We test a DEP exhaust and silencer on a brand new YZ125 and discover hidden bhp.

There’s just something very special about a two-stroke engine and especially a 125 and it’s no secret that I’m a massive fan of them. I’ve often been asked where this passion for 125s come from. I believe it’s because of the relationship on and off the track you have to have with the bike. You have to put the work in understanding the bike and dialling it in just right. And then when riding 125s you need to be dialled in to ride them fast and you get penalised if you’re not. Your mistakes are super-amplified and you lose massive ground over the competition when they occur. And vice versa – when you get it all dialled you could be gone! And I enjoy working on bikes especially as nearly anyone could easily work on a 125.

For me getting my head around what works on the track with the hundreds of different bike setup options is appealing. I’m always amazed how something so little can kick out such a punch of bhp and with a few little tweaks you can achieve even more. And the smallest of changes can make such a big difference on track.

I’ve always set my motor so it’s in the ballpark and it runs pretty safe everywhere without changing jets constantly at the track. I can’t be changing main jets and pilot jets all the time trying to gain 0.04bhp. Plus when your head is always in the engine, it’s not concentrating on the track. You can lose time on the track due to thinking about the motor instead of hitting your track marks.

A stock YZ125 is known to have issues in the sense it doesn’t run too cleanly. It’s impossible for a manufacturer to cater for everyone’s individual needs so for some riders it could be too rich or lean. But once you get the little 125 in that sweet spot, she’s unstoppable and feels great. The DEP torque pipe with a shorty silencer is the ideal setup in cleaning up the woolliness from bottom on a stock motor for most riders, making the sweet spot easier to find and ride in.

DEP has a few options on the YZ125, offering a torque and rev pipe with a shorty or longer tail pipe like stock. I’ve been using the DEP torque pipe and shorty silencer for years because the 125 hasn’t been changed for a long time and I’m pretty confident it’s the best setup to run. It was back in the day and it never fails.

We’ve had the YZ125 a little while so I’ve had time to tweak her up. On track, the feeling in standard is very good and this 125 puts a smile on your face like no other due to the power delivery. But the jetting on this motor was a little out for me and I needed to up jet on the main.

I found it fine off the bottom but a few of the intermediate boys found it a little woolly off the bottom with standard jetting, when running second gear in a few turns. I didn’t feel what they did but I did feel the motor would advantage me more with extra fuel to give it a little more strength and top end. When I fitted the DEP system I knew the power gains would benefit the guys feeling the motor running a little rich and they instantly felt the bike was easier to get onto the pipe.

I instantly felt the motor pulling harder under load and when shifting up the box. It always kept in the meat of the power range and then you have the obvious extra snap out the turns. It really did a good job cleaning up the full power range, making getting to those high bhp numbers easier. The over rev was improved too, even though it’s the torque pipe.

I like the torque pipe over the rev pipe because it’s easier to jet. I found the rev pipe system needs more fuel to keep the engine running efficiently. I just found it too much effort in jetting to worry about the power gains over the torque.

We like the little shorty silencer for a better response, sound and coolness. With this combination you’ve got a 125 which wants to shred out the turns, even with the standard carb settings. The crisp response helps on the jumps as it gives a hit out of the turn, too. If you have a stock YZ125, you need this system!

First published MotoHead Issue 29

By Dave Willet. Photos by Adam Duckworth