This is the first in a series of short blogs we're going to be doing which covers some of the basic queries which we're asked on a regular basis.
This blog addresses the question of whether or not it's important to fit a gear-up kit after fitting a 70cc cylinder kit
Firstly it's important to recap how a moped variator works in it's basic form. If working and set up properly a moped variator allows the engine to run at peak power constantly as the road speed increases. It does this by seemlessly and constantly changing the gearing over an 'infinite' range as the road speed increases to keep the engine rpm constant at a set rpm determined by the choice of roller weight. This means that whenever you mash the throttle the revs jump to the rpm at which the engine makes the most power (provided the optimal weight rollers have been fitted) and keeps them there as road speed increases meaning that the full peak power of the engine is always immediately available. This is in contrast to geared bikes where the revs go up and down in direct relation to the speed of the bike and you are virtually never at peak power rpm but only as close to it as you can maintain by constantly changing the gears.
There is an important limitation to the moped variator though in that it can only keeping changing the gearing in this way until you reach a certain road speed at which the variator is fully changed out. As soon as this speed is reached the only way that the road speed can increase further is if the engine speed also increases... in effect the moped then becomes a geared bike and the road speed can only increase in direct proportion to any increase in the engine rpm.
This 'fully changed out speed' is affected by roller weight but only up to a point (which is why roller weight doesn't affect top speed (see Moped Tuning- ROLLERS)) but it is affected predominantly by the final drive gearing in the gearbox (can be changed with gear kits) and other factors such as tyre or wheel size which do affect top speed.
The following graph shows a dyno printout of a standard Aerox...
Click to enlarge...
You can see here that the variator is holding the revs at around 7000rpms (where a standard aerox makes peak power) *until* it reaches 33mph at which point it can no longer continue changing gear and the revs increase on a 'fixed gear slope' until it gets to 9000rpms at 42mph at which point the standard Aerox is no longer making enough power to continue to accelerate.
If you change roller weight the 7000rpms will change *until* it meets up with this 'fixed gear slope' as in the following graph...
Click to enlarge...
This graph is of an unrelated moped but shows clearly how this fixed gear slope comes into play with different weight rollers. For all roller weights point 'A' shows the point at which the variator is fully changed out and the ped can only accelerate further if the rpm's increase. Although these points are at different rpm's they all meet on the same slope and this slope cannot be moved without changing the final gearing (e.g. gear kit, different size wheel etc.)
Ok, so now let's fit a Polini Sport 70cc kit and a Leo Vince ZX pipe to our Aerox.
The following graph shows how much power the most common 70cc sports kits make at different engine speeds with a ZX pipe fitted.
(click to enlarge...)
We can see that whilst our standard Aerox was making peak power at around 7000rpm's our 70cc Polini Aerox with ZX pipe is now making much more power, but it now makes this peak power at 9,300rpm instead of 7000rpm. This means we have to fit lighter rollers so as the variator runs at 9,300rpm.
If you look back to the first graph you can see a red dotted line graph added at 9,300rpm which simulates where the engine revs will be set when optimised for our new cylinder and exhaust for maximum performance.
However we've already discussed that it can only hold these revs until the variator is fully changed out at which point it hits our 'fixed gear slope line'. We can see on the first graph that this happens at 45mph with standard Aerox gears. We can therefore see it is impossible to exceed 45mph on a standard Aerox under any circumstances without the engine revs exceeding 9,300rpm.
If we refer back to the 3rd graph showing the power curve for our modified engine we can see the power developed by our Polini/ ZX engine very quickly drops off as the revs start to exceed 9,300rpm....
This means that it is therefore impossible to make full use of the power available from a Polini Sport/ ZX Aerox at any speed over 45mph without a gear kit fitted.
Without a gearkit you are very quickly completely out of the powerband over 45mph. At 50mph the engine is doing almost 11,000rpm and making around 6hp which is way down on peak power and although high enough to maintain 50mph, it isn't enough to significantly increase the speed much further. This means that the bike will spend most of its life over-revving at 11,000 just over 50mph. Because these sports kits aren't designed to run so high they are easily damaged by continual over-revving and the crank is put under more stress running at 11,000rpm under 6hp than it would have been running 9hp@ 9,300rpm.
