setting? Chainsaws have a mechanism built into the handle to hold the throttle at 1/2 speed for starting with the choke on. Once the saw starts you close the choke immediately but keep blipping the throttle till the engine is warm. As soon as you touch the trigger the mechanism that holds the carb at 1/2 disengages.

The H/L screw adusters are L for idle and H for high speed limit.

 

[Wamsutta]

The H/L screw adusters are L for idle and H for high speed limit.

Yes and there's bench setting in number of turns out from stop and a running setting that most people in the world have no clue how to do.

 

[peejay75]

Once the saw starts you close the choke immediately but keep blipping the throttle till the engine is warm.

Thanks, that's the part I was looking for! So do NOT let it run like mad at the 1/2 choke setting once it starts, blip the throttle to disengage half-choke to immediately bring it to idle?

 

[2oolhound]

well, yes blip the throttle but you may need to keep your finger on the trigger if it sounds like it may stall. This is more prone to happen in the winter though. Once it's warmed up it will idle.

 

[Steve_P]

H and L screws are mixture settings for fuel and air. H for high speed mixture, L for low. Yes, this changes the engine speed, but they're mixture settings primarily. You should use the H screw to limit WOT RPM to the max value specified, but you need a tachometer to do this. You don't use the L screw to set the idle speed, this is for off idle throttle response, etc.

There should be one idle speed screw in addition to the H and L. The chain shouldn't move at idle. Unless something has changed about two cycle carburetors, this is how it is on what I own.

 

[peejay75]

Thanks all!

I did lower the "T" screw, as the idle was initially too high (chain moved, didn't use a tachometer), then I backed the "L" screw down a 1/4 turn because the saw would bog when giving it throttle, seems to be running ok, haven't touched the "H" screw (yet).

The fast idle (should I even be calling it that?) at half choke was tripping me up, seems dangerous (and harmful to the engine) to let it run like that for any length of time, so from now on I will disengage that as soon as it starts and feather the throttle if need be to keep it running until it's warm enough to idle on its own.

 

[Wamsutta]

H and L screws are mixture settings for fuel and air. H for high speed mixture, L for low. Yes, this changes the engine speed, but they're mixture settings primarily. You should use the H screw to limit WOT RPM to the max value specified, but you need a tachometer to do this. You don't use the L screw to set the idle speed, this is for off idle throttle response, etc.

There should be one idle speed screw in addition to the H and L. The chain shouldn't move at idle. Unless something has changed about two cycle carburetors, this is how it is on what I own.

You're full of it. The H screw is not there to limit wide open throttle. It's there to give you the HIGHEST rpm at wide open throttle.

You can use a tachometer if you want to, but if you have decent hearing, you can listen for the highest rpm between too lean and too rich.

Both too lean and too rich will give you a drop off in rpm. The ideal setting is the highest rpm between those two!

Guess what? When the engine is burning at its peak efficiency, it runs faster! Who would've ever thunk it?

You set the L screw the exact same way, except you're not squeezing the throttle trigger wide open the whole time. You're using the idle speed stop screw.

Why don't people in this world study carburetors? They're not THAT complicated.

 

[5ubtle]

You're full of it. The H screw is not there to limit wide open throttle. It's there to give you the HIGHEST rpm at wide open throttle.

You can use a tachometer if you want to, but if you have decent hearing, you can listen for the highest rpm between too lean and too rich.

Both too lean and too rich will give you a drop off in rpm. The ideal setting is the highest rpm between those two!

Guess what? When the engine is burning at its peak efficiency, it runs faster! Who would've ever thunk it?

You set the L screw the exact same way, except you're not squeezing the throttle trigger wide open the whole time. You're using the idle speed stop screw.

Why don't people in this world study carburetors? They're not THAT complicated.

 

[Tynee]

Did he call it a "whipper-snipper?" That's a new one. I think I like it.

 

[Wamsutta]

He's correct about the base settings since they're straight out of the book, but he's wrong about the running L and H settings.

You can set your idle speed where ever you want as long as the engine is running at peak efficiency for that idle speed.

Using the H needle to richen the mixture past its peak efficiency so that it's always 500 rpm below max speed is just plain wrong.

The tachometer is fine if you're hard of hearing, but setting the mixture by sound is just as accurate, simpler, and quicker.

Wear ear protection while setting the running mixture.

 

[2oolhound]

My 1st husky was a 280 that wound up to 11,000 rpms with power. It lasted years too. I knew other guys with 280's and 380's that would only pull up to 9000 which is what was advertised.

 

[ericm]

Thanks, that's the part I was looking for! So do NOT let it run like mad at the 1/2 choke setting once it starts, blip the throttle to disengage half-choke to immediately bring it to idle?

