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How to get the most out of a Chinese 2-stroke 50cc moped....cheaply!
Chinese 2-stroke 50cc moped engines are basically a copy of the air-cooled horizontal Minarelli engine as used in Yamaha Jog (basically an air-cooled version of the Yamaha Aerox engine). There are a few differences, all the recent Chinese ones use 12mm gudgeon pin instead of the genuine Yamaha 10mm and they also use a heftier crank with larger diameter shafts (the same size as used on Yamaha Aerox 100 in fact).
So far so good, they are copying a decent engine and have even improved a couple of its weak points.
Unfortunately what can let the Chinese engines down from hereon in is the huge variation in the quality of the components used within them. Without years of experience dealing extensively with different Chinese factories and seeing first hand the problems customers face with them it's impossible for the average customer to know which Chinese scooters are the good(ish) ones.
We know of a certain brand Chinese moped where we can virtually guarantee the main crank bearings will fail within the first year, another brand where the rear gear teeth will always strip within 2 months unless a simple modification is made and another brand where the variator gudgeon pin is 3mm too long causing the engine to over-rev and be at constant risk of seizure.
There are also many brands where the engines are built with every single part being sourced simply from the cheapest supplier at the time, so one batch of engines may be decent, the next poor. The list goes on.
For this blog we're going to be using a Baotian Falcon 50cc 2-stroke. The reason for this choice of scooter is that Baotian has very consistent quality control. It's supplied by a single supplier within the UK with a genuine technical and experience pedigree and they seem to have religiously overseen all aspects of its production during the last few years in which they've been the sole UK supplier. They also don't seem to be afraid to spend a little extra money to get the product right. It only takes a few extra dollars at production level to buy decent crank bearings, decent variator, even a genuine NGK spark plug and Yuasa battery, but not many Chinese scooter importers are prepared to do this with most prefering to opt for the cheapest components to the detriment of the end customer and the reputation of import mopeds.
So whilst they still have their faults inherent to all budget scooters the Baotian brand is generally considered to be of decent quality for the money you pay. It probably should be mentioned that there are other Chinese moped wholesalers selling scooters from the Baotian factory in China. These are not allowed to be openly marketed or advertised in the UK under the 'Baotian' name and are not made to the same specifications as the genuine Baotian branded scooters are.
Ok, back to business. For this blog we're going to look at just mild tuning to get a bit more out of the bike. If you want to heavily tune a moped or you are fanatical about peds you are unlikely to start with a Chinese one, but there are a lot of these Chinese mopeds about and a lot of people would be perfectly happy to pay maybe £50-£150 if something modest but significant and reliable could be achieved.
First of all let's run the bike up to see what it's doing. It's immediately apparent that the moped is restricted to 30mph, but surprisingly it's actually restricted much more effeciently than the Yamaha/ Piaggio mopeds. There is a wire running into the transmission side casing - this wire allows the cdi to monitor the actually rear wheel speed (not the engine rpm) from the clutch bell speed. This is clever because it allows full acceleration and full power right up to 30mph at which point it simply won't accelerate any further. The Piaggio/ Yamaha mopeds don't do this, their restriction method restricts bike power from approx 20mph upwards by prematurely raising the revs (with a variator restrictor ring) until the moped is revving too high to make enough power to rev any higher. Also this variator ring method will not work if you tune the bike (e.g. add a powerpipe). The Baotian method would still work and restrict to exactly 30mph (you could just fit a switch) regardless of the state of tune of the bike.
Curiously the Baotian Falcon doesn't pop or stutter as most electronic limiters do at least to some degree when they cut in. We investigated this and instead of cutting the spark (as say a rev limiter would) the cdi is progressively retarding the ignition timing (sparking later and later) as the moped begins to reach 30mph. By sparking later the power produced is reduced. This is more sophisticated than we were expecting, the Italians could learn from this!
Ok, with the wire coming from the transmission casing disconnected the restriction is removed and the Falcon made it all the way up a speedo reading of 48mph. As with all mopeds the speedo reading was a bit optimistic and the dyno showed the genuine top speed to be 42mph.
There was also a variator restrictor ring fitted to the Falcon similar to those fitted to Piaggio/ Yamaha mopeds but it wasn't actually doing anything. It was too thin to be restricting the variator in any way and made no difference to the performance whether it was removed or not. We removed it anyway on principle :)
Ok, here is how the Falcon performed in standard form but de-restricted.
This is actually quite promising, for comparison a standard Aerox would make slightly more (up to 4.5hp) but not a lot different. You can see that the transmission is not holding the revs constant and they are creeping up from 6500rpm to 8000rpm until it gets to 35mph at which point the variator is fully moved out and the only way the moped can go faster is to increase the revs (in the same way as a geared bike or car).
The variator should be holding the revs constant really up unto this 35mph point, a new Piaggio or Yamaha moped would have shown an almost horizontal graph line here but in their standard form most mopeds have a fairly wide powerband so it might not be worth spending time tuning the transmission to straighten it out and we'll leave it as it is for now.
