You've got a stock 49cc 2 stroke and you want to go faster, but you're strapped for cash. Sure, you can save up money until your budget will allow you to get a big bore kit or an aftermarket exhaust, but why wait to get more performance out of your scooter? There are things that can be done for very little cost that can get your scooter moving a little faster in many cases. I'm familiar with the Yamaha / Jog/ Minarelli and Chinese clone engines, but some of the things I mention here can be done to many styles of two strokes. Some scooters have very few parts available for them, so this information may be useful to owners of those scoots.
Let me note that the things you see here probably aren't going to get your scooter perfroming like a big bore, piped, etc... scooter, but you can certainly get gains. If you don;t like to tinker and would prefer to simply bolt on performance, these mods and tweaks probably aren't for you. If you have to have the fastest scooter on the block, go ahead and start having the best parts shipped from overseas. Otherwise, continue.
I'd also like to advise you to be very careful when doing any modifications or tweaks. Getting a scooter out of proper tune, not checking clearances in some cases, and lack of attention to detail amongst other things can cause serious damage to your scooter, poor performance, or dramatically reduce engine longevity. Pay close attention, do farther research if necessary, and don't modify or adjust anything if you don't feel comfortable doing it.
Check For Restrictions
Some 49cc scooters are manufactured with restrictors to limit power output and/or top speed. Since many states require 49cc scooters to drive 35MPH or less the restrictors help to keep the average person from pushing much above that speed.
Sometimes restrictors limit the engine's power output by placing obstructions in the exhaust or intake manifold. These are usually a washer or other object that allows less air or exhaust to pass by it, thus keeping the engine from "breathing" properly. I have seen exhaust restrictors in person on a 4 stroke scooter, but both of my 2 strokes were free of exhaust restrictors. That doesn't mean that your scooter couldn't have one though. I have never seen an intake restrictor in person, but I have seen pictures of intake restriction on a 49cc 2 stroke scooter so they are out there on some models.
Some manufaturers use CDI ignition systems that reduce the engine's output by limiting it's maximum RPM. Reducing RPM can cause a lack of power for acceleration as well as a loss in top speed. Sometime the rev limiter can be bypassed by simply cutting the correct wire. In other instances the CDI may need to be replaced to remove the RPM restriction. If you happen to have a fuel injected scooter, the ECM may need reprogramming. If you believe that your scooter may be rev limited, do research on your particular make and model to find out if there may be a rev limiter and how to go about removing it.
A very common method of restricting top speed involves the scooter's CVT transmission. Scooter manufacturer's are known to use washers, often made of plastic, to limit the variator and drive belt's travel. In order for the CVT drive belt to reach the outer edge of the variator (for the highest top speed) the variator must be allowed to travel to it's maximum reach. Removing restrictor washers will often gain a few MPH and sometimes more. If your scooter has a plastic washer between the two variator halves it is most likely a restrictor. Research your make and model if you aren't sure that what you are removing is a restrictor. Removing the wrong parts can lean to belt failure or other transmission damage.
If your stock 49cc scooter goes 40MPH and above you probably don't have any factory restrictors installed. It wouldn't hurt to do a little research on your scooter online to be sure anyway.
Every scooter, be it stock or heavily modified, needs a carburetor in proper tune to perform at it's full potential (unless it happens to be fuel injected). Your scooter may not be tuned well from the factory. When scooters are manufactured they often choose settings and parts that will work in a wide range of climates and locations. A scooter that ships to Phoenix, AZ may run great while the same scooter may perfrom very poorly in Denver, CO. Elevation changes, differences in temperature and humidity, and other factors make it impossible for a simple carbureted scooter to perfrom at it's best anywhere it may be shipped. On top of that, some Chinese scooters are notorious for having mixed and matched parts. The same make and model of scooter has been known to come with different jetting, clip positions, etc... These are all great reasons to be sure your carburetor is in tune.
