Tune Your Bicycle Motor for WA PAPC Compliance

Tuning Your 48cc Happy Times Bicycle Motor for Power Assisted Pedal Cycle (PAPC) Compliance in Western Australia.

I went through the process of clarifying legislation with one of the W.A. Police Department officers that deal with PAPC matters. Petrol PAPC is fine as long as it’s 200W or less, and you keep all the other equipment intact (pedals etc.) and in good working order. You may be pulled over and questioned about your PAPC vehicle’s compliance, and if the officer is unconvinced, then he may seize the bike for a dynamo-meter run. If found to be over 200W, you pay for the dyno run (~$2000) and are charged with operating an unlicensed motor vehicle. If the bike is found to comply, however, then you get your qualifying dyno test for free. Police officers are instructed to initially go by the engine marking of power rating, so the thing to do after de-rating your engine is to have it marked by an engineer.

My method for determining engine power does not involve direct output measurements, but relies on information derived from a study of measured and calculated drag characteristics. I instrumented my fully fitted PAPC bicycle with it’s drive chain removed, and measured the rate at which it loses speed, therefore kinetic energy, over a span of time. The method is probably not very interesting, but after some calculation, I found that at 23.8km/h, the rate at which my vehicle and I lost energy was exactly 200W. If my engine outputs 200W, then that’s my maximum speed, an equilibrium between thrust and drag, on level ground, with no wind.

I bought the popular ‘Happy Times’ 48cc Chinese 2-stroke motor kit, fitted it, and then did the required number of run-in kilometres below a set speed in a car park near my home. The stock 48cc engine puts out WAY more than 200W, so I installed a short length of 10mm diameter copper tubing within the carburettor, between the throttle slider and it’s service cap, threaded by the throttle cable and slider return spring. It restricts upward excursion of throttle slider, and hence limits throttle opening. Expect to have to cut a number of lengths of tubing, and to have to finely adjust the throttle restriction a number of times to maintain PAPC compliance as the engine further wears-in. Start with 13mm lengths and file them down. I think my last one measured 11.9mm, but every motor will be different. A less fiddly approach might be to replace the standard twist-grip throttle with one of the ATV thumb throttles. They make ones that have an easily screw-adjusted maximum opening, intended to make operation safer for youngsters.

As well as constraining throttle opening, I found it necessary to adjust the engine mixture to prevent ‘4-stroking’, a condition where limited oxidant results in the engine only firing every 2nd revolution. I set the adjustable mixture needle to it’s lowest extent, leaning-out the mixture as much as possible, but this alone wasn’t sufficient. In addition, it was found necessary to solder-up the carburettor injector jet, and re-drill it with a PCB drill sourced from Jaycar. Correct mixture for me was at either 0.5mm or 0.6mm (I forget), but for everyone it will be different, and must be determined by experimentation. Don’t worry about going too lean, as I’m assured that a ‘Happy Times’ engine will be very hard to start if you go that far. You’ll probably want to tackle mixture first, as altering it will change the power output, and necessitate further throttle constriction trimming.

After I was as sure as I could be that my PAPC bike was legal, by attaining a pre-calculated maximum speed on level ground with no wind, I then made-up some engine compliance decals – I’m an engineer and can do that, but others should seek someone who, if the stuff starts flying, will be able to provide an expert’s opinion in court. “Complies with Western Australian Power Assisted Pedal Cycle Regulations. 48cc 195W Max.” is what mine reads.

Amongst other things that you can do to help any curious law enforcement official establish compliance, is to show them your cycle computer’s average speed reading. Reset it for each ride, and an average speed below 25km/h should be a strong indication that you’re doing the right thing, at least in terms of maximum allowable power. Also, needing no tools to do so, and can unscrew the throttle slider access port and show anyone interested that you have a power-limiting throttle constrictor fitted.

Overall, it’s been a good experience, and I’m working on my 2nd bike, a Honda GX35 powered 4-stroke, that promises to be more economical, quieter, and less of an oily mess than was my first excursion into the PAPC realm.

ps. Please, please, please observe all the WA PAPC and Cycling Rules, and don’t do anything to cause a community backlash. This legislation, along with the light, low compression, 4-cycle powered bicycle, can go a long way as a fossil fuel extender, and will help minimise emissions, but not if some innocent pedestrian or their dog gets creamed early on.