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Dr. Pulley Clutch


1) CVT UPGRADES for 250cc ENGINES:Recalibrate your CVT (Continuously Variable Transmission). This can be a big one! Recalibrating your CVT will not increase the actual power output, but it can be one of the best bang for the buck modifications that you can buy. Most mod kits for gas scooters will also come with new parts for the CVT. A properly calibrated CVT can significantly increase the overall performance. The front driver pulley (variator) can be recalibrated with different roller weights to vary to overall RPM of the engine under driving conditions. Lighter roller weights will increase overall engine RPM. Heavier roller weights decrease engine RPM. Roller weights can be found for the 250cc in the range of probably 15 - 26 grams or so. Somewhere around 15 -20 grams would probably be a good place to start to experiment for heavier go karts. Lighter scooters may want to have a little heavier roller weights. Weigh your stock weights using a precision scale and select new rollers weights either lighter or heavier - depending how you want to change your engine RPM range. In addition to the front driver pulley, the rear driver pulley can also be recalibrated with different clutch shoe springs to vary clutch engagement RPM, and also the main driven compression spring can be changed to a stiffer spring which also effectively boosts engine RPM under driving conditions. There is a lot to be said about CVT tuning, but to simplify many people can probably benefit from slightly lighter roller weights in the front driver pulley and also a stiffer torque (compression spring) in the rear driven pulley. These changes will effectively slow down the upshift of the CVT and allow the engine to rev up higher and have better torque under driving conditions. Don't go too far though. Too high an RPM and you might be way out of your powerband. Also, too high of an RPM can be annoying while cruising with low engine loads. It can feel like driving your 5-speed car in 3rd gear on the freeway. The buzzing of the RPM at all times can be annoying to some people.

  • Kevlar Belt- Depending on whether your 250cc engine is vertical or horizontal, you will need to upgrade your belt to a Kevlar belt to accomodate the extra torque from the rollers weights and torque spring.Click here for belt size specifications.
  • Racing Performance Variator Kit- We recommend variator kits either from Dr. Pulley or MRP. Extra diameter and aggressive ramp design for top performance.Click here for racing performance variator kits.
  • Racing Clutch & Bell- Clutch-in at 3000 rev. = Less Slippage > stall in at presetable rev. < 4000 rev. (3400, 3500 or 3800), vehicle starts to move. This clutch is a must if you want to stand a chance moving in the sand.Click here for racing performance clutch and bell kits.


  • Main Jets- Re-jet your carburetor! Modifying your scooter engine for additional airflow will NOT increase performance, UNLESS you have the appropriate amount of fuel to go along with it to produce more power. Depending on the modifications you make to your engine, you should probably look to re-jet your carburetor with a main jet in the 130 - 160 size range. Many of the 250cc scooter engines seem to use a Keihin style CV carburetor (30mm), so check with a motorcycle shop or motorcycle supply source that sells carb jets. It would probably be best for you to bring in your stock jet so it can be visually matched with the a new one that is the right style. Best to error on the rich side (larger jet), because too small of a jet and you could run too lean and damage your engine. If someone were to put on a high flow intake filter and also a high flow exhaust, then you'd probably want to look closer to the 140 main jet size if you have the stock 30mm CV carb on a 250cc engine. You might want to buy a few different jet sizes and then experiment with what gives you the best performance. Read your spark plugs to see if you are running lean or rich under wide open throttle conditions. Just remember, that too lean of an air/fuel mixture can increase engine temperatures and cause engine damage. If in doubt, err on the rich side with extra fuel to help avoid engine damage.
  • UNI Air Filter- The stock airbox can be eliminated and a high flow air filter like a K&N style or UNI foam filter can be used. For very dusty and dirty off road conditions on go kart applications, an oiled Uni foam filter is probably the best way to go.
  • 32mm Carburetor- If you really want to bring your 250cc engine to life, this will do it! You will need to order the performance manifold to install this carb.Click here for high performance carburetors.
  • Racing Performance Exhaust- An exhaust system upgrade can yield a big improvement in performance as long as the additional flow is combined with properly tuned carburetor. Stainless and aluminum exhaust has an awesome fit & finish! The sound is deep and mellow, with very low restriction.Click here for high performance exhausts.


