This article was originally published in the December 2017 issue of Dirt Rider.

In last year's 2017 250F MX Shootout the Honda CRF250R placed fifth. As to why it didn't win, we wrote: "Every little bit of power helps, and the CRF's motor leaves something to be desired. And even with three chambers of air to mess with, we were never really happy with the fork." The winning bike was the Yamaha, and second was the KTM.

A well-known secret in the factory pits is that some teams use the KTM throttle body on non-KTM engines. We got to wondering what it would take to make this mod then see how it performed. While we were at it, we decided to throw on the winning bike’s fork to see what type of machine we’d wind up with. The Long Haul in this issue answers the performance question, but here’s the how-to on the way we did it.

The '17 250R is equipped with a 46mm Keihin throttle body that we opted to swap out for a 44mm Keihin throttle body that is standard equipment on a '17 KTM 250 SX-F. Not only is it 2mm smaller, but it also has a unique injector position that is mounted on the bottom of the throttle body. Many testers have reported that this injector angle and position has substantial benefit on throttle response and overall engine performance.

This mod requires substantial mechanical experience with a good understanding of how both the electrical and EFI systems operate. You will also need to have either the Honda PGM-FI programming tool or an aftermarket ECU along with its programming software.

Your parts list should include a complete throttle body assembly and possibly a wiring harness from the same KTM model the throttle body fits. You will need to have some fuel line available to extend and reroute the fuel line from the fuel pump to the fuel injector.

throttle-body design
Step 1Sean Klinger
  1. Side by side you can see the difference in throttle-body design. As you can see, from where the fuel line connects to the fuel injector the Honda throttle body's (above) injector is located on the top and the KTM throttle body's (below) injector is located on the bottom.
KTM throttle body
Step 2Sean Klinger
  1. The KTM throttle body fits nicely in place of the standard Honda one. Some modifications and rerouting of the wiring harness is required. The rerouting is fairly simple; you will just need to take your time to find the best routing for your bike.
Relocating the regulator/rectifier
Step 3Sean Klinger
  1. Relocating the regulator/rectifier assembly is required. To simplify our test we were able to secure it (and the condenser unit) along the frame rib. We would not recommend this for a permanent installation. Should you plan on making this a permanent installation it would be highly recommended that the regulator/rectifier assembly be adequately secured to the frame or a plate that is secured to the frame so that it will act as a heat sink to help keep the rectifier cool.
relocate the condenser unit
Step 4Sean Klinger
  1. We also needed to relocate the condenser unit. Again, this is our temporary mount for the sake of this experiment, though the condenser unit is encased in rubber and could actually be fine in the location where we mounted it.
KTM wiring harness
Step 5Sean Klinger
  1. We had access to a KTM wiring harness and scavenged the connectors we needed from it. If you plan on trying to complete this mod yourself you might consider ordering a wire harness along with the throttle body.
fuel injector and MAP sensor connectors
Step 6Sean Klinger
  1. The fuel injector and MAP sensor connectors will need to be replaced. Pictured here is the MAP sensor connector.
number of wires for each sensor remains the same
Step 7Sean Klinger
  1. The number of wires for each sensor remains the same. You will need to pay close attention that you keep them in the correct order.
fuel injector
Step 8Sean Klinger
  1. Here you can see the location and spray pattern of the fuel injector of the Honda throttle body. This injector is mounted after the butterfly and sprays directly into the engine at about a 35-degree angle.
KTM throttle body
Step 9Sean Klinger
  1. Here is the location and spray pattern of the fuel injector of the KTM throttle body. This injector sprays directly onto the butterfly valve and is mounted at 90 degrees to the intake.
throttle-body bell crank side
Step 10Sean Klinger
  1. Looking at the throttle-body bell crank side by side you can see the Honda (left) has a straight rate, while the KTM (right) has a cam-style design. The Honda cables fit perfectly into the KTM throttle body.
adaptor from aluminum
Step 11Sean Klinger
  1. For the sake of this temporary test we don't really want to confess how we sealed the intake boot to the new throttle body (think duct tape but stronger). For a permanent mod you should machine an adaptor from aluminum.

In addition to the throttle-body swap we decided to test a coil spring fork on the CRF250R. We felt that the Yamaha YZ250F KYB SSS fork is one of the best-performing stock forks on the market so selected it.

axle offset
Step 12Sean Klinger
  1. Before you just go ahead and try to swap forks you need to take a few things into consideration. Not all forks are the same length, and some may have different axle offset. These measurements can completely change the setup and handling of your motorcycle. For this application we lucked out. The 2017 Yamaha KYB SSS fork is pretty much the same length and has the same axle offset as the 2017 CRF's TAC fork. In addition to that the upper tube diameter is also the same. This means you can pull out the Showa TAC fork and slide in the KYB SSS fork into the stock triple clamps.
forks
Step 13Sean Klinger
  1. We did encounter one difference in the forks. The Honda runs a 20mm front axle and the Yamaha runs a 22mm front axle. This meant the quickest and easiest way to assemble the bike for this project was with the Yamaha front wheel.
Yamaha front brake system
Step 14Sean Klinger
  1. We also went ahead and used the Yamaha front brake system to finish off the assembly.

This fork swap, done this way with a complete front end available to us, was relatively easy. With basic to novice mechanical experience you could do this with a friend’s donor bike at the track one afternoon and test it yourself.

These are two mods that are slightly out of the ordinary for most people, but if you own a CRF250R that has been relatively unchanged for a few years now you might want to try. To see how the mods worked, check out the Long Haul update on page 74.