After some consideration about DRZ400SM carb choices (Pumper vs CV) I decided to that I would like to give fuel injection a go. This has been a project that I have had in mind for a few years thinking that I would be a great way to get the best out of a bike. At the time of inception I didn’t know that it was going to be a DRZ400SM. I did know that I was going to uses a Microsquirt to do the injecting. I have done a couple or car FI conversions – a 77 V12 Daimler Double SIX and a Mitsubishi RVR (4AG63) – Both started out life as FI vehicles making them easy to convert to aftermarket – tuneable ECU’s. Both running MegaSquirt 2’s with outstanding results.
Moving along – when I brought the bike I was a little underwhelmed with the performance off the showroom floor. I got a Yoshimura rs2 for a good price 2nd hand – cleaned it up and rejected the SM’s standard CV carb did the 3x3 mod and that was much better. Then I had a ride on an E – pumper carb -super snappy, so I started to have a look around for one to put on the bike. I was a bit overwhelmed with the price of said carbs new. I got to thinking about fuel injection and how good that would be, as well as fully programable ignition and the room for mechanical mods further down the track. It was an idea I couldn’t leave alone.
So here we are – the bike is just back together and operational. It is still in need of some tuning but preliminary results are it is a success. It is much more “Stompy” than the CV carb’ed version and at the onset looks to be more fuel efficient.
The throttle body come from an injected LTZ400 which is a very close relation in terms of motor to the DRZ. It is made by Keihin and 38mm. It has the same external diameter of the Keihin MX39 carb that it commonly fitted so the boot you buy for that carb conversion also works for the Throttle Body conversion. I made a bell mouth adaptor for the air box side to fit the standard SM air box boot. The throttle body has an integrated MAP and TPS sensors now used by the Microsquirt. Careful bench testing of the sensors enabled a calibration table to be made for each to enable the Microsquirt to read the sensors accurately. I’m not sure of the purpose but the LTZ had an axillary air plunger on a cable system (hot start / cold start?). I found Keihin spares had a choke plunger that fitted with no cable involved – so I now have an auxiliary air knob for fast idle situations.
I also brought the LTZ flywheel with the view of fitting it to gain the required triggering for the ECU. The flywheel is subtlety different in too many ways to fit the DRZ, for one it is 1.5mm larger in diameter and has about 200grams more mass and the stator for the injected LTZ is also of bigger diameter. I liked the 18-1 tooth triggering arrangement though. So I had the existing flywheel modified, taken down by a couple of mm - removing the standard trigger teeth for the CDI unit and a ring made with the 18 – 1 triggers machined into it, and had that slipped on.
I had considered the LTZ fuel system as they have an out of tank pump, but after a good look I decided that there was no room for the swirl tank / pump combo. Other considerations were the extra plumbing required for fuel, low pressure fuel, return fuel and high pressure fuel also the pressure regulator and fittings. I had a good hunt around and found a fuel pump with integrated regulator, this also helps keep the plumbing to a minimum as the fuel connections to the regulator are integrated in the pump. I designed printed some mounts to hang it in the space where the carb float bowel would normally be.
There were a few minor hiccups along the way – the high pressure fuel barb aimed directly at the frame spar, this was rotated on its O-ring with the use of a bracket to become parallel to the frame. The cover for the throttle body interfered with the oil breather box. I split the oil box in two and designed and printed a new side with a scallop in to clear the throttle cover.
I’m a firm believer in the right tools for the job too so I have fitted an innovate wideband O2 meter, while the end game is not to use the O2 for day to day use (i.e. have the bike tuned well) I need such a device to complete the initial tuning. The control box for the O2 sits in on the frame just forward of the tank rubber, in the place that the CDI coil used to sit, the O2 Probe can be seen on the side of the exhaust. The control box is connected directly to the ECU to provide feedback on mixture richness ETC.
The CDI coil has been replaced with a Nissan S15 Coil on cap item. This has an integrated igniter that can be triggered directly from the Microsquirt. It needed a small amount of modification to fit, I had to cut about 10mm from the bottom of the plug cap so the conductor would reach the bikes spark plug. I also printed a spacer / dust cap for the top to make the system as waterproof as possible.
The electrical system I tried to keep simple. One extra relay for fuel pump, injector and spark power and two fuses, one for fuel pump / injector and one for spark. The relay is mounted between the shock body and the back of the air box, the fuses are above it. I selected waterproof items where possible as I do have an 18/21inch wheel combo for off road / adventure riding.