  • 300cc Cylinder Kit- This big bore kit will give your motor that extra CC's that you are looking for. Make sure you are good with a wrench as this installation will take a bit of skill on your part to install it.
This kit does require machine work for proper fit.
  • Camshaft- For better climbing power, use low-end torque.Click here for high performance camshafts.

  • 4) ELECTRICAL UPGRADES for 250cc ENGINES:Advance ignition timing. Another way that people try to increase performance of a scooter engine is to increase ignition advance. Ignition timing is when the spark plug fires relative to the position of the piston (and crankshaft/flywheel). The 250cc engine has a magnet embedded in the outside surface which passes by a pickup sensor that acts as the trigger for initiating the ignition spark. By changing the timing of the spark, it is sometimes possible to increase power and torque of an engine. There are a number of ways that this can be done. One way is to reposition the ignition pickup that is located next to the flywheel. However, to make any significant changes in timing this way, trying to move the ignition pickup may require welding and drilling/tapping of new holes for the ignition pickup mount. This is probably not the best way for most people. Another way is to take a stock flywheel key (that indexes the flywheel on the crankshaft and keeps position secure) and grind an offset so that the flywheel is repositioned relative to the ignition pickup. This is probably also not the most straight forward way of doing this. A different way that is more convenient is to install a performance CDI (capacitive discharge ignition) module that has additional ignition advance programmed into the ignition calibration. These may be included in some mod kits for gas scooters and go karts, or you can often buy one by itself. The CDI is sort of the brain of the whole ignition system and changing modules is as simple as unplugging the old and plugging in the new. Truly just Plug and Play! Just be sure you get a CDI that actually has a remapped ignition curve with additional ignition timing advance. Some modules might simply have a higher RPM rev limit. If you want to advance your ignition timing, then make sure the unit you get actually has the advance programmed into it!

    • NGK Iridium Spark Plug- Iridium is a precious metal that is 6 times harder and 8 times stronger than platinum. It has a 1,200(F) higher melting point than platinum and conducts electricity better. This makes it possible to create the finest wire center electrode ever.Click here for high performance spark plugs.
    • LED Lights- By using these LED lights, you are conserving battery usage. Because your scooters and some go karts use a relatively small battery, it is vital that you reduce your electrical consumptions since some of your other high performance gadgets will consume most of it.Click here for LED lights.
    • High Performance CDI- It allows for increased RPM from stock to no rev limit.Click here for high performance CDIs.
    • High Output Coil- By adding this coil it gives you a much stronger spark that is much more reliable than stock coil. It improves combustion, electrical performance, and ignition performance.Click here for high performance ignition coils.
    • Current Stabilizer- It connects to the positive and negative terminals of the battery. It stabilizes the current allowing for quicker firing during high RPM's. Also, it reduces fuel consumption by increasing the burning efficiency! Use in addition to coil, racing spark plug, magneto, and other items for maximum electrical performance. It helps on some models if you have weak lights during idle.Click here for current stabilizers.
    CONCLUSION:Because of the popularity of the 250cc engine in the scooter world, there are many mod kits for gas scooters and go karts to boost 250cc engine performance. There are even big bore kits that boost engine displacement up to 300cc -- stroker cranks, larger carbs, different intake manifolds, even aftermarket 4-valve heads. So, if you have a go kart, then check out the mod kits for gas scooters because many of the parts may work on your go kart applications. In addition, if you own a scooter, then check into some of the go kart engine mods because they may also work on your scooter. Regardless of what machine you own, performance upgrades can help increase the power output (and fun factor) of your go kart or scooter!

    Disclaimer:This page is solely intended to be used for information only. We will not be held liable for the outcomes of your mod projects. The information was collected from many skilled and experienced mechanics and technicians. We highly recommend you consult with an certified or skilled mechanics or technicians before proceeding.