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This blog is a follow-on from our short blog introducing the basics of dynos "Dyno Moped Tuning" and is intended to show how a dyno can be using in the workshop to very quickly setup a modified moped.
The following is only a very basic setup which can be done very cheaply (approx £40) but still normally makes a very significant difference and takes about 30 minutes. Our workshop does dozens of setups like this every month.
The moped used is an '08 Yamaha Aerox 50cc '08. The customer bought the following parts...
Leo Vince ZX powerpipe
Stage 6 Sport Pro 70cc Kit
Polini Open Air Filter
Malossi Gearup Kit
and fitted them himself...
He also rejetted up from size 62 main jet to size 82 (~32% increase) and dropped the roller weight from 5.5gr (standard) to 4g
He'd been driving it around for a few days to run it in and wanted us to check it was all set up properly and not going to blow up (!)
The first thing we do is run the bike up on the dyno to see how it performs. This gives us the graph below...
(Click on the graph to enlarge it)
Ok, so what does this tell us ?
First of all from the top graph we can see the engine is making almost 10hp at the rear wheel, which is a perfectly good peak reading for the setup. What the top graph also tells us though is that it isn't actually making this peak power for very much of the time. You can see it's making more than 9hp only between 18 and 26mph, at 30mph it's dropped down to around 5hp and creeps back up to over 8hp between 39 and 42mph after which the power drops off very quickly.
If we look at the 3rd part of the above graph which shows the engine revs we can get a good idea of why. In a previous blog Malossi Sport vs Polini Sport vs Stage6 Sport Pro 70cc we ran up a few different cylinders on an Aerox with the same Leo Vince ZX pipe which gave us the following graph...
(Click on the graph to enlarge it)
This graph is a special test run with the variator locked which shows us that the Stage 6 Sport Pro with the ZX pipe makes maximum power between 8900 and 10500rpm. In fact this is a very wide peak powerband if you compare it to the Malossi or Polini Sport kits, so it should be easy to setup. All we have to do on our customer's moped is make sure the revs stay between this range and we should have 10hp available all the time. If we look again at the top graph we can see that where the bike is not performing correctly corresponds exactly to where the bike falls out of this rev range. For example up to approx 25mph the revs are above 9000rpm and consequently we have high power output... at 30mph we can see the revs have dropped right down to 8000rpm which is simply too low, the Stage6 70cc kit is only making 5hp here.
It's the job of the variator to hold the revs as dead steady as possible. Unfortunately the Aerox standard variators are notoriously poor at doing this. On a standard cylinder with a wide powerband it doesn't matter so much if the revs wander about, but on a modified bike with a narrower powerband this becomes a real problem because the moped can easily drop out of its powerband and it's normally necessary to ditch the standard variator and replace it with something decent to hold the revs steady to get good results. Incidentally our variators of choice are Malossi Multivar or Stage6, both of which perform superbly well. We used to also rate Polini variators very highly until they were recently replaced by the new cheaper 'High Speed' model.
You can see in the top graph that our standard Aerox variator allows the revs to wander between max 9500 and min 8000 revs, which is a range of 1500rpms and a huge change- however because we've already established that the Stage 6 Sport Pro Kit makes peak power between 8900 and 10500rpms (a range of 1600rpms) we might just about be able to make it work with the standard variator.
It's worth noticing though that the Polini Sport and Malossi Sport kits make peak power only for a far smaller range of around 500rpms and there is no way they could be made to work properly with this standard variator, they would need a decent aftermarket variator which can hold the rev range within 500rpms across the whole speed range of the bike and the bike would have to be set up carefully to make sure it stayed within this range.
This is one of the reasons we like the Stage6 kits so much, they make more power than the other kits in their class and are far easier and more forgiving to setup.
Ok, now let's take a look at the fuel mixture (jetting) this is in the 2nd part of the top graph. From our previous blog "Dyno Moped Tuning" we know that we want the AFR to be running around 13:1, from the graph we can see that our moped is spending most of its time running at over 16:1, this is much too lean and the jet needs to be bigger even although the customer has already increased from 62 to 82. Although the bike will seem to run ok and makes good power there is a serious risk of the engine seizing on a long run with this sort of reading. Having more fuel present helps to keep the cylinder cool, if the bike is too lean the piston will expand until it is big enough that it starts to no longer fit within the sides of the cylinder. This additional rubbing friction very quickly overheats it more and in turn the piston becomes so big that it jams against the sides of the cylinders often causing the engine to lock. On a manual geared bike this would involve either a locked rear wheel at high speed (scarey) or a trip over the handlebars, luckily on a moped the transmission usually just disengages harmlessly. If you're paying attention you may notice the warning signs as the moped starts to loose power before it actually seizes (due to the additional friction between the enlarging piston and the cylinder walls) and back off or let it cool down. If you're lucky the bike might run again after leaving it to cool down once it's seized but probably with reduced power.
When you take the cylinder apart you can usually identify this type of seizure by the fact that the piston rings are often 'melted' into the piston ring grooves and consequently no longer spring out at all.
There is a lot of mis-information being spread about moped jet sizes. If you fit a sponge type air filter you will normally have to increase the jetting by around 50% to avoid risking engine damage or unreliability (Piaggio even bigger, a Piaggio 50cc will amazingly sometimes need over 100 jet to run well and safely- basically you want the biggest jet size the moped will perform well with, just keep increasing the jet size until the performance starts to drop off and then go back down one step.
Here are some jetting increases for Yamaha Aerox *only*.
We obviously can't take any responsibility for people using these jettings but they based on literally hundreds of runs and are normally a good starting point so long as there are no other problems with the bike such as air leaks etc.
So... as in the example of our customer's Yamaha Aerox with sports pipe, 70cc kit and Polini open sponge air filter we're going to rejet it with a 96 jet and see how it goes. We're also going to drop the roller size by 0.5gr which isn't much, but we're expecting to see a power improvement from the larger jet so 0.5gr might be all that is needed. We're also going to slightly change the weight of the rear torque spring back to a standard one as it seemed to have had a heavier one fitted which we think could be adversely affecting the variator function. You only really need a stiffer torque spring for very high power bikes, basically the spring just has to be strong enough to stop the belt from slipping, any heavier it will just grip the belt too tightly and lose power due to the additional drivetrain frictional losses.
Also the customer had fitted a standard Aerox short reach plug, but the Stage 6 kit is designed to run with a long reach plug. Using the short reach plug lowers compression and can easily result in a drop of 0.5 to 1hp at certain rpms.
So... despite all the talk all we've actually done is change the spark plug, changed the main jet, tweaked the roller weight and changed the torque spring, let's run the bike up again and see what we affect this has...the green line is our new run
(Click on the graph to enlarge it)
We can see straight away that we now have more than 9hp available all the way from 16mph right up to over 65mph whereas before it was only between 18 and 26mph. In fact before the bike wouldn't run much over 50mph (probably showed 60mph on notoriously inaccurate moped clocks), now makes a genuine 70mph and with almost 10hp immediately available at any speed the acceleration is obviously massively improved. With the drivetrain tweaks the variator is now able to entirely maintain the engine revs between 8900 and 10,000rpm which is exactly within the peak powerband of the Sport Pro kit as discussed earlier.
We can also see that the mixture is now firmly in the safe zone so no risk of seizures.
This dyno work is obviously at the very basic end of tuning, we could make more improvements by spending more time and money on the bike, but for just 30 mins work and a charge of £40 +rollers and a jet the customer is certainly going to notice a substantial difference for his money with the bonus that it's not like to seize as it was before.
Next time we'll progress onto some more advanced performance mods ! Happy tuning !
