Bigger is better, so why not go even bigger? The Yamaha Zuma 125 is a fairly powerful scooter of the box. Its electronic fuel injection system has made it easier to ride and keep maintained. Parts are easy to find because the Zuma 125 is also the BWs 125 that is sold in around the world. Adding the many Yamaha Zuma parts to your Zuma 125 is also easier because of the EFI system. So, does the EFI make it easy to install a big bore kit? It should since you don’t have to jet the carburetor because there isn’t one anymore, right? Well, that’s not completely true and we’ll detail what you could use even after buying a Taida Big Bore Kit, the most complete kit on the market!
Complete 155cc big bore engine package for Yamaha Zuma 125
The DROWSports Big Bore Kits, both the 155cc and the 164cc kit, come with everything you could need for your cubic conversion. From the manifold, to the injector, to the cylinder; you come away with a kit that is easy to install and doesn’t require much more after putting one in. Even the 164cc kit, which comes with a stroker crank, is simple as the case requires no special tools to split it, unless a rubber mallet is a tool you don’t have. Seriously, in the Factory Service Manual, it tells you to tap on the case where a ribbing support is present and apart it comes. You don’t even hit it that hard! Even though they are so complete, there are a couple of things you could still need to finish off the kit.
Complete Taida 164cc engine package with 58.5mm cylinder and 3.5mm stroker crankshaft
The first part is an exhaust, why go through all the trouble to get the engine to inhale more air if it can’t exhaust it out? The OEM exhaust isn’t horrible, but it is also not designed for anything over 125cc. You will be adding anywhere from 30cc to nearly 40cc and that is a large increase for the exhaust system to handle. Most all aftermarket exhausts are larger and flow even better than the OEM piping and muffler. They are also provisioned for the OEM Oxygen Sensor. So, upgrading the exhaust should be done as soon as possible.
Two Brothers Racing Zuma 125 Juice Box Pro Fuel Controller.
The next part is a fuel controller. Since this is an EFI system, there are no jets to change. This makes maintenance less of a chore because now the bike can compensate for changes in weather and atmosphere faster than changing the air/fuel mixture screw on a carburetor. It is also more efficient than the Constant Velocity (CV) carburetor at making those adjustments. However, because you are now changing the amount of air flowing in and the cylinder volume you need to adjust how much fuel goes in.
The 6 and 10 holes injectors do help, but it’s not just about the amount of fuel that goes in, but it is timing and duration that you need to change as well. You may need an extra .01-.10 of a second for the injector needles to hold open. You may need them to open a .1-.0001 sooner to get more fuel in. You may also need to change the injector duty cycle as well. When you start messing with fuel injection, it’s all about the milliseconds. That is where a fuel controller comes into play, as it makes changes to the ECU to compensate for those needed changes. Some are as simple as turning screw potentiometers to as complicated as using a laptop. Really, it’s not that hard and with a good wideband oxygen sensor (not the OEM narrowband oxygen sensor), making changes to the injector timing isn’t hard but it is a little more tedious.
