Almost dead 200T carburettor

[MattG]

it's on my super bee ,modified 200t and that saw never ran so good

Wish it were mine!

 

[Terry Syd]

I'm a bit late to this thread, but I'm interested in working up a good carb for a 200T that Randy is building for my neighbor.

If someone will post a picture of the inside of the carb (wetside without the diaphragm), I think I have a fix for this carb.

Not just a fix, but an upgrade.

 

[Lone Wolf]

I'm a bit late to this thread, but I'm interested in working up a good carb for a 200T that Randy is building for my neighbor.

If someone will post a picture of the inside of the carb (wetside without the diaphragm), I think I have a fix for this carb.

Not just a fix, but an upgrade.

Check valve? I cant find any.

 

[Terry Syd]

I'm not referring to a 'check valve'. I have a C1M carb in front of me, which I believe is the reverse of the C1Q.

The circuits in the carb are (once again) quite different from other carbs. The fuel flow to the low speed circuit is entirely controlled by the brass jet in the carb. The low speed needle only controls the fuel flow to the idle port.

The brass jet sits above the two air bleed/transition holes.

The low speed circuit must be running lean if it requires an acceleration pump to get good throttle response. You should be able to drill out the brass jet from .25mm to .3mm (or even farther if the engine needs it).

Increasing the fuel flow in the low speed circuit will eliminate the need for an accelerator pump and give more torque when the revs drop.

 

[MattG]

If someone will post a picture of the inside of the carb (wetside without the diaphragm), I think I have a fix for this carb.

Hi Terry, the bad carb I've got claims to be S126. I can't remember the C designation. I can certainly get you piccies later on today if you like.

 

[Wonkydonkey]

The welch plug that I fitted to that trimmer, was oval not triangular.

I just got a punch that fitted in and gently flattened it a bit, as it's slightly raised flattening it makes it bigger and sealed it. But I can see that using a bit of nail polish, makes sure it's sealed but after sometime I'm sure if you put/leave to much on/around the plug the fuel will break it down, as I've seen it in another carb.

The carb ( chineese ) I got is supposed to be a C1Q S126A the one that is in my 200t is the C1Q S32.
So as this carb s126a is not up to par, ( butterfly valve has no cut out, or fits to well, thus LA screw needed to be screwed almost all the way for it to tick over). I'll pull the top cover off, and post a pic

 

[MattG]

So as this carb s126a is not up to par, ( butterfly valve has no cut out, or fits to well, thus LA screw needed to be screwed almost all the way for it to tick over). I'll pull the top cover off, and post a pic

Yeah I've got a Asiani one with no cutaway aswell. I recall in an AS thread @Terry Syd saying something about filing one out yourself. That and maybe a few tweaks to the spring preload and the L screw setting, might make a half decent carb out of it.

 

[Wonkydonkey]

 

[Wonkydonkey]

Yeah, while reading a few other threads about Asian carbs, (194 for the 260) I swapped the butterfly from a duff fixed H jet carb, it then worked like it should have.
The one I swapped had a half moon cut out, and I had read about filing a slot.

In the pic I just posted I noticed it has an oval/elongated Welch plug (the same as I was saying about using a punch) but I thought yours and the oneS in the IPL had a triangular plug.

 

[MattG]

@Terry Syd
This is my f**ked Zama which this thread is about...

Identity numbers




Inside:

(I've photographed the other side if you need that Terry).

Outlet holes:


What's really weird about the Zama, I've got is the hole for the impulse is (deliberately by factory) closed. You can see that the right hand of the above picture. And though this photo is a bit blurred, you may be able to see that the impulse signal side of the carb is opaque to the flow of air (???). No hole, no mesh....


and once upon a time the saw ran like that too.

 

[MattG]

More welch plug information:



If anyone is more interested, the whole web page seemed to have a lot of good carby info:

http://www.tecumsehcarburetorguide.com/how-to-guide/

 

[Terry Syd]

Neither of those carbs looks like the C1M in front of me. The above picture shows that the carb is just a very basic High and Low speed needle adjustable carb. There is one jet on the back of the nozzle, but that is just to put the needle adjustment in a usable range.

As MattG stated above, you may end up tweaking the low speed needle (which provides fuel to the ENTIRE low speed circuit) and then shim the existing spring or put in a stronger spring. If these carbs are running as lean in the low speed section as you guys have indicated, then it might be better to find a longer and stronger spring and tweak the needle to the new spring.

It would be very informative to see what size the air bleed/transition holes were under the welch plug. It may be that the early 020T carbs had slightly larger holes. The larger holes allow more air to bleed through during idle, thus the low speed needle can be tuned out further.

Since these carbs are so basic, that may be the defining difference between the 020T and the later 200T carbs.