For these reasons it's imperative that a gear-up kit is used with Malossi or Polini Sport 70cc kits.!!
The addition of a gear kit (up to 20% for this level of tuning) would shift the fixed gear slope significantly to the right allowing approx an additional 8 or 9mph of being held at 9,300rpm. This means we now have the full 9hp available right up to almost 55mph whereas without the gear kit we're down to 6hp (and falling very fast) at just 45mph.
Because once you get up around 55mph you need *significantly* more power to continue to accelerate than you do at 45mph the sports kits tend to limit themselves nicely as the power drops off when the revs finally do increase over 55mph with a gear kit fit and it's therfore not really possible to over-rev them
It's interesting to note that the 'mildest' sports kits i.e. malossi sport and polini sport are the ones where it is most important of all to have a gear kit fitted
The reason for this is that high power kits do not drop off in power at such low revs - e.g. if you look again at the power curve graph for the sports cylinder kits (our 3rd graph) you can see the Stage 6 one continues to make peak power all the way from 9000rpm to almost 11,000rpm. For this reason these sorts of kits will always destroy the Malossi and Polini sports 70cc kits on final top speed although at the price of running at higher revs (they should not however be run on a standard crankshaft as opposed to the Polini and Malossi kits which will happily run on a standard crank).
There is a widespread belief that you will loose acceleration when fitting a gear kit which is a trade off for the higher top speed but this isn't entirely true. This would be true on a single geared bike, if you had to choose only one gear to use on a geared bike then obviously 1st geared would accelerate best but have the lowest top speed... most people think therefore that fitting a gear kit is like selecting 2nd gear instead of 1st gear where you have slightly worse pull off but higher top speed.
Because of the variator mopeds don't work quite like this... the revs are still held at peak power, so in theory if your bike is making 9hp peak power and the bike is set up correctly then when you will still have 9hp available at 10mph when you mash the throttle regardless of what your final gearing is because the final gearing is being compensated for by the variator to have the same overall ratio as is necessary to keep the engine speed at our 9,300rpms. A bit like fitting a smaller gear on the back of a BMX only to compensate it with a smaller gear on the front also - net result is the same only you have the advantage of the higher overall top gearing.
In reality though the variator works less efficiently on initial pull away and struggles to be efficient until the moped is up and moving. Because of the higher final gearing the moped has to initially accelerate through a higher difference in speed until it is fully engaged and working efficiently which can affect pull away initially especially if the engine is not particularly powerful or well matched to the final gearing.
This however means that as soon as you exceed approx 15mph there isn't really any perceivable acceleration difference between a bike with or without a gear kit fitted. So unless you're frequently stopping and starting and in contrast to popular belief you can even quite happily use a gear kit on a standard 50cc bike with a decent powerpipe fitted.
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
This article deals with Piaggio ignition fault finding where the scooter has no spark and covers Piaggio Zip 4-stroke 50cc and 100cc
Testing for a spark
We've already covered testing for a spark in our previous article Piaggio Ignition Fault Finding which dealt with 2-stroke Piaggio Moped ignition, so we won't cover it again but suffice to say it isn't enough to look at the sparkplug to see if there is a satisfactory spark or not. In the workshop we use a simple spark tester or you can make your own as described in the linked article.
As described in the previous article you need to see a spark of at least 6mm otherwise there is an ignition problem.
Click to enlarge...
The ignition components and wiring is the same on the 4-stroke 100cc zip as the 4-stroke 50cc zip and is almost completely independent to the rest of the scooter wiring/ electrical system. This means that the ignition can continue to function and the engine continue to run regardless of any faults that occur in the lighting/ battery/ other electrical functions of the bike.
Once you've established there is no spark or an inadequate start it's fairly simply to start narrowing things down, in the previous article we explained the basics for using a multimeter so we'll skip straight to the testing.