If it's cold out I will let the saw run on high idle for a few seconds. The idea is to get it off fast idle as soon as it will idle normally. Then I let it warm up on idle until it will take throttle without lean bogging.

I set the L mixture so the saw idles properly when it's warm, without turning the chain. I set the H screw slightly to slightly richer than the maximum rpm (as per Stihl's recommendation). The saw should four stroke just a bit when you lift in the cut but not under load. This is a safe setting that will make good power but not seize the saw should conditions change in a way that makes the mixture leaner (i.e. cooler temperatures). This would be a rich setting for a racing motorcycle but we're doing work here, not racing.

 

[Treeman]

You're full of it. The H screw is not there to limit wide open throttle. It's there to give you the HIGHEST rpm at wide open throttle.

You can use a tachometer if you want to, but if you have decent hearing, you can listen for the highest rpm between too lean and too rich.

Both too lean and too rich will give you a drop off in rpm. The ideal setting is the highest rpm between those two!

Guess what? When the engine is burning at its peak efficiency, it runs faster! Who would've ever thunk it?

You set the L screw the exact same way, except you're not squeezing the throttle trigger wide open the whole time. You're using the idle speed stop screw.

Why don't people in this world study carburetors? They're not THAT complicated.

I've been running saws professionally for over 4 decades. I have always adjusted the H screw like this (below the highest rpm):

"The high speed screw is then turned clockwise to reduce fuel (clockwise reduces fuel). As the fuel mixture is leaned out, the saw will run faster until it sounds as if it is screaming. At this point, turn the high speed screw to the left to allow more fuel until a "flutter" is heard. This is the proper RPM for your saw. " https://www.forestapps.com/tips/tips.htm At this setting under no load, when your bar gets into the wood, the "flutter" will settle out and the saw will be operating at the correct mixture to avoid damage from too lean. Maybe we are on the same page, but semantics differ??? It seems you are stating to lean (turn in) the H screw to the highest rpm under no load?

 

[Kenskip1]

I for one would like to know what make and model of the saw we are talking about. With the chain brake on and at the fast idle setting this may cause the clutch to over heat and cause serous damage. Are there limiter caps on the adjustments? Is this a new saw? if so simply take it to your nearest dealer. I suspect that it is running to lean. How old is the mix? Is the saw running premix or his own brew?

 

[5ubtle]

He's correct about the base settings since they're straight out of the book, but he's wrong about the running L and H settings.

You can set your idle speed where ever you want as long as the engine is running at peak efficiency for that idle speed.

Using the H needle to richen the mixture past its peak efficiency so that it's always 500 rpm below max speed is just plain wrong.

The tachometer is fine if you're hard of hearing, but setting the mixture by sound is just as accurate, simpler, and quicker.

Wear ear protection while setting the running mixture.

Stihl MS 440 Service Manual

STIHL MS 440 SERVICE MANUAL Pdf Download

View and Download Stihl MS 440 service manual online. MS 440 chainsaw pdf manual download.

www.manualslib.com

Section 12.4.2, manual page 73 (PDF page 74).
Right column
4. Set the engine speed to 2,500 rpm with the low speed screw (L).
5. Set the maximum speed to 13,500 rpm with the high speed screw H.

 

[Wamsutta]

Maybe we are on the same page, but semantics differ??? It seems you are stating to lean (turn in) the H screw to the highest rpm under no load?

You want the highest rpm under no load in between too lean and too rich. The rpm will drop off at both too lean and too rich.

The engine will run the fastest at a point in between too lean and too rich because the engine is burning fuel and air at the best ratio to make power. However, if the saw's speed drops off under load, richen it up until the saw maintains speed.

Note : Too lean and too rich can also be explained by saying full lean and full rich.

 

[peejay75]

I for one would like to know what make and model of the saw we are talking about. With the chain brake on and at the fast idle setting this may cause the clutch to over heat and cause serous damage. Are there limiter caps on the adjustments? Is this a new saw? if so simply take it to your nearest dealer. I suspect that it is running to lean. How old is the mix? Is the saw running premix or his own brew?

Gulp, was hoping to get away without mentioning the brand name for fear of condemnation...but it's a Poulan...Pro...

Now, I had it running good once (so I thought), but the last time I used it, it would bog, die when revving, etc, but I was done with the task so I left it alone...for some years!

Needed it again so dug up some YouTube vids on how to adjust it...one of the more popular ones recommends setting the "L" screw with the throttle wide open/max RPMs, and while this worked for the YouTuber it felt "eh" to me. So I found a good article on how to adjust the carb at low/idle RPMs.