O.K., so I want to make sure my carburetor is giving me the best performance... but how do I do that? I suggest using feel and spark plug checks to give you an idea of your scooter's state of tune. If your scooter hesitates or bogs when you try to open the throttle, has trouble starting, bogs at wide open throttle (WOT), refuses to idle, or stumbles when you let off of the throttle there is a very good chanve that it is not tuned well. If you notive that your spark plugs are excissively light or dark you may also need to take a look at your carburetor if you want to go faster or simply operate it with ease.
If you have a tachometer it can be a valuable tuning tool as well. If you would like to use a tachometer to tune, but don't want to shell out the money for an aftermarket tach, I came up with a pretty simple solution that may benefit you. If you have an automotive timing light with a build in tachometer, you may be able to connect it and secure it to your scooter long enough to get your tuning finished. My timing light has leads that clip to the postive and negative battery terminals and one that clips around the spark plug wire. My timing light has 2 and 4 stroke settings to get accurate RPM readings, but I suppose you could even use a simple 4 stroke only light to give you an idea of your state of tune. A 2 stroke engine (and "wasted spark" 4 stroke engines in many scooters) fire once per revolution of the crankshaft. Traditional 4 stroke engines only fire once every other revoltuion. For that reason, using a standard 4 stroke tach/timing light will cause it to read only half the actual RPM.
I'm not going to go into great detail of how a carburetor works and how to tune one, but here is a link that should explain anything you need to know to get your scoot's carb in tune.
Check out other articles for more information in the 49ccScoot.com tech section as well.
The engine's tune will need to be checked after nearly any engine modification. Air box modifications, aftermarket air filters, big bore kits, exhaust modifcations, and more can change the jetting and settings necessary for your carburetor to run properly. If you do need to change jets they can usually be found pretty cheap online or via a motorcylce shop. If you cannot find the jets you need you also have the option to drill jets using special bits. Jet needle position can be changed for free by simply moving the clip up or down as described in the link above. You won't usually need to change other parts fo the stock carburetor, so tuning it can be done for the price of a few jets in most cases.
Intake Manifold Modification
Just because there are no actual restrictors in your intake manifold, that doesn't mean that it is as good as it can be. Some stock intake manifolds have material left over from casting or designs that don't promote maximum performance. Intake manifolds do not need to be smoothed to a mirror polish inside to maximize potential. In fact, intake manifolds may benefit from a slightly textured surface. A mildly rough surface reduces fuel "pooling" that may occur on smooth surfaces. I would imagine fuel pooling should be minimal given the short intake manifolds that most scooters use, but if you find any other reason to modify your intake it is worth creating the right surface finish. Rough as cast is too rough, but sanding with around 100 grit sandpaper provides and apporpriately rough finish.
Intake manifolds can also be ported and modifed to create a larger or more direct path to the reed valves. This can involve removing and/or adding material. Stock scooters with stock carburetors most likely will show little gain by increasing intake diameter via porting. Opening the intake manifold up too much will reduce intake velocity and reduce low to midrange power as well as degrading throttle response. Reshaping an intake manifold for a more direct path can be beneficial, but can be a very time consuming and tedious task.
Refining A Stock Cylinder
Much like intake manifolds, stock scooters can often benefit from modifying the stock cylinder. HERE is an article I wrote about smoothing out a cylinder.
If you wish to go beyond simply smoothing out your scooter's cylinder, you can do porting and milling to modify port size, port timing relative to TDC, port duration, angle of port entry, and more. If you wish to redesign your cylinder's ports, I strongly suggest consulting and expert and/or reading up on two stroke porting. Porting can make or break and engine. The right changes can make an engine perform much better than stock, but the wrong changes can damage your engine or cause very poor performance. I would suggest reading A. Graham Bell's book, "Two-Stroke Performance Tuning", and Gordon Jennings' "Two-Stroke Tuner's Handbook" are two great sources to start gathering information.
Squish And Compression Modification
It is pretty common for scooter engines to have a greater amount of clearance between the cylinder head and the piston's crown than absolutely necessary. Depending on the style of cylinder head that your scooter uses, this can affect compression and possibly squish band clearance. Most stock scooters do not have squish band style heads, so most likely it will only affect compression.
