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Almost dead 200T carburettor

Terry Syd

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DND - good post. It says that that brass jet is the supply to the low side.

I still like to figure out all the circuits in the carb before modding - I've been fooled way too many times. However, if the Stihl document is correct, then all we have to do is start drilling that jet out.

The low speed needle must only change the fuel mixture for the idle circuit (like most Zamas) and not the whole low speed circuit. I say that on an ASSUMPTION. As I said, I still like to figure out all the circuits before modding.
 

Al Smith

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As I said before it appears on those fixed jet carbs they were so equiped evidently for the elevations they we being sold at .I've only seen one on my buddys 020T .Evidently it was sized correctly because it ran very well .That praticular saw had a leaky oil tank which I replaced the entire crankcase using a 200T .So it now has one flippy cap and one screw cap .Still never figured out why the oil leaked .
Before the aftermarket cylinders and piston were made for these they came up,entire crankcases with crankshafts on flea bay for 40-50 dollars .I think most of them died from leaky main seals caused by worn out bearings not from faulty carbs .They blamed the carbs but I'm not so certain that was entirely the fault .
 

Terry Syd

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Al, this isn't a 'fixed' jet carb. It has both the low and high speed needles. The jet under the metering lever 'limits' the fuel that can flow through the low speed circuit.

If the jet is drilled out, then the low speed circuit will be richer. The LOW SPEED NEEDLE appears to be configured like the C1M I have where it only adjusts the idle mixture and not the entire low speed circuit.

The low speed circuit is set up for a lean burn and the acceleration pump is there to give it a shot of fuel when the throttle is opened. We can eliminate the acceleration pump by increasing the fuel flow in the low speed circuit. In doing so we will have very good acceleration with the bonus of increased torque.

The stock jet is probably .25mm. We can take it out to .3mm and check the results, then .35mm and check again.
 

tree monkey

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

Identity numbers

View attachment 104008
View attachment 104009

Inside:
View attachment 104012
(I've photographed the other side if you need that Terry).

Outlet holes:
View attachment 104013

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....

View attachment 104015
and once upon a time the saw ran like that too.

well it can't run like that now can it. it has no impulse
 

MattG

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well it can't run like that now can it. it has no impulse
It does run like that! Very badly now though.

It actually used to run very well just like that. (Tell me how somebody!!).

But the port in the carb for the impulse signal is visibly opaque to fluid flow. Yesterday I put a drop of fuel on the outer port (i.e. where it aligns with the top handle body's hole) and the droplet does not weep into the inside of the carb where it should.

I'll challenge anyone who is a carb expert to explain, what Zama were playing at.

If there is anyone else who's interested and has a Zama C1Q-S126B, please can you share a piccie of the impulse signal outer port so I can compare?
 

tree monkey

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if there is pressure in the fuel tank, then it will run without a pump.
 

Wonkydonkey

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Umm this is a good point, but I'm sure there's a very simple answer.

I remember connecting the impulse hose to the place where it should (under the back of handle) I followed where it came out but it eludes me now. At the time I thought errr... Well that's how it was
I don't know if the compensator valve bit helps out a little ?
 

Wonkydonkey

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I found my simple answer to this impulse conundrum.
You can see in the first pic there is a little hole that goes through into the carb body.
Number
1, is where the hole exits
2, is the chamber where no1 enters (pulse chamber)
3, is the other side of membrane/diaphragm where fuel is

image.jpeg image.jpeg
 

MattG

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@Wonkydonkey and anyone else for that matter

Hi Sean, Yes I know exactly where the impulse pipe on the 200T goes to the top handle, then through passages in the plastic etc. etc. then terminates at the back of the inside of the handle. But on my Zama C1Q-S126B the matching port on the outside of the carb (which would align with the back of the handle like I mentioned above) is BLANK.

Yes, it's BLANKed off by Zama....

Yes, it's opaque to airflow...

Yes, I've tested with a drop of fuel...

No, it's not a micro mesh....

No, it's not a dirty/blocked....

Pictures of outer impulse port on my Zama :

2017-12-30 13.52.37.jpg

(if you can make out a slight marking on the port, it's not hole, it's where I've tried to investigate it with a pick)

2017-12-30 13.53.22.jpg

ANYONE ELSE GOT A GENUINE ZAMA C1Q ......

2017-12-30 13.48.26.jpg

??

If I get the time in the week I'll pull off the impulse hose, block it with plug and fit this carb, and make a vid of it running - if possible. That would prove it either way I guess.
 

Wonkydonkey

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So have you taken off the cover (same as I have) and looked , at. Those flaps that reg the fuel, the chamber that holds the fuel, and the other side of the membrane chamber. And followed it back ? To where it should get a pulse. If there's no pulse it can only be a dead carb..
Lol..
Idk cant rember if you said it worked before. But if not, someone's changed a few parts.? A frankingcarb?? Maybe...?
Or ...idk.. I'd just get another carb lol, then try it. ..

