Stihl 026 rebuild

[Terry Syd]

Yeah, once you get your head around it, it's bloody addictive. Take a look at Rich and now Brewz, they're hooked! All it took was a little bit of time and maybe a few bucks for some parts and they got some great results.

That Walbro spring kit is a excellent idea. When you work out a spring for a particular model, you can just order more springs when one of those saws come in to be modded. The springs are probably around a buck a piece - that's modding on the cheap.

I've got most of my power from my saws from a muffler mod and dialing in the carburetion and timing. I've done the same thing to a few friends saws. We're all wood cutters not cookie cutters, so making a stronger work saw is the objective and to be able to do it cheap and easy is the path we take.

 

[MattG]

The idea is to supply more fuel via the LOW circuit increase torque when the saw speed drops under a lot of load.
With the stiffer spring, I actually had to lean out the HIGH side as the LOW was supplying more fuel.
The way I see it is that it doesn't matter where the fuel comes from at WOT, as long as there is plenty of fuel when the engine speed drops under load. If your LOW circuit is set very lean to give better throttle response, it wont supply enough fuel at higher revs.
I'm not really sure what the ratio's are for H and L fuel delivery at what rpm's but it sure worked on my 039 with a larger carb that pumped in more fuel on the LOW circuit.

Hi Brewz

Sorry but I'm a little bit on this thread - but still find it and the linked threads very interesting.
So does fitting a stiffer spring make BOTH the H and L circuit run richer? I just wondered - since the emphasis seemed to be to richen the L side - for torque boost when the the revs drop.

Matt

 

[Brewz]

Hi Matt.

I am still in the learning phase, soaking up all the info I can myself, but here is what I make of it and I am sure Terry can add or correct my thoughts. In fact I encourage him to.

H circuit supplies more fuel at high RPM's and drops off fast as revs drop
L circuit supplies fuel through the whole rev range on a ramping scale, keeping things leaner at idle and increasing fuel as revs rise.
I am not 100% on this and happy to be corrected to enrich the info in this thread. It is what I see from what I have learned thus far.

The first thing to remember is that both circuits supply fuel on different curves depending on RPM's. Its not a case of L is low speed and H is high speed.

When you load the saw, the revs drop. As the revs drop, the H circuit fuel drops off and if you have a lean restricted L circuit, the saw will have reduced power in the cut due to lower fuel/air ratio.
What we are trying to do is provide the saw with an optimal fuel air ratio in the lower "loaded" range of the band to increase torque in the cut. Most saws and carbs (especially the newer ones) are limited to conform to environmental regulation emission levels. This means less power, bogging in the cut, and slower cut speeds.

By adding the stiffer spring, it leans out the H circuit because it takes longer for it to open on each impulse of the motor. Longer to open means less fuel.
This means you can richen up the L circuit to make up the lost fuel.
The advantage of the richer L circuit is that when you load the saw and RPM's drop, the richer L circuit will keep the fuel up further down into the RPM range than a rich H circuit will.
The name of the game is to find the balance where we maintain optimal fuel/air mix for max power throughout the rev range.

Remember that we use both the H and L circuits in the cut. Balancing them for max power in the cut is what we are working towards. Factory settings work, and work well, but are balanced for reduced emissions.

I cannot claim that the stiffer spring will make the saw stronger in the cut yet as I have not tested it, but I do plan to test it and report my findings.

Once again...... Would like to hear from Terry on how close I am with my knowledge thus far. Happy to be corrected....... You dont know till you have been taught!

 

[Brewz]

To add to that, when i fitted a larger MS460 carb to my 039, (76cc carb on a 64cc saw) I increased the revs with the LA screw and richened the L screw to get a smooth idle.
This meant I didn't need to enrichen the H screw as much at WOT because the L circuit was supplying more fuel.

When it loaded up in the cut and dropped RPM,s it had more power from the extra fuel supplied via the L circuit.
and when I say load, I mean a full 20" bar buried in hard timber, lifting up through a half cut rolled log. I had the rear handle propped on my knee and was pulling the handle up as hard as I could. It never slowed at all.
With the stock carb it would have dropped speed and cut slower.

I was kind of weird being able to lean on it so much and have it ask for more

 

[CR888]

Saw carbs are a bit crude in the way that midrange fuel is not cared for as much as ldle/transition & WOT. Bike carbs do midrangs power better.

 

[Terry Syd]

Brewz, you got everything right except this -

By adding the stiffer spring, it leans out the H circuit because it takes longer for it to open on each impulse of the motor. Longer to open means less fuel.
This means you can richen up the L circuit to make up the lost fuel.

The stiffer spring only affects the 'ramp' (or slope) of the fuel coming in from the low speed circuit. The spring affects the 'ramping' in of that low speed circuit fuel. If the diaphragm section (diaphragm, metering orifice and valve and the spring) weren't in the carb - then the low speed circuit would be full on as soon as you started the saw. You need to 'ramp' in the fuel slowly as the revs rise so the engine doesn't flood. Think of the slope of a ramp for something like a loading dock, some ramps are steep and others are much shallower - that's what the different springs and such are doing, they are changing the slope of the ramp.

If the ramp is too steep, you will get too much fuel too quick and the saw will bog with a rich mixture. If it is too slow, then the saw will hesitate with a lean bog.