Now, I haven't tested it in the cut yet, but it is idling and doesn't bog and will run at near full throttle without cutting out "in the air" so that's an improvement to me! Keeping me from buying the Ryobi 40V...for now...

(Oh, no limiter caps, 40:1 homebrew mix, ethanol-free, it's probably at least 2 years old, I know, I know, I'll get some fresh fuel for it.)

 

[peejay75]

If it's cold out I will let the saw run on high idle for a few seconds. The idea is to get it off fast idle as soon as it will idle normally. Then I let it warm up on idle until it will take throttle without lean bogging.

That right there (again), I need to see how quickly I can get it off fast idle (partial choke) once it's started, instead of letting it run for 30 seconds, then taking it off fast idle, like I thought I was supposed to!

(All of my cranking has been done with the chain break disengaged, I previously thought that was just an "oh snap!" safety feature, now I know different!)

 

[Wamsutta]

Stihl MS 440 Service Manual

STIHL MS 440 SERVICE MANUAL Pdf Download

View and Download Stihl MS 440 service manual online. MS 440 chainsaw pdf manual download.

www.manualslib.com

Section 12.4.2, manual page 73 (PDF page 74).
Right column
4. Set the engine speed to 2,500 rpm with the low speed screw (L).
5. Set the maximum speed to 13,500 rpm with the high speed screw H.

"2. Turn low speed screw (L)
clockwise or counter clockwise
to obtain maximum engine
speed."

That's what I do for both L and H using my ears.

If you want to go one step further using the tachometer, knock yourself out.

 

[5ubtle]

... You should use the H screw to limit WOT RPM to the max value specified, but you need a tachometer to do this ...

You're full of it. The H screw is not there to limit wide open throttle. It's there to give you the HIGHEST rpm at wide open throttle.
...
Why don't people in this world study carburetors? They're not THAT complicated.

"2. Turn low speed screw (L)
clockwise or counter clockwise
to obtain maximum engine
speed."

That's what I do for both L and H using my ears.

If you want to go one step further using the tachometer, knock yourself out.

 

[Wamsutta]

Now what are you pondering over? Do you ever use words?

 

[ericm]

"2. Turn low speed screw (L)
clockwise or counter clockwise
to obtain maximum engine
speed."

That's what I do for both L and H using my ears.

If you cherry pick one step from a multi-step set of instructions and miss-apply it, you're not following the instructions.

 

[Treeman]

Sorry Wamsutta, but you are not making sense and any newbie following your directions will ruin their saw.

I have also confirmed with a tachometer that the 13.500 rpm mentioned above is lower than your "highest rpm under no load".

Here's yet another VERY respected site that states you back down (richen) the H screw lower than maximum rpm:

Carb Adjustment on Pro Chainsaws Info

"After revving it up to max RPM and holding, we adjust the high speed rich, causing it to slow down and blubber, developing little power. Then we adjust it lean causing it to sound smoother and then it starts to starve, also creating little power and creating a LOT of heat. As we back it off from lean to rich again, the sound becomes slightly rougher. This slightly rough sound (or 4-cycle sound, as it's sometimes referred to) is where you want it to be. We like to say, as "close" to the smooth as you can get it, but still just slightly into the rough."

Can you reference ANY sources that recommend your way? = "to obtain maximum engine speed."

 

[2oolhound]

Gulp, was hoping to get away without mentioning the brand name for fear of condemnation...but it's a Poulan...Pro...

WHAT? Did you say a Poulan?!!! Oh Poo Poo pj75. We're discussing a Poo-lan, OMGawd no.

OK PJ, I know you've got thick skin (you're still here), things can get rude around here can't they. FYI I got beat in a bucking contest at loggers sports by a poulan (the shame the shame), I was running a 2100 husky. I also worked along side a faller who used a poulan back in the late 70's. He put a lot of wood down with those green beans and they only cost half of what mine cost.

And for the record I replaced my 090 stihls with 2100 huskys because of the rubber mount system but both those saws 4 stroked when you leaned on them. Dig the dogs in and feed the chain in carefully until a clean trough was formed through the bark and into the wood then lean your body into the saw and they'd drop down a gear (4 strokin) and just dig the wood out in thick gobs of sawdust. You had to be real careful not to cut all the holding wood out on the back side. When chain brakes came out most fallers stripped them off the saw because they were in the way. When they became mandatory I would never purposely engage it.

 

[peejay75]

WHAT? Did you say a Poulan?!!! Oh Poo Poo pj75. We're discussing a Poo-lan, OMGawd no.

That's Poulan PRO, mind you, the YELLOW one...you almost got away with short-"chain"ging me!