This is a blog to show how restrictive the standard 12mm carb is as fitted to a mildly tuned Yamaha Aerox 50cc moped, and subsequently explain why standard Aerox's sometimes struggle to pull away after a powerpipe has been fitted

The following dyno graph is for Yamaha Aerox which was standard apart from a PM Tuning 360 exhaust, Polini 70cc sport cylinder big bore kit, and gear up kit.

click here to enlarge...

Everything else on the Aerox was completely standard apart from the variator restrictor ring being removed.

The standard carb was of course a standard Aerox 12mm carb, the 17.5mm carb was a budget but very capable 17.5mm carb , and the 19mm carb was a budget 19mm TNT carb
The standard air filter and airbox was used on the standard and 17.5mm carbs and an open filter was used on the 19mm TNT carb. The only reason for using the open air filter was that the standard airbox doesn't fit onto the TNT carb. There is however no performance benefit whatsoever of using any air filter other than the standard airbox at this level of tuning (see this article for more details)
The jetting and transmission was tweaked with each carb to ensure the mixture and revs were identical for each carb for a fair comparison, and the runs were done on a load cell dyno which allows a load to be programmed to accurately recreate wind resistance, driver weight and other real world circumstances to ensure the comparison would be identical to that experienced in real life use.
We could have professionally optimised the transmission to improve all the results all round but instead left it basically untouched to better replicate real life user experiences. Also it wouldn't have made any difference to how the graphs compared relative to each other

As you can see, although there is substantial benefit in increasing from 12mm to 17.5mm there is no performance benefit in this instance from increasing thereafter. This relates in part to the fact that as the carb inlet area increases the velocity of the airflow signicantly decreases to flow the same volume of air unless the bike is able to utilise a higher volume airflow (which it can't at this level of mild tune)
The higher revving kits e.g. Malossi MHR rep or Stage6 Race benefit more from a 19mm carb but realistically 21mm is the maximum optimal size even at this higher level of moped tuning.
It's interesting to note that Piaggio mopeds already come with this larger 17mm carb size fitted as standard, and as such there is no real performance benefit in changing from the original Piaggio carb unless you are tuning past the level of 70cc sports cylinder + sports pipe.

This graph is also interesting in that it shows insight into a problem which is common in standard 50cc Aerox scooters which have a slightly revvy sports pipe fitted e.g. Leo Vince ZX.
This is probably the most common problem we see in the workshop with Aerox mopeds where the customer has fitted a sports pipe to a 50cc Aerox and involves the bike no longer pulling away properly.
The customer will complain that whilst the bike worked fine with a standard exhaust, it will no longer pull away properly with a sports pipe fitted. When cold the bike will often be fine, but once warmed up the bike will struggle to pull away (especially on hills). On the flat the bike will often pick up speed very slowly until it eventually suddenly picks up and takes off all at once at around 20mph+... until the bike comes to a stop again at the next set of lights at which it struggles to pull away again
We can see this starting to happen if we look at the graph above at point A. You can see that the green graph for the standard carb is consistently almost exactly 1hp lower than the 17.5 and 19mm carbs at all speeds *except* below 25mph. Below 25mph the power is very significantly down on the standard carb, at 10mph the 17.5mm carb is allowing the engine to develop a massive 70% increase in power of the standard carb which is completely out of proportion with the 1hp (approx 12%) increase seen at higher speeds but why is this ?

The answer is that it's mostly due to the fact that the original Aerox moped variator is particularly poor when used with anything other than a bog standard Aerox.
The function of the variator is of course to hold the revs constantly at the revs at which the scooter makes peak power, thus ensuring peak power is always on tap instantly whenever you mash the throttle, but we can see from the lower point A on the graph that the variator is initially struggling to get the engine up to peak power (9000rpm in this case). This lower revs obviously explains the lower power until the revs match that of the other carbs at 25mph. If we look at this graph here which is for a 50cc standard cylinder...
click here to enlarge...

and look at the graph for the standard Aerox exhaust you'll see that at anything below 6800rpm the standard exhaust makes more power than a Leo Vince ZX, TT, and Stage6 Pro rep, or indeed any other half decent sports powerpipe which has a high (but narrow) powerband.
Although ideally the variator would allow the engine to rev straight to peak power whenever you throttle, the standard aerox variator tends to always engage progressively meaning that it requires a fair amount of power to initially get it there. On the high performing pipes, especially in combination with a worn standard Aerox variator this progressive application means that due to the pipe developing lower power than a standard pipe at low revs it sometimes simply can't pull through to get to the higher revs.
The problem is also particularly prevalent on Aerox mopeds because of the additional power restriction from the 12mm carb. As you can see from the Point A section in the top graph, even with a 70cc Polini sport kit fitted the slightly lower power of the 12mm carb can be seen to substantially increase the spin up time of the variator and consequently the whole performance up to 25mph is significantly affected as it struggles to make it through the progressive application of the variator
If we were to repeat the runs with a decent variator fitted such as a Stage6 Sport Pro Variator we would see Point A significantly improve and the rpm graph for the 12mm carb come much more into line with those of the other carbs.

So in conclusion...!!! When tuning an Aerox it is particularly beneficial to ditch the standard carb and standard variator even at the most basic level of tuning even although these rules would not be such an early priority when tuning other mopeds such as Piaggio/ Gilera