Testing the flywheel pickup
The flywheel pickup sends a pulse to the cdi everytime the flywheel completes a full rotation and from this the cdi can calculate the speed the engine is spinning (rpm) and the exact position of the crankshaft (and therefore the piston) so as it can make an informed decision of when to make the spark plug spark.
To test it unplug the connector block from the cdi which is found behind the panel which sits behind the rider's heels (close to the spark plug). There should be a reading of between 120 ohms and 180 ohms between the brown pickup wire and earth point (e.g. the - terminal on the battery)- you may need to push a pin into the connector block to access the pins.
If you get this reading the pickup and pickup wiring are almost certainly ok, it's very rare for there to be a problem if you get this reading.
If you don't get this reading unplug the 2 wire connector coming from the stator and test the wire coming directly from the stator/ pickup assembly (by the fan cover) which was connected to the brown wire- if you still don't get the correct reading directly from here then the pickup is faulty and will need replacing.
If you are getting the correct reading at the cdi then remove the fan cover. As you turn the flywheel with your hand you'll see it has a 'nobble' which passes the pickup every rotation, the gap between this nobble and the nobble on the pickup should be no more than 1mm, any more than this and the pickup may not be triggered.
Testing the killswitch wire
When you turn the ignition key off this connects the green wire to earth and tells the cdi to stop sparking. If you cut or disconnect this wire you won't be able to turn the moped off with the key, so to test this all you have to do is check for continuity between earth (e.g. negative at the battery) and the green wire at the cdi connector (with the block disconnected)- there should be no continuity whatsoever (test on a high range e.g. 10 million ohms) when the key is in the 'on' position and there should be 0 ohms (full continuity) between this wire and earth when the key is in the off position. Check carefully, if the ignition switch is faulty it can have a 'medium' resistance which may allow the bike to start but cause poor running or cutting out if say the switch gets wet. If you're unsure you can temporarily disconnect the green wire from the cdi to eliminate it for testing.
Click to enlarge...
Testing the earth wire
This one is easy, just disconnect the cdi connector block and check with your multimeter there is continuity between the black wire and earth. You should have a reading of 0 ohms.
Testing the stator coils
The 4-stroke Piaggio Zips are a little unusual. Normally on mopeds there is one stator coil for the ignition which powers the cdi, and completely separate charging coils to power the lights and charge the battery. On the 4-stroke Piaggio Zip scooters there is just one set of coils (the blue/grey wire) which powers the cdi *and* also goes to the regulator to charge the battery and power the lights. This is why there are only 2 wires coming from the stator assembly- one for the pickup coil (brown) and one for the stator coils (blue black). The coils also connect to earth internally through the bolts holding it to the engine.
To test it disconnect the cdi block connector and check the resistance between the blue/grey wire and earth, it should be approx 0.7 ohms. If you then crank the engine with the multimeter set to AC volts (not DC!) you should get a good 8-9volts AC between this wire and earth, this reading should not drop substantially if you test it again with the cdi plugged in.
If you don't get this reading disconnect the regulator connector block temporarily as the blue/ grey wire also goes there and this eliminates the possibility (although unlikely) that the regulator is faulty and shorting out the wire and then repeat the test. The regulator is located under the front fairing panel and can be recognised by its cooling fins.
If all of the above tests check out ok there is no other real option than to replace the cdi module
Polini tuning parts prices in the UK are already highly competitive and probably the cheapest in the world !
Because of this Polini have prohibited their legitimate dealers from selling their scooter parts at any more than 20% discount off the Italian retail price.
However, as the exchange rate constantly changes so does the Italian retail price.
Therefore in order to continue offering you the cheapest Polini parts prices possible in the UK our Polini moped parts prices will now constantly adjust themselves throughout the day on the Pedparts website as the exchange rate changes always guaranteeing you the cheapest prices available anywhere !
Couple this with genuinely impartial buying advice, our unbeatable next day service (free shipping over £60), constant high stock levels, and our unrivalled expert technical support from our fully qualified workshop staff (combined 15yrs specialist dyno tuning experience) we believe this is a truly unbeatable package !!
You can search for Polini part numbers directly from the search bar (leave out any dots!) or browse in the normal manner (click here)
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Part 1: Guide to jetting a Moped ! An introduction to the basics.
