Have I been tuning saws wrong

[Chris-PA]

All the la does is open the butterfly which adds air and makes you add more L
I'm just cutting out the middleman.

A normal carb is designed to add fuel in proportion to how much air flows through it. If you open up the throttle plate to flow more air you don't have to add more fuel - that's what the carb does. If you back off the LA all the way so the throttle is fully closed and your saw still runs then you must have some other air leak.

 

[MattG]

Can you inform us unfamiliar ppl what a bing is?

Dude a Bing is a make of carburettor.... Usually the 064s sported a Walbro carby... but apparently some rarer model runs featured one manufactured by this here "Bing" company.

 

[jmssaws]

A normal carb is designed to add fuel in proportion to how much air flows through it. If you open up the throttle plate to flow more air you don't have to add more fuel - that's what the carb does. If you back off the LA all the way so the throttle is fully closed and your saw still runs then you must have some other air leak.

There is a hole in the butterfly that let's it pass air over the idle jet.
If you start a saw and turn the la in you absolutely have to add fuel to it.
The more you turn the la in the more L you have to give it.

When I get a saw in 99% of the time it's say 1.5 + out on the low and runs ok
I back the la out to just touching and it will tune close to 1
Opening the butterfly just makes you add more L
I build several saws with a 1" walboro wb23 that has no idle adjustment period
You set idle with the L needle.

 

[jmssaws]

Every wj carbed stihl I tune this way
I seriously doubt every single one has a air leak.

 

[Chris-PA]

The more you turn the la in the more L you have to give it.

This would imply that a carb jet puts out a constant amount of fuel regardless of air flow/velocity, and the more air that flows the leaner the mixture gets. This is simply incorrect.

 

[Big_Wood]

This would imply that a carb jet puts out a constant amount of fuel regardless of air flow/velocity, and the more air that flows the leaner the mixture gets. This is simply incorrect.

don't you know your supposed to turn those needles while in the cut? your tuning wrong

 

[Andyshine77]

Every wj carbed stihl I tune this way
I seriously doubt every single one has a air leak.

Unless the hole in the butterfly is huge something else is going on IMHO.

 

[jmssaws]

The only saw I've tuned that won't tune like this is a husqvarna.

I probably build 100 1122 stihls a year and have been porting saws for a long time and I have yet to see one that won't idle with the la removed.

Obviously if you turn the la out on any saw it will die unless you lean the L

The only difference is that the saws I tune will be a 1/2 or so turn less than most.
Why would there be something wrong?

Do you think that there is something wrong or a air leak on every one of the hundreds I've tuned in 10+ years?

I don't know what you guys do for a living 40 hours a week but I port saws everyday and tune saws everyday.

 

[jmssaws]

I just checked 5 saws that I have tuned and the la is around a 1/4 turn in from touching. Maybe it's from having a big ignition advance and the crap ported out of it? I don't know but that's the way they always tune for me.
Maybe I'm doing it wrong but everything I've read on tuning says to start with the la just touching and find your best idle with the low and fine tune with the la.
But I never need the la.

 

[Amp4027]

I just checked 5 saws that I have tuned and the la is around a 1/4 turn in from touching.

So you're saying that on the --< of the idle screw, the butterfly with the linkages is touching the -- a quarter turn from actually engaging the <?

How hot is the exhaust on your forearm around 4" away from the port? Enough to burn you pretty much instantly, or enough that it gets uncomfortable in less than 1second?

 

[Andyshine77]

I just checked 5 saws that I have tuned and the la is around a 1/4 turn in from touching. Maybe it's from having a big ignition advance and the crap ported out of it? I don't know but that's the way they always tune for me.
Maybe I'm doing it wrong but everything I've read on tuning says to start with the la just touching and find your best idle with the low and fine tune with the la.
But I never need the la.

I've had a few older saw that would run "just" without the LA screw touching, a few times I found the throttle wasn't closing all the way.

If what you're doing works, than it works. We're all just hear to share and give advice to the best of out ability.

I personally turn the L in until the saw starts starts to run out of fuel, than back it out about 1/4 and check for stumble free acceleration. At that point I set the LA, normally have to back it out a little. This is the way I believe Stihl and other manufactures recommend tuning the low and I find it works extremely. You definitely went to be on the lean side with the L or acceleration can suffer, too lean and hot starts will be more difficult, and again you get a hesitation. Being rich can cause the saw to load up during idle, and can make acceleration sluggish. I'm sure you and most here already know this.

This is how I set the low. Don't mind the H tuning on this saw that's how theses bypass carbs seem to run and it's rev limited.

 

[MattG]

There is a hole in the butterfly that let's it pass air over the idle jet.
If you start a saw and turn the la in you absolutely have to add fuel to it.
The more you turn the la in the more L you have to give it.

That is a reasonable point. I do wonder though that by tuning this way you reduce the contribution that the L circuit makes at the higher air flow part of operation. I am not talking about idle here... I am referring to the point where the L and H circuit systems both have influence... this would be where the saw is WOT but under high load and thus the revs have dropped... In that area there will be less air flow, hence the H circuit will have *less* influence than at high revs (and hence airflow).