Here is a standard cylinder head design. This particualr head is from a 90cc Yamaha / Jog / Minarelli clone.
Here is a cylinder head that utilizes a squish band desgin. The head shown here is from a 54mm big bore kit.
The squish band is the outer ring of the combustion chamber in the bottom cylinder head pictured above. Cylinder heads that have a squish band can offer a few benefits. The first is the most obvious. The combustion chamber of a squish band head tends to have a smaller volume than that of a conventional cylinder head. The smaller volume creates higher compression, and usually makes more power. Squish band designs have also been know to improve cylinder cooling and promote faster combustion as well as a more complete burn. For more information on squish band's I would suggest reading A. Graham Bell's book, "Two-Stroke Performance Tuning".
If you have a squish band cylinder head it is very likely that the factory squish is set to a wider gap than necessary for optimal performance. First off, you'll need to find out what your current squish band clearance is. This can be done with some thick solder (preferably with a diameter of around 2-3mm) and a caliper to measure the solder. Remove the spark plug from the cytlinder head and place the solder into the cylinder so that the tip of the solder will be touching the cylinder wall. Make sure the solder that you use is long enough to allow you to hang on to it with your fingers and hands clear of the cylinder. Kick or crank the engine over while holding the solder to the edge of the cylinder wall. Now remove the solder and you should see that the solder has been compressed by the piston squishing it up against the cylinder head. Measure the smallest part of the smashed area to determine your current squish band clearance.
Here are the specifications provided by A. Graham Bell for squish clearance depending on engine displacement. 50 to 80cc engines should have 0.6 to 0.8mm of clearance. 100 to 125cc engines should have 0.7 to 0.9mm of clearance. 175 to 250cc engines should have 1.0 to 1.4mm of squish band clearance. In my personal experience, those numbers may be a little low unless the combustion chamber volume is increased.
If you find that you have way more squish band clearance than optimal, there are low cost options to decrease the distance from your piston's crown to the squish band. A lot of stock engines use a flat metal head gasket. Measuring the thickness of the compressed head gasket and subtracting that number from the squish band clearance that you measured will give you a good estimate of your squish band clearance wihout a head gasket. If the clearance figures that you come up with here are within or still above the suggestions for clearance listed earlier, you may want to install your cylinder head without the gasket. You will probably need to use a copper gasket spray to seal the head to the cylinder. Another option is sanding down the stock metal gasket until it reaches the thickness you desire. If using this method I would sugest having a replacement head gasket or two handy, just incase. Another option is to have the cylinder head milled to remove enough material to reach your desired squish band clearance. I suggest this method as teh last option or to be done only once you know exactly what clearance you desire. If you mill too much off of the cylinder head you will need a replacement head, which is much more expensive than a few spare head gaskets, or you may need to use multiple head gaskets which can cause sealing issues.
No matter what method you use, if you modify squish band clearance you must always check it again before starting the engine. If the clearance is too small your piston could end up contacting the cylinder head and causing major engine damage. I also recommend letting the engine warm up at idle after reinstalling the cylinder head. Once the engine warms up to operating temperature, turn it off and re-torque the cylinder head nuts being careful not to contact any hot engine or exhaust components. Once the engine cools, remove the spark plug and check your squish band clearance once again. Sometimes heat cylcling the engine will allow the head gasket to compress farther when re-torqued and cause a tighther squish band clearance than your first measurement. It pays to be safe when dealing with such tight tolerances.
Once you are pleased with your meaurements, you need to do some real world testing. When you take your scooter out for it's first test ride with your newly modified clearance, it is not simply a joy ride. You need to pay close attention to the sounds that your engine makes. More specifically, you are listening for detonation. Detonation occurs when the fuel in unburned gases ignites from excess pressure and/or heat. Detonation can damage your engine and/or cause it to seize. Detonation usually makes a high pitched crackling sound or pinging sound, which is why some people call detonation simply "ping". You may also hear it referred to as spark knock. If you don't hear detonation under your normal driving conditions or even on witde open throttle runs, I suggest trying to induce detonation to be sure it's not present. I have found that running at wide open throttle for a stretch and then slightly backing off the throttle will make detonation the most detectable. If you notice detonation at any point, you should seriously consider increasing your squish band clearance until it is eliminated or having the combustion chamber enlarged to drop compression a little. If you don;t already, use premium fuel and it may be nough to make the detonation disappear.
