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Discussion Starter · #1 ·
A few months ago when I bought my 2000 TW 125 I noticed that it was not running as smooth as it could, I had the feeling that the mixture was too rich but I ignored it temporarily as I was waiting for some parts and would take advantage when they arrived to do a tune up.

Well, I did a valve adjustment, carburetor cleaning, spark plug change, oil and air filter change, carburetor adjustment according to the manufacturer's specifications and after that I did a small run, I noticed a small improvement but not the one I expected so I checked the spark plug just put in and indeed the head was already darkened.

The carburettor has two main adjustments, on the one hand the mixture adjustment screw and on the other the clip that regulates the needle height. Of course I could adjust the mixture screw but since both the screw and the needle height are according to the manufacturer's specifications, two turns for the mixture screw and the clip placed in the 3rd slot of the needle, the mixture theoretically should be correctly adjusted.

Before checking further I would like to know your opinion and if anyone has had similar experiences.

Thanks to all who want to contribute
 

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The specs from the manufacturer are usually just a base line or place to start. Your settings will vary depending on altitude, humidity, engine condition, fuel quality and so on.
Adjusting the mixture screw will affect idle to about 1/8 throttle. Moving the jet needle clip will affect mixture around 1/4 to 3/4 throttle. Moving the needle clip up a notch will lean out the mixture.
Try adjusting one thing at a time and then recheck to see if anything improves.
Check to be sure the plug is getting dark from unburned fuel or soot and not oil.
A compression check on a new to you engine is always a good idea too.
 

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Discussion Starter · #3 ·
Fortunately it is not oil, it does not have that characteristic sheen but is soot.
I have been trying to modify the mixture through the mixture screw and I am surprised and not for the better, generally with the carburetors that I have experience with the slightest variation of the adjustment screw I notice a change in rpm, however variations of more than 1/4 turn seem to have no response and the same with the idle adjustment screw.
 

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Let me preface by saying I have zero experience with TW125's. This said, it's probably similar to a TW200 in function.

If you're not seeing a change by turning the pilot screw, it can be an indication that the pilot jet is the wrong size. Do you know if the pilot jet has ever been replaced? Also, when you cleaned the carb, did you mechanically clean the pilot jet with a guitar string, dental file, welding tip cleaner, etc? It's often not enough to simply clean it with carb cleaner.

Does the bike have any modifications, such as deleting the original air filter?

Is there any popping on deceleration, or backfiring? Does it run well, or are there flat spots or cutting-out?
 
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If you arent getting any change in idle rpm with the mixture screw and the engine up to operating temp it usually means one or more of the air or fuel passages has dirt, varnish or some other kind of foreign gunk. In your case maybe a blocked air passage is causing the rich mixture.
 

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Discussion Starter · #6 · (Edited)
If you arent getting any change in idle rpm with the mixture screw and the engine up to operating temp it usually means one or more of the air or fuel passages has dirt, varnish or some other kind of foreign gunk. In your case maybe a blocked air passage is causing the rich mixture.
I'm more surprised about the idle speed adjustment screw, I don't see any way it could be dirt.
 

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Discussion Starter · #7 ·
Let me preface by saying I have zero experience with TW125's. This said, it's probably similar to a TW200 in function.

If you're not seeing a change by turning the pilot screw, it can be an indication that the pilot jet is the wrong size. Do you know if the pilot jet has ever been replaced? Also, when you cleaned the carb, did you mechanically clean the pilot jet with a guitar string, dental file, welding tip cleaner, etc? It's often not enough to simply clean it with carb cleaner.

Does the bike have any modifications, such as deleting the original air filter?

Is there any popping on deceleration, or backfiring? Does it run well, or are there flat spots or cutting-out?
The carburetor is the original and at first glance I do not see that they made any modifications and the different parts look like the originals.

