Is Something Wrong with My DT 125 RE?
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Hello everyone,
I’m new to this forum, I’m from Italy and I own a Yamaha DT 125 RE. I need some help because I feel that my bike is not performing as it should.
The bike is fully derestricted: it has the ground wire connected, a full Giannelli exhaust, and I also blocked the small bottle near the air filter (not the one on the intake manifold). After doing this, it seemed to give slightly better performance.
Regarding the carburetor, the idle is a bit unstable: sometimes it’s lower, sometimes higher. The air screw is almost completely closed, even though the air filter is completely stock and unmodified. I’m not sure if this could be related to the performance issues.
Another thing I noticed is that the YPVS valve is not aligned with the exhaust port. It is actually “reversed”, meaning it stays more open than the fully open position. When the valve is correctly aligned with the exhaust port, the bike becomes very slow, especially in acceleration (I don’t know why). With the valve set as it is now, the bike reaches about 122–123 km/h (around 77 mph).
For reference, the engine has been fully rebuilt and has about 5,000 km on it.
Even so, when I compare my DT 125 with others I’ve seen on YouTube, mine seems slower, especially in acceleration.
Do you think there is something wrong with my bike, or is there anything else I can do to make it more powerful without spending too much money?
Thanks to anyone who can help. -
Hi @matt0084,
77MPH sounds pretty decent to me! My DTRE certainly doesn't reach those speeds anymore and I've just about tuned everything on it. I swear when I got the bike bone stock it did 80+ and went off the clocks. But alas, I suspect I've put on weight and so has the bike and that's what I'm left with.
I'm not sure what restrictions Italian models would have had, are they the same as the UK models.
Of course, making sure the YPVS is working is going to be your first port of call. The way I check this to rule out other possibilities, is to align the valve with the exhaust port so that it is flush and then disconnect the powervalve system.
Go and take your bike for a ride. Without the valve you'll notice it's lost all bottom end, but after 6K RPM you should feel the bike come alive and scream to the red line in every gear. This is going to be what your bike would be like if the valve was working (minus the bottom end loses).
If your bike is like this, then you know the issue lies with your valve timing (you've indicated it's reversed so unreverse it).
If your bike is still sluggish and slow, then your problem lies elsewhere.
It's a misnomer about the second bottle on the boot. Please restore this bottle to the air boot, it is not a restriction.
On the DTR engine there is free performance to be gained by flowing the crankcases. If you remove the reed block and look down the inlet port, you will see that the spigots on the bttom of the barrels protrude into the ports. There is around 12mm to be removed from the spigots.
Also, port matching the bottom end to the cylinder is very important. If the crankcase transfers don't match the barrel transfers then you're just dropping performance on the floor.
Sadly both of these free performance improvements require you to dismantle the engine, but worth bearing in mind next time the engine is stripped.
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Hi, thank you very much for your advice and for taking the time to explain everything.
I’ll check the YPVS as soon as possible following your suggestions.Just to clarify, the valve is not physically installed backwards. It is only set slightly more open than the position where it is flush with the exhaust port.
As far as I know, I think the Italian models have the same restrictions as the UK ones.
Unfortunately, I don’t have the knowledge or the proper tools to do serious work inside the engine, so I won’t be able to attempt modifications like port matching or working on the crankcases myself.
Thanks again!
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@matt0084 Yeah, I only say that as it is a known issue where the cables get mis-installed and the servo doesn't operate as intended.
I find that the alignment hole in the cylinder doesn't allow the valve to sit flush in the port. I always align the valve with the exhaust off to guarantee alignment.
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Hi everyone, sorry for the late reply, I’ve only just had time to check a few things on my DT.
I checked the YPVS and it is working correctly.
I had noticed that the bike had a strange and unstable idle, so I removed the carburetor and cleaned the jets. After cleaning it, the bike feels more responsive and I managed to reach around 80 mph.
I also reinstalled the bottle on the air boot, restoring it to stock.
However, I noticed a problem with the carburetor: the air screw won’t unscrew completely. After about 3 or 4 turns it gets stuck. I tried using some lubricant but it’s still stuck, and I didn’t want to force it and risk damaging it. Does anyone have any advice on this?
I also noticed that the bike still seems unable to fully pull the last gear. Do you have any simple and inexpensive suggestions to slightly improve performance? I was thinking about opening up the airbox a bit and rejetting, but I’m not sure if it’s worth it.
Regarding the advice about removing the reed block and checking the inlet port: if I decide to remove the reed block and work on those protruding spigots, what kind of tools would you recommend to safely remove or smooth those edges?
Thanks to anyone who can help!
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Opening the airbox has proven to improve performance, but there are a whole raft of lightweight mods you can do here.
Firstly don't just drill holes into the airbox it is true that this can increase performance if done a certain way but it's not what you want.
Instead you could do with fitting the larger snorkel from the DT200/DT230, this keeps turbulence to a minimum whilst improving airflow and to be honest you could just easily design one kn CAD and 3D print this part meaning it's literally cheap as chips.
Secondly on the reed valve, whilst the OEM valves are very good, it's likely they're old/aged and prone to fluttering. Consider fitting some carbon reeds to restore health and improve resistance to fluttering.
Equally, ensure the air filter is in good health as this will stifle performance and ensuring the exhaust is well cleared as well. Gunk in the exhaust really hurts performance.
I know there are limitations with a lot of Yamaha reedblocks and that they seem to often be way over engineered (in a good way) or inadequate. I don't know where the DTR sits here, however I know the DTLC basically ran YZ sizsd reed valves and fell into the former category.
The spigot can be modified carefully with a hacksaw, but it's probably best to follow a guide from a reputable source rather than winging it.
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Hi, thanks again for the detailed reply and for all the information.
I had already heard about the larger snorkel from the DT200/DT230, but I’ve seen that they are very difficult to find. Designing one in CAD could be possible, but I don’t have any experience with CAD programs and I also don’t have access to a 3D printer.
Regarding the carbon reeds, I had heard about them before but I’m not sure which ones would be a good option at a reasonable price.
I’ve also read that some people install a VForce4 reed block for the Yamaha Blaster on the DT125 with small modifications. However, it’s quite expensive. I did find a cheap copy on AliExpress, but I’m worried that the lower quality materials could damage the engine or even perform worse than the original reed block.
The air filter was replaced a few months ago with a TwinAir filter, and the exhaust was also cleaned a few months ago.
If I eventually decide to modify those spigots in the inlet port, do you know where I could find a good guide or tutorial to follow?