Now I've explained squish band clearance, but what if your head doesn't have a squish band? One option would be to find a cylinder head intended for your engine that does use a squish band design. You may be able to find one pretty cheap online. If you don't want to go that far, you can still increase your compression. You would go about increasing compression much like decreasing squish clearance. You can try removing the head gasket and use only copper spray to seal the cylinder and head. Decreasing the distance from the cylinder head to the piston will reduce combustion chamber volume and therefor increase compression. Just as explained with squish band clearance, you will need to take measurements and test rides to ensure there is no risk of contact between the piston and head and no detonation is present.
Some cylinder heads may use an o-ring style gasket to seal the head to the cylinder. If your scooter uses an o-ring head gasket your options are not quite as easy. The o-ring style gasket does not raise the cylinder head higher above the piston so there is no quick way to test with lower squish clearances or higher compression. You could replace the head with a different head designed to fit your scooter's engine if available. You could also have the cylinder head milled far enough to remove the o-ring's reciever groove from the head and then use standard gaskets or no gaskets. Be careful not to mill the cylinder head so far that it will ruin the combustion chamber while doing this.
Once again I would like to stress the importance of taking lots of measurements, working slowly, and checking everything twice when dealing with this type of modification. It's very easy to destroy an engine if you don't perform these mods cautiously.
Opening up your stock exhaust can help to free up a little power. I wouldn't expect big gains, but many stock mufflers can be modded fairly easily if you have basic tools. Some will require welding to do the job properly. Stock mufflers are often very restrictive in order to meet various standards and to keep the scooter quiet, so there is a lot of room for improvement. HERE is one option for modifying one particualr style of muffler. Searching the 49ccScoot.com forums and the internet may provide more information on other styles of exhaust or different methods of modification.
Affordable CVT Tuning
Before I even start to talk about the CVT, it is important to have a general knowledge of how it works. HERE is some great CVT info.
The stock CVT setup does a good job of keeping your scooter's engine revving lower for increased fuel economy and longevity, but it is not designed for the best performance. If fuel economy and maximum engine longevity are your primary goals modding is not for you... and you probably haven't read this far on the page. Fuel economy and engine longevity don't have to be affected by any great degree, depending exactly what is done.
One of the easiest ways to pick up a lot better acceleration is by making the clutch engage at a higher RPM. Clutch engagement controls the initial take-off. The clutch does not engage when you frist rev the throttle because clutch springs resist the centrifugal force that tries to push the clutch shoes against the clutch bell. Once the clutch shoes contact the clutch bell the rear wheel will begin to move. Making the clutch engage at a higher RPM means your engine is making more power when the clutch engages. This can cause your scooter to accelerate from a dead stop much faster. A scooter that once lurched away from red lights and stop signs can often be made to jump away from stops. Making your clutch engage too late can make your scooter difficult to drive, but upping the engagement 1000RPM or more usually offers a good boost.
You can make the clutch engage later by installing stiffer clutch springs that will hold the clutch shoes closed and away from the clutch bell longer and/or you can lighten your stock clutch. HERE is more information on lightening a clutch. Clutch engagement will depend on just how much material is removed from the stock clutch. Be careful not to go too far and weaken the clutch or get it out of balance. Clutch springs are commonly found in 1,000RPM, 1,500RPM, or 2,000RPM designations. These numbers do not represent the RPM that your clutch will engage, but rather the additional RPM that it will engage at. For example, if your scooter's stock clutch engages at 2,000RPM a set of 2,000RPM springs should make it engage at 4,000RPM or 2,000RPM above stock. I suggest going to the 2,000RPM springs for a nicely improved kick off the line.
