Orkid: I'm surprised no one has chimed in yet? Fuel injecting these old SOHC engines has successfully been done by some on this site. There are also a couple members that are currently in the process of fuel injecting their bikes.
Do a"search" on fuel injection on this site......some of the best information to be found anywhere regarding making the Honda I-four electronic fuel injected.......be prepared to spend a few hours!!!!
It is quite easy to put fuel injection on these engines, but there are a few hurdles in the process. I`ll try to give you a brief rundown on what to expect. You have to be serious to do this...it is not plug and play!!!!
1: Fit a set of throttle bodies to the SOHC intake. You may find the spacing has to be changed...there are many different ways that folks have mounted the throttle bodies and lots of different ones to choose from. If you use throttle bodies from a Donner 600cc to 750cc, you will have injectors the right size. Keep the original MAP (manifold air pressure) and TPS (throttle position) sensors from the Donner. Of course, you will need different throttle cables.
2: You will need a Microsquirt ECM (engine control management) computer to drive the injectors. You have to program this ECM yourself using a laptop. The programming is not difficult, but it does take patience and a good understanding on how fuel speed density works. If you don`t want to mess with speed density, you can use an alfa-n program that just uses throttle postion and engine rpm, but you will be quessing a lot on where to set the VE (volumetric efficiency) table that is used for the fueling equations for the injector pulse widths. To start off with, I would advise using a heated O2 sensor for initial tuning. With the O2 sensor, you know in real time whether the engine is lean or rich....without the O2 there is a good chance the engine could be damaged before all the programming is done. Once you are happy with the tune you can disable or remove the O2 all together...saves a bit of amps as well.
3: The charging system on these SOHC is marginal to keep up with the added fuel injection stuff. Again, there are a few different ways to address this issue.
4: Fuel pump...once again, many different ways to install and regulate. You will want a accurate 40 psi to 45 psi of fuel supply to the injectors fuel rail at all engine loads.
5: The wiring...Very easy to do...there are many wires that come with the Microsquit harness that we will not need on our SOHC engine. Just cut them short and seal the ends. Be extra carefull with the ground wires and the 5 volt sensor circuit. You will also need the IAT (intake air) and CLT (coolant temp...actually cylinder temp) sensors if you use the speed density system. The CLT is only used in engine warm up.
6: You will need a GOOD ignition or tach signal to send to the Microsquirt ECM. The ECM uses this signal to control EVERYTHING...so it has to be very clean. If you do not have a clean signal it will be almost impossible to do the programming. There is not a lot of room to spare on a motorcycle, but try and keep the ignition and charging system wiring separate from all the sensor and ECM wiring. This is to keep unwanted signals from happening. Use good 8mm or bigger solid core spark plug wires and make absolute sure the coil and plug ends and boots are install correctly.
This is the basics...if I missed or miss informed you...others will correct me.
Take care; Clifford