This blog is intended to help answer some of the questions we're most commonly asked about moped jetting....
- My moped has a brand X air filter and brand X pipe fitted, what size jet do i need?
- Will my moped go much faster with an open air filter fitted?
- Will my moped use more fuel with an open air filter
- Can I jet my bike by doing plug chops? If so how ?
First of all the boring stuff- feel free to skip straight to the 'ANSWERS TO THE QUESTIONS'
INTRODUCTION: WHAT IS JETTING ?
'Jetting' your bike means tuning or adjusting the carburettor so that it is always supplying the correct ratio (mixture) of petrol/ air at all times to the engine. Roughly speaking it's necessary for the mass of air to be approximately 13 times as much as the mass of fuel for the engine to burn all the available fuel and develop it's full power potential (and run safely and reliably).
If the mixture ratio exceeds this (perhaps the jet is too small or there is an air leak) then the engine is said to be running lean, or in other words the fuel/ air ratio needs to be altered so as there is more fuel in relation to the mass of air.
The problem with a bike running lean is that not only are you down on power but more seriously the engine can run much too hot. This can cause seizures (where the piston expands from excess heat so much that it no longer will move within the cylinder) or can even melt the piston.
On a 50cc moped we find that once the mixture reaches around 15:1 this is the point where there is a real risk of seizure and engine damage especially on long full throttle rides.
In contrast if the mixture ratio is much less than 13:1 the bike is said to be 'rich' whereby there is not enough air present for the fuel to be fully burnt. This could be caused by for example a restrictive or dirty air filter or a jet that is too large.
Running rich isn't as potentially catastrophic as running lean because the excess unburnt fuel acts to cool the combustion chamber, but again you won't be reaching the full power potential of the engine and wasting fuel.
On a 50cc moped we find from experience that once the mixture drops much below 12:1 it can start to cause spluttering and significant power loss.
Normally when tuning we would aim for a mixture around 12.8:1, this seems to make the most power whilst being without much risk of seizure.
One of the main problems with carb tuning is that the mixture it supplies can be affected considerably by many factors such as throttle position, temperature, engine revs and the load on the engine (how hard it is working at any instance), and as you adjust the mixture for one cirumstance it affects the mixture for other cirumstances and consequently *real* motorbike carb tuning is quite an art requiring considerably experience and knowledge.
This means that bike manufacturers have to tune their carbs so as the correct mixture is supplied at all times. The mixture ratio needs to always remain within an acceptable range whether the bike is ticking over at the traffic lights, cruising at half throttle, or powering full throttle up a steep hill.
The shape and position of the needle largely dictates the mixture between 1/4 and 3/4 throttle, the idle jet circuit affects the mixture at up to around 1/4 throttle and the main jet affects the mixture at all throttle positions. Some carbs also have extra jets which come into play at high revs or high load as a method of fine tuning. At full throttle however the main jet generally has the most significant effect on the mixture.
Fuel Injection
Fuel injection has considerable advantages over carbs. You can program the injectors to inject the exact amounts of fuel you want depending on throttle position, rpm, load, temperature etc. and have a far more accurate control over the 'jetting'. Some more expensive bikes are now equipped with 'closed loop' fuel injection systems.
A closed loop system incorporates a lambda sensor in the exhaust system which is able to continuously and accurately read the mixture ratio and instantly instruct the bikes computer to correct the mixture by injecting more or less fuel under any circumstance.
This effectively means the mixture is always perfect at all times whatever you do. Tuning fuel injection bikes is much more predictable anlyd logical than carb tuning which tends to be lots of trial and error but is obviously (unfortunately) out of the realms of most diy'ers due to the specialist equipment and understanding needed and often requires expensive add-on control modules to allow modification.
It's all starting to sound a bit complicated !
Actually, no. Mopeds have one significant advantage which makes moped carb tuning very accessible for the home tuner.
Because 50cc mopeds are relatively low power they tend to spend most of their life with the throttle mashed all the way open. This is especially true of the mopeds owned by the sort of person who is going to tune it ! Also because of the way the variator works we have the bonus that the engine is spending most of it's time at a fixed rpm.