Another clutch related issue is clutch fade. There is a lot of heat inside your CVT case, lots of it from the clutch. As the temperature of the clutch rises the clutch and springs cannot work properly. The springs temporarily weaken, allowing earlier clutch engagement and slower launches. The clutch shoes may not grip as they should and can glaze over causing the clutch to slip. If your clutch shoes become glazed and slip you will need to remove the clutch and bell and lightly sand both surfaces to remove the glazing. The additional heat inside the CVT case can also cause premature belt wear or failure, roller weight/variator glazing, and more.
Luckily, reducing the amount of heat within your CVT transmission's case is easy and cheap as long as you have some pretty basic tools. Creating cooling vents can make a big difference in CVT temperatures. You can be creative with your venting to achieve the results and look that you desire. HERE is more info on CVT cover venting.
Now that you know how to get your scooter taking off faster from a dead stop, you'll probably want to make it pull harder all the way to it's top speed. This can be achieved by changing roller weights and/or swapping in a new contra spring. It's a little harder for me to suggest an ideal setup here. Roller weight and spring stiffness often need to be tailored to the rider's weight, riding style, and preferences. Rider weight will affect the way the transmission acts, so a 140lb rider and a 200lb rider will probably end up using different setups. Some people want maximum acceleration and will sacrifice some top end speed, and some want the fastest top speed they can get out of their scoot even if it slows down acceleration.
Lighter roller weights or a stiffer contra spring (sometimes called a main spring or torque spring) will improve acceleration, but may reduce top speed. If the weights are too light or the spring is too heavy the rollers won't be able to overcome the contra spring and push the belt to it's outermost point in the variator to reach top speed. If you are way off with the tuning you could go incredibly slow. I once used 2g roller weights and a 2,000RPM contra spring for a test. My scooter took off fast but topped out at around 15MPH. If your scooter revs too freely you may have lighter rollers or a stiffer contra spring than is ideal. Heavier weights or a lighter contra spring will give the scooter a higher top speed, limitied by power and belt travel. If the weights are too heavy the scooter may be sluggish and may not have enough power to pull itself to it's top speed. If the engine is staying at low RPMs all the time it's likely that your rollers are too heavy or your spring is too light.
Before deciding on any upgrades, I would suggest evaluating your stock transmission. If you plan to do motor work, wait until that is finished. A 70cc scooter would likely need different CVT components than a stock 49cc scooter. Try to find out what roller weights your scooter has stock. You may be able to find this information online or you can weigh the weights using a scale that mueasures in grams. Measure the outside dimensions of your rollers as well. They are available in 15x12mm, 16x13mm, and more sizes. Most minarelli scooters use 15x12mm stock. Take a look at your variator face to see how far your belt is traveling. Belt travel is usually pretty obvious if after a bit of riding. If you cannot tell how far your belt is traveling in the variator you can use a black marker and draw a line from the center hole in the variator to the outer edge of the face the belt rides on. Assemble the CVT again and take a ride to get your scooter up to max speed for a moment. The mark you made should wear off anywhere the belt contacts it. If your mark is more than a few mm from the outer edge of the variator you may want to consider heavier rollers or a lighter contra spring. If the mark is not far from the edge you might be able to use lighter rollers or a heavier contra spring to improve acceleration.
Aftermarket variators, clutches, clutch bells, and kevlar belts can be great improvements as well but they don't fit in with budget tuning as easily as the afore mentioned CVT mods. Autotech355 on eBay sells a package including 3 different clutch springs sets and 3 different contra springs. I believe they are each 1,000RPM, 1,500RPM, and 2,000RPM. It sells for right around $30. Shop around for roller weights and you can probably them between $5-10 per set or eBay seller daywot offers large tuning sets with rollers varying from 2 grams to 13 grams for around $30. For somewhere around $60 you can have all the parts you would need to get your stock CVT performing the way you would like. If your transmission is very close to what you want you may be able to but less parts and tune it very cheap.