The carburetor was completely disassembled and cleaned part by part with cleaner and compressed air. To say that it was quite clean already, I have seen them much worse. The needles I clean them with a rag after leaving the liquid to act. In my opinion, dirt in general has the opposite effect as it hinders the passage of gasoline making the mixture poor.

In the short time since I have it I have not noticed any strange modification, everything seems to be standard, except the exhaust that seems to have been changed but it is a similar model.

As I said, I just put a new air filter.

The sensations of the bike since I bought it is a little uneven at idle and lack of verve, what I transmitted is excess gasoline and this is confirmed by the soot on the spark plug (and also consumption after spending the tank).
 

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Discussion Starter · #8 ·
The only thing I don't know for sure is if it comes from the factory with the needle clip in the third slot (in the middle) since the manual does not say so, but from what I have read everything points to yes.
 

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The exhaust modification can certainly throw off the carburation.
Is it a factory air filter assembly, or aftermarket?
Again, are you experiencing any popping on deceleration or backfiring?
Any flat spots in the throttle?
Necessary to leave the choke on for it to idle correctly?
 
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Discussion Starter · #12 ·
The exhaust modification can certainly throw off the carburation.
Is it a factory air filter assembly, or aftermarket?
Again, are you experiencing any popping on deceleration or backfiring?
Any flat spots in the throttle?
Necessary to leave the choke on for it to idle correctly?
The air filter is not the original spare part but it is a specific one for this model, it is not generic.

No exhaust backfire, no black or white smoke. I would say that at idle the sound is a little irregular than it should be instead of a constant sound (of course partly natural as it is a single cylinder engine).

The throttle is fairly linear

And regarding the choke that I didn't mention, since I bought the bike I've found it strange that I never had to use it for cold starts, the other day I put it on with the bike running to see if it affected it and nothing. All this leads me to think that the mixture is so enriched that neither the changes in the idle screw nor the choke alter the air flow enough to notice an appreciable difference in the engine.
 

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OK, that information is really helpful, and that video is fantastic!

This is a different carb, than the TW200, but still made by teikei.

According to the comments in that YouTube video, the pilot jet should be a size 38. I'd both verify this is correct, and also verify the size of jet installed in your carb. When a pilot jet is upsized in a TW200 carb, it results in less need for use of the choke, or even no need for choke at all. Assuming a pilot jet with a larger orifice would let more fuel through, this could be the source of the rich fuel mixture. Note that the pilot jet only controls the flow of fuel at the low end of throttle, and the main jet controls the high demand, so it's conceivable you could be experiencing multiple issues. As I mentioned previously, if the pilot jet is an incorrect size, it can result in no result when you turn the pilot screw.
 

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...One other thing to consider... If your bike is running well and you're not experiencing negative symptoms, you may want to leave well enough alone.
 
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I checked the size of the pilot jet and it is indeed the original (38).
You already have some really good replies...just a few more thoughts:
Stamped 38 does not preclude it being reamed or aggressively probed to clear, resulting in a larger orifice.
If it is rich, at idle, then possibilities include:
Partial choke applied (butterfly not fully open or enrichener not closed or enrichener leaking)
idle jet not fully seated
idle jet too large
mixture screw too rich
carb passages not fully clear
float level too high/float valve not seating

If rich mid-range/off idle, then possibilities include
Needle too rich or clip set too low.
If clip position is a concern, lower needle (leaner) by placing clip higher on needle
 
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I'm more surprised about the idle speed adjustment screw, I don't see any way it could be dirt.
If I read correctly/fully, Salabim reference the mixture screw, not idle screw.
Turning mixture screw past ideal should drop idle speed.
 
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Discussion Starter · #20 ·
I have tried:

- Disassemble the choke and close it manually to rule out a bad operation of the same

- I have looked for possible air intakes

- I have checked the valves again.

-second cleaning of the carburetor

- I have verified that the slide was well positioned and it rises when tightening the idle screw


for the moment without result, we keep trying
 
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