Pro Circuit Works Pipe And Silencer–YZ125

What it is

• A racing pipe and muffler combination to boost the performance on the Yamaha YZ125.

Pro Circuit makes this stamped and welded pipe and silencer combination for the Yamaha YZ125 from 2005 to the present. It claims an increase in horsepower and torque throughout the RPM range. It is made from stamped AKDQ high-quality carbon steel with hand welded and pounded seam and a reinforced mounting brackets and stinger. The Works Pipe (tested here) has a bare metal finish and takes extra care to keep from rusting but is also available in a platinum version that has an electroless nickel plating. The  R-304 silencer is a shorty design and made for closed course racing only. It is made with Hex head screws on the inlet cap that can be removed easily for quick re-packing of the silencer. Additionally the intake tube is 304 stainless tubing with a double-walled mounting bracket on the 6061 T6 brushed aluminum silencer body including a stainless steel end-cap. The muffler is tig and mig welded for strength and durability. E-Glass internal packing is used for maximum packing life and the silencer is made in the USA.

How it works

• Easy bolt-on in less than 10-minutes.
• Power gains you can feel without needing any jetting or fuel changes.
• Easily improves the biggest issue for most riders, it gives the bike more top-end and more over rev.

First off, the Pro Circuit pipe and muffler bolt right on and everything lines right up with minimal fuss. About the only complaint we have is having to exchange the O-rings out of the stock pipe and into the PC one and that is nothing. The smaller and lighter muffler uses a single mount with the grommet swapped from the stock muffler.

The oiled bare metal finish looks works on the bike and if you treat it with a little WD40-type oil it will color nice and blue on your first ride if it isn't too muddy or wet. The sound of the bike is a little sharper and has a little more snap to the ear of both the rider and the people close by. Not enough to complain about in a world of four-strokes but it is louder. We ran our bike on pump gas and started with stock jetting as a baseline even though Pro Circuit recommends mixing in a little VP race gas to get the most out of the pipe.

On the track we were instantly impressed with the pipe—way more than we expected. The jetting and crispness of the stock bike is very impressive and we didn't lose hardly anything here. Just a trimming of the fuel screw (leaner) got the bike to where it was on par in crispness and response with the stock setup right off the bottom, the only place it was changed at all. The PC pipe and silencer have the YZ pulling harder with torque from the bottom that we just didn't expect. It really allowed our lighter riders to short shift on occasion and our heavier riders could make mistakes and not be penalized so much. The bike pulls and builds power smoothly and as authoritatively as the stock pipe through the mid, which is excellent. Then on top, the PC system is noticeably stronger and pulls harder and longer with a less abrupt taper and much less of a sign off. It feels like the bike pulls and makes power for an additional 500-1000 RPM. Yes, it is that good and for a simple pipe bolt-on, impressive.

If there is any downside to all this good power was one odd trait that we could not totally explain. If you put a huge load on the bike when landing from a jump, the kind of jump that you need to do at least a downshift, maybe two on a 125, landing from a jump going up a steep hill, for instance, the YZ will fall off the pipe for a second before screaming back to life. It wasn't a bog--it just felt like the pipe didn't like the drastic change in RPM so suddenly. We played with jetting but this trait stayed and we are guessing race gas might be the cure here, hence the recommendation. In comparison to the stock pipes and others we tested, this issue was unique to the PC pipe, but not enough for any of our riders to want to change it to any of the others.

We ended up with a final jetting setting in California at tracks in the 1000 FT. elevation range with a one larger main jet (440) and a one-two size smaller pilot jet (38 or 35) depending on the temperature. The pipe toned itself to a blueish brown over time and as long as we kept a water repellant oil on it after washing it stayed rust-free. And with an acidic type cleaner you can get that white like-new look again with a little elbow grease. If you don't like the extra work, the platinum finish pipe (same price) will make you happier. The muffler’s packing stayed consistent over the 10-hours we put on it and we suspect it will easily last 20-hours before needing a repack.

Overall this pipe and muffler is an excellent addition to the YZ125 and did nothing but compliment the already great power. There wasn't a track or condition we didn't prefer it over stock.

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