This means that all we really care about when jetting a moped at a basic tuning level is making sure the mixture is correct whilst at full throttle, full load, fixed rpm and we don't much care about anything else. The mixture under these conditions can essentially be entirely controlled by the main jet size. Happy days !
For the purpose of the rest of this article we'll therefore only be considering full throttle/ full load when we talk about jetting or mixture ratio
ANSWERS TO THE QUESTIONS !!
- My moped has a brand X air filter and brand X pipe fitted, what size jet do i need?
For now we'll just consider mopeds up to the level of tune of sports 70cc with sports pipe fitted. Whilst there is truth that all tuned mopeds should be custom jetted individually, at this fairly mild level of tune you can jet predictably without using trial and error or any other method.
The basic rules are that if you've fitted a sport pipe the jetting should be increased by 10%, and the only other thing that substantially effects jetting at this level is fitting an aftermarket filter. If you go from 50cc standard cylinder to 70cc sports cylinder you sometimes need to upjet very slightly by perhaps 2, but rarely enough to be of importance unless you're fitting a more highly tuned high revving kit.
As an example we ran up an Aerox which was completely standard except there had been a Leo Vince ZX pipe fitted
As we know from earlier we want our mixture ratio to be around 12.8, and the graph shows a helpful guide 'safe zone' area as anywhere between 12.5 and 13.5
As standard an Aerox is fitted with a 62 jet, and on this moped it had not been changed after the pipe was fitted. Although a 62 jet would have run perfectly with a standard pipe we can see that with the ZX pipe fitted the bike is actually running lean at almost 16:1. Although the bike will normally seem to run ok like this, 16:1 is at the point where the bike is at real risk of seizing, especially if you ride it hard. It may work for months like that but it's definitely in the danger zone. We often see mopeds seize which haven't been rejetted after fitting a sports pipe, frequently it happens on a long (e.g. 15 miles) bike ride.
Applying our 10% increase rule if we swap the 62 jet out for a 68 we can now see the jetting is running at a much healthier mixture ratio peaking at around 12.8, and we've also gained a significant 'free' increase in power of almost 15%.
99% of the time an Aerox with a sports pipe will run perfectly like this with a 68 jet. Sometimes in cold weather or with a 70cc sports cylinder kit we would maybe have to increase to 70, but still very close.
How is the jetting affected if we now fit an aftermarket filter ?
Aftermarket air filters affect the jetting more than any other factor. If we completely remove the air filter the carb needs a 104 jet to run. We ran up some of our air filters on the Aerox with the ZX pipe to see what jet would be needed, these are the results... you can see how restrictive they are by how close they come to the maximum 104 figure where no filter is fitted
Small Polini Covered Air Filter- 88 jet, Large Polini Covered Air Filter- 96 jet
Malossi replacement for standard airbox- no change, identical jetting to standard filter
PM Tuning Filter- all PM filters are virtually straight through and ran with 102 jet with our Aerox
Polini open foam filter- 88 jet- this is quite a small filter, the large open Polini filters needed closer to 96 jet
STR8 covered filter- 96 jet- this has very high flow rate for a 'covered' filter due to its 'corrugated' filter giving a much larger surface area than you'd expect
Koso (partially) covered filter- 100 jet- like the STR8 filter this has very high flow rate for a 'covered' filter due to its corrugated filter
These figures are valid only for Yamaha Aerox with sports pipe running anything up to a sports 70cc kit as this is probably the most common setup. We'll try to build up a database to cover other bikes as time goes on.
- Will my moped go much faster with an open air filter fitted?
Not until you get to quite a high level of tune. If we run a bike up at any state of tune up to 70cc sports cylinder with sports pipe the dyno results will always be virtually identical when comparing a sports open filter to the standard airbox. It's unlikely to run any worse with an open air filter fitted but you aren't really going to see any advantage. Straight through filters come more into there own at much higher levels of tune with engines running at high revs where the more restrictive original filter becomes less able to allow through the volume of air required.Keep in mind that airboxes actually serve a useful purpose which we'll cover at a later date. Some bike manufacturers spend considerable amounts of money researching and perfecting airbox design for super and sport bikes. Some superbikes will drop 2% power if you remove the manufacturer airbox in favour of unrestricted plain filters.