Perhaps I'm being a bit too theoretical... 25 years ago me+mates tuned DT 175 MX motor crossers... and I used to have about 4 different points of adjustment (screws, needles, jets etc.) and balancing them up right meant that all the overlapping regions in the FA behaviour played nice together. Granted a saw is NOT as fussy... and presumably your technique still ensure correct FA at all the points in cutting load.

This would imply that a carb jet puts out a constant amount of fuel regardless of air flow/velocity, and the more air that flows the leaner the mixture gets. This is simply incorrect.

I think you over simplified things here. Carburettor venturi effects are horrifically complex, and due to venturi effect L screw will deliver MORE FUEL as the flow increases to a POINT (before H screws effect takes over). The complexity of a carb is why the automotive manufacturers switched over using a microprocessor to control fuel injection many year back... but in that field the demands are much more complicated too.

 

[jmssaws]

I start with the la just touching the butterfly linkage,maybe a 1/4 turn opening the butterfly and everything else is 1 and 1 and tune from there.
I rarely ever have to touch the la
Once hot I turn the low in to lean surge then back it off just off surge then set the high and recheck lean on the low.

My 395 ran pretty well with no idle screw at all. It fell out
It fell out of my 394 also but it wouldn't idle.

I tune probably 5 saws a week on average pretty much year round and I'm almost always leaner than it arrived,even after porting.
I mostly port 1 kind of saw but I've done a ton of them and they always tune just over 1 on the low and just under 1 on the high. That's for around 14..2k

 

[Chris-PA]

I think you over simplified things here. Carburettor venturi effects are horrifically complex, and due to venturi effect L screw will deliver MORE FUEL as the flow increases to a POINT (before H screws effect takes over). The complexity of a carb is why the automotive manufacturers switched over using a microprocessor to control fuel injection many year back... but in that field the demands are much more complicated too.

Of course I did, because those details were not relevant to the point which was that fuel output is not constant and varies (roughly) in proportion to air velocity. I'm quite aware of how carbs and venturis work, and have been discussing how these uncorrected venturis give a fuel output that increases drastically as air velocity increases (causing 4-stroking) for years. Still, within range of the L outlet port/venturi and over a small rpm band the fuel outlet will be close to proportional to air velocity.

We're still discussing a saw that will idle on only the air that flows through the hole in the throttle plate, and it would have to pick up enough fuel too. Not having seen this throttle plate it is hard to tell - it may be one that has a large notch by the idle fuel outlets. With a very open muffler and appropriate timing I suppose you could pull enough air back in through the exhaust port through reversion - but this air will have no fuel, so whatever fuel made it though the closed throttle plate would have to pick up all the fuel. That's not consistent with leaning the L to make it work though.

 

[MattG]

Of course I did, because those details were not relevant to the point which was that fuel output is not constant and varies (roughly) in proportion to air velocity. I'm quite aware of how carbs and venturis work, and have been discussing how these uncorrected venturis give a fuel output that increases drastically as air velocity increases (causing 4-stroking) for years. Still, within range of the L outlet port/venturi and over a small rpm band the fuel outlet will be close to proportional to air velocity.

We're still discussing a saw that will idle on only the air that flows through the hole in the throttle plate, and it would have to pick up enough fuel too. Not having seen this throttle plate it is hard to tell - it may be one that has a large notch by the idle fuel outlets. With a very open muffler and appropriate timing I suppose you could pull enough air back in through the exhaust port through reversion - but this air will have no fuel, so whatever fuel made it though the closed throttle plate would have to pick up all the fuel. That's not consistent with leaning the L to make it work though.

I know what you are saying Chris... Yeah his setup does strike me as being odd... Not something I'd do just for the sake of it.... I assume you read my remarks re. adjustability?

 

[jmssaws]

Butterfly is opened a 1/4 turn on them
Which barely does move it but would let some air past.

 

[jmssaws]

If your 1122 is tuned close to 1 on the low then I bet it's about a 1/4 turn in on the la too.

The notch in the butterfly is there for a reason. It makes air flow over the idle jet so it will idle.

 

[MattG]

Butterfly is opened a 1/4 turn on them
Which barely does move it but would let some air past.

If your 1122 is tuned close to 1 on the low then I bet it's about a 1/4 turn in on the la too.

The notch in the butterfly is there for a reason. It makes air flow over the idle jet so it will idle.

What you therefore have is low L and low butterfly opening on these WJ carbs. Perhaps if tried I similar setup with mine it might reduce it's sensitivity to L screw tweaks? Mere speculation... mind you...

 

[jmssaws]

Some carbs are sensitive but like I said
Cut a couple shims from a soda can
Try 005 or 010
I generally like 22 to 24lbs of pop off on a strong ported 064 hybrid. Most are 20lb from the factory.
It will help with the sensitivity
Also after a hard cut it lean lopes for a second more pop off will help that also.

The 98.5cc 064's I build are amazingly finicky and require a good bit more pop off

 

[MattG]

It will help with the sensitivity

I guess that would be cos a stronger vacuum is require to flex diaphtragm enough to pull the needle.

Also after a hard cut it lean lopes for a second more pop off will help that also.

Not too sure why that is but take you at your word