The whole diaphragm section of the carb is for IDLING AND TRANSITION. If you want to prove that, pull the pump diaphragm out of the carb and start the engine with the choke on. It will start, but don't let the revs drop too low or it will stall.

However, there is a limit to how much flow there is in the low speed circuit. In the Zama it is the two jets, in the Walbro it is the low speed needle. The ramping in of the low speed circuit will continue until you have opened up the metering orifice to the same size as the jets in the Zama or the low speed needle in the Walbro. After that the flow in the low speed circuit is determined by the pressure differential caused by the air flow. Since the low speed circuit is essentially a fixed jet, the flow will start dropping off fairly quickly - but by then the venturi in the high speed circuit should be coming in to pick up the slack.

We're trying to get the best air/fuel mixture we can for power no matter what the revs are.

If you go back through the links I posted, you will find some air/fuel mixture charts that show the power available at different air/fuel mixtures.

I used to make my own jets, needles and slides for Mikuni carbs. Figuring out these diaphragm carbs was a COMPLETELY different experience. I'm surprised that such a simple device can be made to work as well as it does.

 

[Brewz]

Mate, great post!
Discussion is bringing forward more and more information.

I am lovin it!

 

[MattG]

I'm at work so can't get too into debate right now....



I'll have a another read later tonight! It's all GOOD stuff, guys.

 

[Terry Syd]

On this side of the Earth, we're on our way to bed. See ya, tomorrow...

 

[MattG]

On this side of the Earth, we're on our way to bed. See ya, tomorrow...

Alright for some!!!!

 

[Brewz]

Yeah its 11pm tomorrow here for you folks in the US

We are always WAY ahead of you and there is nothing you can do about it!

 

[MattG]

Was kind of playing catch up a bit by having a read of

http://www.arboristsite.com/community/threads/dolmar-7900-carb-upgrade.229528

Like this thread, it's fascinating stuff. I'm no expert, but it seems to me that what with EPA, cost-cutting manufacturers, that a number of compromises are evident in a stock carburettor. So we can achieve gains in both standard and tuned saws by being mindful of this.

I'm not in the tree service business and I have only rebuilt 2 saws (stihl 064 and ms200t), but a couple of days ago I was up our eucalyptus/gum tree doing some lopping work. At times, whilst cutting a thicker section, up in the tree, with the 200t, I could feel the torque falling with the revs. Reading these threads is now making me wonder what (carburetion) tweaks I could do to reduce this bogging... without resorting to porting or muffler mods etc.

(If you're interested some of my eucalyptus work is just here [album=medium]1666[/album] )

EDIT: I didn't want to derail the thread, regarding the cool 026 work, just mention why I'm finding the carb talk so interesting...

Matt

 

[Brewz]

Was getting all excited with plans to try the 026 out tomorrow and realised I still don't have the chain catcher or the boss that holds it in the rubber AV mount.

I might pinch the bits off my dads saw to get it going so I can run it in some wood

 

[MattG]

dialing in the carburetion

What are you guys referring to when you say "dialing in" ?

 

[Brewz]

Adjusting them to find the optimal setting giving best power over the broadest RPM range

 

[MattG]

Adjusting them to find the optimal setting giving best power over the broadest RPM range

Oh right... as simple as that!

 

[Brewz]

Sounds easy doesn't it.

 

[Brewz]

Today I bolted the chain catcher from my dads 026 onto my saw and gave it a run in some timber with the stiffer 039 carb Spring fitted.

There was a large eucalyptus branch on the side of the road near their front gate that blew down last year, 12 to 14" thick

First couple cuts in 12" timber, it fell through with little effort.
I then walked up the other end and ran it in the thicker part and put some load on the bar in the cut.
The chain would bite and stop spinning but the saw would keep on revving, so the clutch is slipping in the sprocket.

The sprocket is quite worn inside and I do have a new one in the mail.

I shut it down content that it ran strong and I had found the weak link needing repair.
When I got it to the shed the entire side and frame was very hot from the slipping clutch.

Question...... How do I know if the clutch needs replacing?
They are about $100 for an OEM unit.

Has anyone used and had any luck with the cheap Chinese clutches?

 

[Brewz]

I asked my old man, who was looking on if he thought it was faster or slower on on par with his stock saw.

All he did was shook his head and said, mine is nowhere near that fast and you have a slipping clutch.

It now has a lot of power and is so light and handy.
Once I get a clutch and sprocket that will hold, I am fairly confident it will pull an 18" 3/8 bar, which is what it will likely end up with, just so I can standardise on 3/8 chains on all my saws.

 

[Terry Syd]

The Chinese clutches can be a hit and miss affair. I had one where one of the shoes was sitting a bit further out than the others and would tend to drag at idle. The other one I put on the 029 was a complete stuff up, the drum was actually out of round.

I had to 'lap' in the clutches with some valve grinding compound. The clutch with the high shoe I found with the engine warmed up, took the clutch off and put a layer of valve grinding compound in the clutch and then started it again at idle (don't rev it). The high shoe showed some where it was hitting the valve grinding compound and I could then file it down a bit. I then did the same drill all over again, but gave it some power to lap in the shoe - no problem after that.

The out of round drum took a lot of time to get right. I couldn't believe how out of round it was. I actually had to use a Dremel to take down the high spots. The valve grinding treatment eventually got it round and now it runs fine. However, if I had known how much work it would have been, I'd probably just paid Stihl their ridiculous price for a new drum.