Carb Tuning Tip that Works

[Farmer_Nate]

I watched a video of everybody's favorite man, Billy Ray Bucker, or whatever about saw tuning. The man is funny and a little goofy, but his tip about carb tuning the low jet I think was right on and helped me a bunch.

I used his tip to fine tune my old 266xp. The main thing he noted was that it is important to get the idle speed set screw positioned so that it just touches the butterfly throttle plate stop thingy to hold open the butterfly valve the least bit. The low jet should then be adjusted the normal way by listening for idle speed to kick up as the screw is adjusted. Don't crank in that idle speed screw. And make L a bit lean at idle. He made a great point that if the idle jet is set too rich, when the throttle trigger is punched the saw has to burn that extra bit of fuel first before it can get the saw going. We have all heard that little bog, then acceleration upon giving the saw throttle. That hesitation is no good. Is point was if the idle screw is just holding the butterfly open a hair, and the L screw is a bit lean, the best throttle response is attained. I tried it and boy do I like it.

The saw runs way differently now.

Of course I still have the H screw set for warbling under no load, 4-strokin', and then smooth clean-up sound under load in the wood.

The key was getting the L set with as little idle screw engagement as possible. Many people don't consider the position of the idle set screw when adjusting L.

 

[Mastermind]

I see this a lot. Throttle plate open too far, idle mixture screw does little or nothing to affect tune. When you have a saw that is acting odd at idle, start with the throttle plate adjustment first.

 

[Farmer_Nate]

It sure helped me to pay attention to that idle screw and get it set to the "just barely cracked open" setting. From now on I look forward to better running (old-fashioned and old-school) saws.

Another point to make is that to achieve this it is best to leave the top over off while doing this so you can visually note the position of the idle screw and it's effect on the throttle plate. Get that screw to just kiss, then turn maybe another 1/8 turn in.

Thanks

 

[Stump Shot]

I see this a lot. Throttle plate open too far, idle mixture screw does little or nothing to affect tune. When you have a saw that is acting odd at idle, start with the throttle plate adjustment first.

This or when slammed shut and leaned out to compensate for correct idle speed, can be problematic as well.
Usually causes engine to have a severe stutter or die altogether when going form wot back to idle operation.
Neither are good and sometimes seemingly easy for folks to get out of synch.

 

[Farmer_Nate]

Yeah, my saw would not run well without that idle screw holding open the throttle plate just a slight bit.

 

[drf256]

Jeez, I don’t think I agree.

Never had a saw with an idle plate open enough to not allow the L screw to work. That would be so open that the saw would race like crazy. If the plate is too far open, there isn’t concentrated flow over the idle and transition holes in the bore (generally a notch in throttle plate just in front of them). It just never happened to me.

In addition, poor tune either way can add to an off idle hesitation. Generally, you want it on the rich side for good throttle response. Thats why accelerator pumps exist, the sudden influx of air can’t be suddenly met with fuel for a good stoichiometric ratio. The only time too fat makes it hesitate for me is when fuel puddles.

I use a tach for idle set. Find highest rpm with tune on L, turn down rpm, repeat the series. Then I fatten it up till throttle response is good and reset rpm to factory specs so that the clutch engages correctly.

Guess it just works for me. I do have a tendency to go through every part of a saw before I run it. Maybe it’s why I’ve never had the issue. I’d notice a throttle plate that was open a quarter of an inch.

 

[Vintage Engine Repairs]

I stick to what Stihl suggests and use it on all 2 stroke ope. With that said, each engine acts differently, I actually don’t richen the L for off idle on rotary carburetors, they run best with the L set to fastest rpm, but they deliver fuel totally differently. I made a video here, it may help hearing it. Below I have put a summary from Stihl.

 

[wcorey]

This is first assuming the saw idles with the plate near closed (or at all), can't tune the L on some yard sale find if the saw won't stay idling to begin with. I find often enough that (after the L goes to a generic one and a quarter turns out), the throttle plate needs to be cranked open however much so the saw runs somewhat on it's own, then find a happier spot on the L, then back the idle screw off as much as the saw allows, then go back to the L.
Rinse and repeat...

 

[Farmer_Nate]

Yes, an iterative solution back and forth works well also.
What I was pointing out was that I never paid much attention to the location of the idle screw before. Now when finished tuning I try to have it just touching the butterfly cam or lever, then screwed a hair inward clockwise.

 

[DigB]

If the idle screw is set so it just touches the butterfly throttle plate stop, and I then I adjust the L mix screw for max rpm, or leaner, how do I get the revs back down so the clutch is not engaged?

 

[hacskaroly]

Carburetors are my biggest weakness when it comes to working on saws, I definitely need more practice on tuning them. I can take them apart, clean them, put new diaphragm/gaskets/needles in them no problem, but when it comes to getting them tuned up, it seems like I end up chasing my tail trying to get it dialed in most of the time. Definitely going to reread whats above to see what I can do differently.

 

[ErnieG]

Here is a great video on how to tune a chainsaw...
Ernie

 

[Vintage Engine Repairs]

Here is a great video on how to tune a chainsaw...
Ernie

Ha that’s my video, thanks for sharing

 

[ErnieG]

Ha that’s my video, thanks for sharing

I have to saw your video is the best on how to tune a carb !
No BS just straight to the point and rite on .

Ernie

 

[Vintage Engine Repairs]

I have to saw your video is the best on how to tune a carb !
No BS just straight to the point and rite on .

Ernie

Thanks Earnie, I appreciate the feedback

 

[Farmer_Nate]

@Vintage Engine Repairs is also my favorite carb adjust vid.

 

[mainer_in_ak]

Always tend to set saws a bit richer at idle than typical carburetor settings, but not so rich that you're choking on rich fumes or that the saw loads up and dies when tipped.

It's more of a "cooling" effect after a hard cut that heats up the saw. For example: an 18 ft long milling rip with the milling saws, or cutting down a large stump. They hot-start better too, if a smidgen rich on the low jet. Like an 8 inch pull on the starter, they fire right up.

Not that show-boatsy you-tuber deal with a lean jet, where a guy has to reef on the starter like he's trying to fkn break it.

Also, cutting frequently in below-zero temps here in interior Alaska, the saws cold-start better a smidgen rich on the low jet.

The guy who advocates for as lean as possible on a low speed jet, he does burn up saws.

 

[Vintage Engine Repairs]

@Vintage Engine Repairs is also my favorite carb adjust vid.

Thanks Nate!