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Discussion Starter #1
I have a bone stock 2003 Yamaha TW125 with a fresh head, valves, piston and rings, the bike runs great overall. But I've had the carb rebuild TWICE hoping it would fix this issue but in 10,000KM I've always had this issue. When I reach about 90KPH for extended periods of time the engine starts stuttering quite badly and inconsistently, which leads me to belive it's fuel starvation. I can't find anyone with the same issue as me so I need someone to help me figure this out, I've had 3 different mechanics go over the bike and there's legit NOTHING wrong with it but it still does this.

Pretty desperate for help!
 

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First up – welcome to the board …..

There were two methods of restricting the top speed of the 125 – an attachment to the speedo that feeds back to the CDI – and a simple washer in the exhaust output to restrict flow

Other than that – possibilities include a faulty CDI, or a faulty coil, or a short on the HT at full pelt. But, always go for the simplest thing first (and the most inexpensive), and as such I’d recommend checking your ignition earths (sounds like you’ve had it apart), while chucking something like “Redex” in the fuel, to blow the carb out

If it is restricted by the “washer up the spout” method, if you take the exhaust pipe from the head, you should be able to see it (if it’s there) – it’ll be at the start of the header pipe, in the pipe itself

Other things to check (cheap fixes), include checking the float height, and replacing the main jet (regardless) – both simple precautions that will eliminate those areas

The carbs on these things are so small, it may appear to be clean, but the slightest thing can affect them – having said that – it may be something completely different, so check everything ) without spending money) first …….
 

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Welcome!! :D
 
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Discussion Starter #4
First of all thanks for the warm welcome.

On another note the bike runs PERFECTLY otherwise, I checked the spark and it's fine, I changed the ignition wire and spark plug, the bike runs WONDERFULLY, the spark plug looks perfect which shows me that the mix is just fine, compression is 10.1:1 so basically factory spec. I got the whole carb cleaned and left it overnight with carb clean, like it got cleaned perfectly.

It's fine when revving the nuts off of it in lower gears it's just when I'm shooting for top speeds that it happens. Secondly I doubt that it's speed limited when I hit 120KPH before.
 

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First rule of motorcycling, no matter what kind of engine problem you are having switch to reserve and see if it goes away. Try it.

Next, check your gas cap to make sure it is venting enough. Leave the cap loose and go for a ride and see how it runs.

If you have a fuel filter change it.

Good luck!
 

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Discussion Starter #6
First rule of motorcycling, no matter what kind of engine problem you are having switch to reserve and see if it goes away. Try it.

Next, check your gas cap to make sure it is venting enough. Leave the cap loose and go for a ride and see how it runs.

If you have a fuel filter change it.

Good luck!
Considering I didn't even HAVE anything but reserve till a couple of weeks ago (I didn't have the hose and I couldn't find one for the life of me so I just used an old fuel hose from another bike), I'm 100% sure it's not that, the bike doesn't have a fuel filter but I did actually try to FULLY clean out the tank and it did absolutely 0, took out the gas afterwards and checked it aswell, 0. It's a really, really 'freaky' thing to say the least, it seems like nothing is wrong yet it doesn't run right. I tried 95 RON, 98 RON and 100 RON and that didn't do anything either, it feels like the bike is surging from fuel starvation but it's got the stock carb and most of it is stock so I don't really know what it could be! If it's the CDI I'm honestly scared to know the cost of a new unit so, I'm hoping it's not that.
 
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