- Will my moped use more fuel with an open air filter
Let's find out ! We set our Aerox up correctly jetted firstly with a standard airbox (68 jet) and then secondly with an open Polini foam filter (96 jet).
To compare fairly we need the bike to experience an identical load each time we ran it.
To do this we set the dyno to lockdown at 45mph. This means that once the dyno drum starts to reach 45mph it will automatically calculate and apply a braking force to the drum (with electromagnets) to ensure that the drum remains at 45mph and cannot exceed it no matter how hard you accelerate. This is basically the equivalent to finding a hill which is exactly the right slope that the moped remains exactly at 45mph at full throttle all the way up.We gave the moped exactly 200mls of petrol through a special tank and ran it at wide open throttle at 45mph first with the standard and then the open filter.
With the standard filter it ran for 4mins 15secs and with the open ran for 3mins 40 secs with the same amount of fuel. This works out at 71mpg for the standard filter and 61mpg for the unrestricted open air filter. In both runs the dyno showed an identical 7.0hp +/- 0.2 throughout
So, yes an open filter does use more fuel but maybe not by so much as many people believe.
- Can I jet my bike by doing plug chops/ check the colour of my spark plug? If so how ?
In a word, no.
In the shop we have lambda air/fuel meters which means we can get an accurate mixture reading whilst the bike is running at full throttle and load on the dyno but obviously you can't do this at home.
It's widely suggested that you can get an accurate idea of whether your bike main jet is running rich or lean by checking the colour of the plug. The theory is the plug 'colour' can give an indication as to the mixture at the time the engine was cut out. The idea is you hold your bike at full throttle for 2 or 3 minutes and then immediately kill the engine and remove the plug. Unfortunately this simply doesn't work.
To demonstrate this we took photos of the plugs on the Yamaha Aerox used in our earlier dyno graph example.
We established earlier that with the 62 jet fitted this moped was running lean to the point at which the engine was in danger of seizing and with the 75 jet it was considerably rich. With either one of these jets the graphs show the moped was significantly down on power compared to when correctly jetted with the 68 jet.
From the pictures you can see there is virtually no discernible difference between the plug photos and there's not a chance that you'd be able to accurately jet your moped using this technique.
On some engines you can actually get some valuable information from this but you need to use a brand new plug everytime. On a brand new plug the white porcelain will start to form a discolouration ring when it is first used. The colour of this band will relate to the maximum attained temperature within the combustion chamber at the time of the engine cut. This band therefore does not depict the jetting mixture but the temperature it depicts is *related* to the jetting mixture. This band will only form once on a new plug, after a few more minutes use the whole of the white porcelain will be discoloured (as in the above pics) and no further useful information can be learned from it (unless there is something *seriously* wrong!) hence you need a new plug every time you do a plug chop. This band forms at the base of the porcelain insulator inside the end of the plug so can be a little tricky to see on a moped plug.
So the procedure would be... 1) warm up bike 2) fit new plug 3) immediately run bike to full throttle under normal load and hold it there for 2 to 3 mins 4) immediately kill engine and remove plug
The insulator should still be white at the end but will have started to discolour further inside. This colour can give an indication to the jetting but there are a few reasons (we will discuss another time) that this method is still not particularly useful on the sort of moped engines we are discussing here. In reality this method of jetting has no real usefulness for 2-stroke single small cylinder moped engines for the home mechanic.
In a future blog we'll cover the practical aspects of how you can reliably and accurately custom jet mopeds at home for unknown setups.
Lighter rollers = better acceleration, heavier rollers = higher top speed?
In the shop we are asked on an almost daily basis for lighter than standard rollers for improved acceleration or heavier rollers for higher top speed. There is widespread belief that rollers are a compromise whereby the rider can choose between higher acceleration at the expensive of some loss of top speed, or higher top speed at the expense of some acceleration. This therefore implies that there are no actually 'correct' rollers for a specific bike, and the best choice of rollers is ultimately down to user preference.
