Affordable tuning of 48cc 'Happy Times' for W.Aust PAPC compliance.

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Local time
1:03 PM
Apr 5, 2013
Perth, West Aus
My experience: Affordably tuning a 48cc Happy Times for PAPC compliance.

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 dynamometer 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.

I'm an engineer with two additional science degrees, one of which is in physics. My method for determining engine power does not involve direct measurements, but relies on information derived from a study of calculated drag characteristics. I instrumented my fully fitted PAPC bicycle with it's drive chain removed, and measured the rate at which it loses kinetic energy over a span of velocities. It's probably not very interesting, but after some calculation, I found that at 23.8kh/h, the rate at which my vehicle and I lost energy was exactly 200W. If my engine puts out 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 throttle slider upward excursion, and hence 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 dropped 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 I soldered-up the carburettor injector jet, and re-drilled it with a PCB drill 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 25kh/h should be a strong indication that you're doing the right thing, at least in terms of maximum allowable power. Also, you won't need any tools to do so, and can unscrew the throttle slide access port and show anyone interested that you have a power-limiting throttle constrictor fitted.

Okay, 2000km down the road everything works fine. I only ride late at night as I can't deal with everyone staring at me during the day. The Police seem to be well informed and haven't raised an eyebrow, even when I have pulled up right next to them at intersections. The public, on the other hand, are totally misinformed by eastern states media reporting the menace of the 'gas bike', and I've actually had some ***** tailing me very, very closely, shouting and blowing his horn. I ride very late at night indeed now.


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 PAPC and cycling regs, and don't do anything to cause a community backlash. This legislation and the light, low compression 4-cycle powered bicycle can go a long way as a fuel extender and emission minimiser, but not if some innocent pedestrian or their dog gets creamed early on.
Excellent! I don't think I've seen anyone yet who has dug in and actually calculated their output, etc.

I've had a notion for a home-made bicycle dyno in mind for years. I haven't actually built it because I haven't felt enough need. No doubt, it would take a bit of fine-tuning anyway. But if I lived in a place where the police might seize and dyno my bike, then maybe I'd feel a bit more urgent about it.

I once, inadvertently, limited the movement of my throttle slide when modifying my throttle handle and cable. And it definitely did cut down on total power. I left it like that because it was still as powerful as I needed. Other than on this one particular hill. But I could see myself doing it again if that were needed to stay legal. It wouldn't even be a big problem.

I am sorry to learn that you're neighbors on four wheels are that hostile to motorized bicycles. Here in the USA we do get some intimidating behavior toward all bicycles. But it's not as intense as it sounded like in your description. And there's a certain number of folks who talk like real fools when they come near a motorized bicycle. In some cases, some hostility is evident. In other cases, they're fools who don't really mean wrong and don't realize that they're being fools.

Those of us in the USA and elsewhere can take warning from this; if our neighbors come to perceive us as 'undesirables', then we won't be safe. Even having the law on our side won't be of very much help.
Hi bluegoatwoods,

Thanks very much for reading my post.

Yes, there are plenty of suitable dynamometers that I would like to have made use of locally. Trouble is getting their owners to help out. Moped dynos are for mopeds, and motorcycle dynos are only for Harleys, apparently. Building a dyno looked like too much of a hassle, so logging the falling velocity of a rolling bike and rider, along with time data, eventually won out. Once you know what everything weighs, and a little about the wheels, then the math for a drag on velocity curve is pretty uninvolved.

You're right, BGW, extra power sure helps on hill climbs, and a bike that has had power limited as a means to control top speed is not very helpful under such circumstances. I would have been happier with a set top speed, and a maximum displacement with which to achieve it, but then sensible legislation is not what Australia is known for. It's reasonably flat along a north south coastal corridor where I live, so I'm still pretty happy with the 200W limitation.

Oh, I was probably just letting off steam, and without much good reason, when I mentioned the one unhappy experience I've had to date. Mostly people just want to ask you if what you're doing is legal, and are quite surprised when you tell them it is. We have never had Whizzers and the like in AU, so the whole motorised bicycle notion seems quite novel to most people here. The only thing that would truly bother me is if the police were not on side. I'm pleased to report that the Western Australian Police have been exceptionally helpful in dealing with my enquiries, and I haven't had so much as a disapproving glace from any of their patrol officers out on the road. Pleased as punch is me!

Thanks again for your reply, BGW. I've been away from the forum for some time, and appreciate such a welcoming message on my return.

T. :cool:
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Good! I'm glad to hear that your neighbors aren't as hostile as it had seemed.

I've been aware for some time that Australia has restrictions on powered bikes that we would consider to be severe. But I'm also glad to hear that the police are good to you.

I'm happy to report that the police here are really pretty unconcerned over these bikes. Not a single one has given me any sort of trouble, or even a furrowed brow, in my estimated 7500 miles of motorized riding.

I'm just a bit busy right now. But I'll try to remember this thread and come back. I'll draw a little diagram of the dyno I have in mind. It's really quite simple. Anyone who can build an MB could build one of these.

Then you can have your own dyno, if you'd like.
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