You will need a 'micro drill' set to measure the holes. You just turn the drill upside down and find the one that will just slip inside the hole. You will DEFINITELY need a set of calipers or micrometer to see what size the drill is. These are very small drills.

If there is a difference in the early carb air bleed/transition holes, then the micro drill set can be used to modify a later carb. You will need a 'pin vise' to hold the drill bit.

 

[Terry Syd]

This carb is just as basic, but it has an additional 'jet??'/hole between the needles. What is that hole going to?

 

[Terry Syd]

So as this carb s126a is not up to par, ( butterfly valve has no cut out, or fits to well, thus LA screw needed to be screwed almost all the way for it to tick over).

Where you put the cutaway will affect the mixture. If the cutaway in on the side of the idle port, the air flow will go over the port and pull fuel out. If the cutaway is on the side opposite the idle port, the air flow misses the port and will give a leaner idle (which then allows you to tweak the low speed needle a bit to richen the idle)

 

[Wonkydonkey]

This carb is just as basic, but it has an additional 'jet??'/hole between the needles. What is that hole going to?

I just had a closer look, at that hole between the needles, it don't go anywhere, it seems to be a dead end, I got a pipette with some water and put it in every hole and looked where it came out. With that hole it did not want to go anywhere
One thing I did learn was while pushing water into the low jet needle screw, it came out of the brass hole under the meatering leaver.

 

[Terry Syd]

One thing I did learn was while pushing water into the low jet needle screw, it came out of the brass hole under the meatering leaver.

WHOA! I think you just found something important. I didn't notice a jet under the metering lever. That could be a 'limiter' jet on the low speed system!

I thought the low speed needle got its fuel from the hole in the carb body. We need to find out what that jet does and what the hole in front of the low speed needle does.

Thanks for this guys, I've never even run one of these saws before, let alone looked into the carbs. If you guys can figure the carb out before I get the saw that would be great.

Here's a 'Rule of Thumb' I use to set up the low speed fuel rate. It works for me, but may not be applicable to all saws. - I like to have the engine rev cleaner when it is COLD. Then as the engine warms up it doesn't quite rev as crisply.

When the engine is cold, it can take a richer mixture, but not when hot. That simple test tells me that the low speed system is running on the rich side where I want it for more low end torque.

 

[MattG]

One thing I did learn was while pushing water into the low jet needle screw, it came out of the brass hole under the meatering leaver.

WHOA! I think you just found something important. I didn't notice a jet under the metering lever. That could be a 'limiter' jet on the low speed system!

I don't know the answer to this. But I'll hazard a guess: such a scheme would permit some fuel to flow into the idle orifices even when the vacuum is not sufficiently powerful to open the needle-controlled metering hole. Perhaps allowing this would enable other design decisions to work? It does sound weird, but that's all I've got...

 

[Terry Syd]

Let's find out how this carb is designed. There are so many variations of carb design it boggles the mind.

We need to pull the welch plug and figure out where the passages/circuits run to. Once we know how this carb is configured, THEN we can mod it.

This carb could turn out to be an easy carb to mod for more throttle response, mid-range torque and grunt off the bottom.

 

[Wonkydonkey]

Matt I'm not sure, from what you said... And me how I wote it. If you or I got the right or wrong end of the stick, so to speak,

The engine pulses a pressure wave. This wave pumps/sucks fuel via the carb bit with those flappy oneway valves.
When there's enough positive pressure ( this bit looses me on some carbs) from fuel the meatering leaver opens and or the diaphragm pushes on the leaver allowing some fuel (more or less depending on pop off pressure) into the chamber. And or while the fuel is being used it draws a slight lower pressure to allow the diaphragm push the leaver.
In this chamber (not all carbs are the same, I think ?) this carb has a hole for the H jet and a L jet (the brass bit under the meatering leaver).
Let's start with the H hole as it's more simple.
Fuel gets sucked into this because of a venturi effect going through the mouth over the carb, the H needle screw is the bottle neck, thus opening the needle more fuel.
Now for the L jet. Umm...

So as before the engine is running and there is a Venturi, A lower pressure.
Because of the butterfly only being partly/slightly open this lower pressure is in a different place.
Somehow fuel is drawn into the Welch plug. But fuel has to come from the meatering leaver chamber and go through the Welch plug and pass by the L screw, (this carb as Terry said has a limiting jet. No matter how much you open the L screw the bottle neck becomes is this brass thing under the leaver)

Iirc, the reason there is the Welch plug, it's made to enable the manafacturing processes.

That made my brain work a bit to hard...

If I missed/skipped or got it wrong..... Feel free to correct my 2 cell size brain, I'm sure one cell was still half a sleep.

Edit: when I say Venturi, it is a reduced relative pressure

 

[DND 9000]

On the file attached you can see what is what on this C1Q-S126 carb.