But is there actually any truth to this ?
First of all we need to consider powerbands. All engines make their maximum power at a specific rpm. The further you are away from this rpm the less power your engine is developing at that moment. As you tune an engine to develop more power the power increases but the peak power tends to occur at a higher rpm and tends to drop off much more quickly as the engine speed moves above or below this rpm, this is what's known as a narrow powerband.
A 2-stroke motocross bike is a good example of a narrow powerband engine, they normally have 6 close ratio gears to allow you to keep the bike revving highly all the time and to remain as close as possible to the rpm where all the power is developed. Because the powerband is so narrow there is virtually no power at lows revs and you have to work the bike to keep the revs within the narrow powerband all the time.
Similarly if you fit a power pipe to a ped without changing the rollers the bike at first seems slower because although the engine now has a higher power output the cost of this is that it occurs at higher revs and the powerband has narrowed. So whilst your bike might have developed enough power to happily pull away at 6500rpms with the standard pipe, the tuned and now more powerful bike with the pipe fitted actually develops less hp at 6500rpm than it did with the standard pipe and it struggles to pull away.
Because the engine only actually develops peak power at one specific rpm a geared bike or car is virtually never utilising it's peak power, it's only actually achieved for a brief moment as the revs pass through peak power rpm whilst accelerating. The best compromise for maximum *useable* power is to manage the gearing so as the engine can be kept as close as possible around this rpm for as long as possible.
Even a standard moped is actually in quite a high state of tune and already has a narrow powerband. A 1.0 litre car developing 50hp actually only develops half of the power per litre that a 50cc moped does producing 5hp.
To keep the moped within it's narrow powerband it uses a variator system. The idea is that if you know at what rpm the engine develops its maximum power, the variator system can be tuned to keep the engine at that specific rpm all the time meaning that peak power is available immediately and continuously at any time you mash the throttle.
So back on topic...
Ok, we all know that lighter rollers allow your engine to rev harder and vice versa, and that if the variator is working well it should hold the engine at a constant rpm dictated largely by the weight of the rollers.
But how does this effect top speed? It actually doesn't. Roller weight doesn't have any effect on top speed (except in unusual circumstances)
It's important to realise that the variator can only hold the rpm constant until it has adjusted itself all the way out, as soon as the road speed is reached at which the variator can no longer keep changing out the transmission becomes fixed gearing.
The printout below shows a Yamaha Aerox, pretty much standard with a Viper pipe and Polini variator fitted. The only difference between the graphs is that each run up is done with different weight rollers- 3.5g (blue line), 5g (red) and 6.5g (green).
(Click pic to enlarge)
If you look at point 'A' on each of the runs this shows the point at which the variator can no longer change the gearing (to keep the rpm constant) for each roller weight. As soon as this point is reached the gearing becomes 'fixed linear' meaning that the only way the speed can increase is if the engine rpm's are also increased in direct proportion. At line B on the printout the variator has fully changed all the way out for all the different roller weights and from this point on the engine rpm will always be the same relating to the road speed and consequently the power developed will be the same and so will the ultimate top speed.
After point B the engine rpms simply continue to increase for all examples until the power drops away as we move further and further away from peak power rpm until it no longer has enough power to accelerate further - this occurs at 60.5mph for all roller weights on our printout when the power developed by the engine has dropped to 3hp.
Having established that roller weight has no effect on top speed it becomes clear that there is only really one ideal roller weight for a specific bike when tuning for both acceleration and top speed. In a later article we'll discuss finding it and general variator tuning in more depth.
It's worth noting that the Polini variator is a vast improvement on the standard Aerox one which is particularly poor on even a mildly tuned Aerox. (You could equally use the Stage 6 or Malossi variator kits). It's able to keep to revs constant far more uniformly meaning the engine is utilising it's available power much better and it's able to adjust out to a higher gearing than the standard one often meaning a significantly higher top speed even without a gear kit.
Note: there are special circumstances where the above information can be disputed, but it can be considered reliable for general use and road peds.