Sometimes saws are a pain,,,.
 

Lone Wolf

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@Wonkydonkey and anyone else for that matter

Hi Sean, Yes I know exactly where the impulse pipe on the 200T goes to the top handle, then through passages in the plastic etc. etc. then terminates at the back of the inside of the handle. But on my Zama C1Q-S126B the matching port on the outside of the carb (which would align with the back of the handle like I mentioned above) is BLANK.

Yes, it's BLANKed off by Zama....

Yes, it's opaque to airflow...

Yes, I've tested with a drop of fuel...

No, it's not a micro mesh....

No, it's not a dirty/blocked....

Pictures of outer impulse port on my Zama :

View attachment 104430

(if you can make out a slight marking on the port, it's not hole, it's where I've tried to investigate it with a pick)

View attachment 104431

ANYONE ELSE GOT A GENUINE ZAMA C1Q ......

View attachment 104433

??

If I get the time in the week I'll pull off the impulse hose, block it with plug and fit this carb, and make a vid of it running - if possible. That would prove it either way I guess.
I have some of them what do you want to do?
 

MattG

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So have you taken off the cover (same as I have) and looked , at. Those flaps that reg the fuel, the chamber that holds the fuel, and the other side of the membrane chamber. And followed it back ? To where it should get a pulse. If there's no pulse it can only be a dead carb..
Lol..
Idk cant rember if you said it worked before. But if not, someone's changed a few parts.? A frankingcarb?? Maybe...?
Or ...idk.. I'd just get another carb lol, then try it. ..

Sometimes saws are a pain,,,.
Yeah, it used to be a good carb Sean. It's what this thread is all about. It gradually got worse a few years ago. I replaced it ages ago :risas3:

It's a regular zama s126b, no sh!t parts or nothing

Periodically I get bored and try to make it work properly. BTW I've been aware of the impulse port weirdness for ages.

The last thing I did was block down the accelerator pump piston and soak in gas for a couple days and then reclean.

The saw starts fine with the carb, and if you turn the LA in long way it will fast idle indefinitely. But it's ALMOST IMPOSSIBLE to rev high......which is NOTHING to do with the impulse pulse by then, since at ~4k the engine vaccuum is quite capable of slurping fuel through without a pump. I believe part of the L speed jetting is blocked, hence my curiosity in welch plug work......the impulse issue, is a side issue for me.... ;)
 

MattG

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I have some of them what do you want to do?
Check the impulse port on the outside of the back of the carb body where it backs onto the matching hole in the inside of top handle back.

See the piccies in #49 to see the familiar port

Post some piccies of genuine zama c1q-s126b impulse ports, up here Steve, and we can if they are similar to my piccies...
 

Wonkydonkey

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I've been,,,, looookin around,,,,, I found a few carbs like yours on eBay no impulse hole and a few others with the impulse hole like mine.
Many other links to c1q 126a/& b's for 02t's & 200t's

I'M sure there's an answer but I don't have it :mad: ;):):):)

Best of luck with this :biggrin: I think I will just carry on :drink9:a few
 

MattG

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@Lone Wolf Cheers pal.

Yeah, it's plain to see that one you've has got the drilling where mine aint. I see our carbys do have slightly different model designations mind you, your's says "S126A 042A", but mine says "S126B 305A". Guess that's where the difference lays as far as Zama is concerned.
 

MattG

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drill the impulse hole in the carb
Yeah, that thought did occur to me. Thing is, like I keep telling people here, that carb used to work good, with the blanked off port as it is!!

I found a youtube from a couple of years ago, when I first rebuilt a saw, and a clip of it's first running....yes with that very carb. LoneWolf and AngeloC might be able to remember this - they were instrumental to my chainsaw mechanics education!


Yes, I know the idle is too low, it's the first start after the build, but you can hear the response is great. The carb started to die a couple of months after...... :(
 
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Lone Wolf

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Yeah, that thought did occur to me. Thing is, like I keep telling people here, that carb used to work good, with the blanked off port as it is!!

I found a youtube from a couple of years ago, when I first rebuilt a saw, and a clip of it's first running....yes with that very carb. LoneWolf and AngeloC might be able to remember this - they were instrumental to my chainsaw mechanics education!


Yes, I know the idle is too low, it's the first start after the build, but you can hear the response is great. The carb started to die a couple of months after...... :(
Are you sure the carb is bad did you try a different one? Vac leaks can drive you crazy if you dont eliminate them with a test, a new test if you don't have a spare carb to try anyway.
 
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