To have a better idea of what's going on we need to see an engine power curve which shows what power the moped is actually making at any given rpm. Because of the way the variator works by constantly change the ratio between the engine rpm and rear wheel rpm scooter dyno graphs normally only show what power the scooter is making at different road speeds.
In order to get a fixed gear ratio graph (power curve) we need to modify the transmission so as it is fixed drive (like a geared motorbike). For this we use special hubs to replace the variator and rear torque drive which connect together using a toothed belt (like a car cambelt).
This gives us the following graph...
click to enlarge...
From this graph above we can see the engine has a good wide powerband until it starts to exceed 8000rpms at which point the power starts to drop off quite quickly. In the first graph we can see that 8000rpm comes at 35mph and as you would expect the power starts to drop off quite quickly in the first graph after 35mph. Because the variator is fully changed out at this point this means that it is impossible to exceed 35mph without exceeding 8000rpm regardless of any change of rollers or other transmission tweaking. This means it will be impossible to improve the top speed from 42mph (48mph clock speed) unless we either alter the power curve of the engine above 8000rpm or change the gearing. Although Stage6 have now started to produce a range of gearup kits for Chinese scooters (the Aerox/ Jog ones won't fit), at this stage it's unlikely the scooter is making enough power to benefit from this so we can dismiss this option for the moment.
The first option which normally offers a substantial performance benefit for an otherwise standard 50cc 2-stroke moped is of course to change the exhaust.
The standard Baotion Falcon exhaust looks like a sport expansion powerpipe already. But it isn't !
It is in fact incredibly restrictive. It's weight gives a clue as to the large amount of internal workings contained inside, and some forums have mentioned that you can cut into the can, remove the internals, re-weld it back up and it then becomes a modestly decent sports pipe. Unfortunately this isn't true. Whilst it is true the exhaust contains a large amount of baffles, internal plumbing and a catalytic converter the actual shape of the pipe isn't right for a sports pipe. The angle and design of the cones are made purely to aesthetically mimic a sports pipe but are in the wrong places to actually work as an expansion pipe. To prove the point we cut open and completely removed the internals of the standard pipe and then rewelded it.
The exhaust became significantly louder but offered no performance increase whatsoever as shown in the next graph below.
In fact by the end of the day it was clear that the standard Falcon exhaust was its single most restricting factor and it is virtually impossible to get any significant gain out of the Falcon without ditching the exhaust in favour of something better.
We tried a few different exhausts to see what sort of power curve would result. It was quickly clear that some of the higher revving pipes were too much for the standard 50cc cylinder with it's fairly mild timings and consequently the mild sports pipes where the ones which worked the best.
An unexpected surprise was how well the Motoforce standard replacement Yamaha Aerox exhaust performed on it.
The Motoforce exhaust (pictured above) is a small, discrete quiet 'standard' replacement exhaust normally sold for Yamaha Aerox for £49.95. It is however different to a standard exhaust in that it actually contains a half-wave expansion pipe within. A half-wave pipe is similar to a sports pipe and can be contained within a much smaller volume than a full wave expansion pipe. Consequently they can be made to look like standard exhausts. The disadvantage over a full wave sports pipe (e.g. Leo Vince ZX etc.) is that although they normally make good power this power is normally made at fairly low revs. Because the Falcon standard cylinder is designed to make power at low revs also they seem to compliment each other well. In fact there was no pipe which would make a higher peak power when fitted to the standard Falcon cylinder than the Motoforce one.
The following graph show the power graphs for the Baotian Falcon with standard pipe and then with the Motoforce pipe and then Leo Vince ZX (most of the sports pipes we tried e.g. Stage6 Pro Rep gave a similar graph to the ZX).
click to enlarge...
As soon as the standard pipe was gone the engine would happily rev a little higher which resulted in a slightly higher top speed of 45mph (52mph clock speed) with both the ZX and Motoforce pipes. All the powerpipes made a peak of around 6.3hp on the Falcon standard cylinder which is a substantial improvement over the standard 4hp.
For comparison a Yamaha Aerox or Piaggio Zip would make around 7hp with a ZX or similar sports pipe fitted, so this really isn't bad at all for a Chinese moped.
It's clear however that the Motoforce pipe is the out and out winner for this bike for the following reasons...
Below are the final results. We also fitted a 70cc Malossi cast iron sports cylinder kit for good measure.
It should be noted that in all cases including the standard de-restricted bike the main jet size was increased to 65 (6mm dellorto jets will fit) in final testing. This is necessary otherwise the moped starts to run a little lean. In summary...
1) Baotian Falcon + Motoforce Exhaust (£49) = *almost* but not quite the same performance as Yamaha Aerox + decent sports pipe(e.g. Leo Vince ZX/ Stage6 Pro Rep)
2) Baotian Falcon + Motoforce Exhaust (£49) + Malossi 70cc kit (£108) = same performance as Yamaha Aerox + decent sports pipe(e.g. Leo Vince ZX/ Stage6 Pro Rep)
As an after-note with both the Malossi 70cc kit and Motoforce pipes fitted we tried swapping the standard Chinese carb with something larger but only very modest gains were noticed so it probably isn't worth changing.
