There are two commonly used drivetrain setups. You can loosely split these into two main categories, petrol and electric. The petrol drivetrain comprises of the engine, clutch, torque converter, drive chain, drive sprocket and rear axle. The electric drivetrain is the same but does not require a torque converter.
Petrol Engine
Most commonly the Honda GX200 petrol engine or a clone version is used such as a 'Predator' or any number of different online generic brands. However, not all engines are made the same. The Honda unit is a bit better quality than the Predator followed by the generic units. However the Honda unit is usually around three times the cost of a generic clone unit. Living in Australia we have the benefit of many different types of generic engines commonly being used in farm equipment and sold by farm stores, so the units sold by your local farm supply and equipment store are generally already tried and tested. You can of course elect to buy online from eBay or Aliexpress and save even more money but you definitely pay your money and take your chances if it is a generic unknown brand.
There is a fella on youtube who did some back to back tests and found that the clone units were a mixed bunch. at the top of the tree the units put out more power and held up just as well as the Honda units, but at the other end of the scale the units tended to use a lot more oil and fuel indicating that manufacturing tolerances were not as good. None of the units failed under testing however, and he had them hooked up for 24 hour on load tests. Make of that what you will. Some people prefer to pay the extra dollars for peace of mind. Some like to save the dollars for other things. You could even buy two cheap clone units and keep one as a spare, plus you would still have change.
Govenor
The engine can be used out of the box as-is but has a built in govenor that will limit the engine speed to about 3600rpm. Removal of the govenor is pretty straightforwards but requires that the engine case is undone and the actuating gear and lever is removed. Be wary of simply disconnecting the governor and letting the actuating lever hang free as it can hit the crankshaft with catastrophic results. There are a bunch of Youtube vids and blogs that cover the govenor removal procedure should you want to attempt it
Exhaust
The standard exhaust that comes with your engine may not have a spark arrestor fitted. Whilst not ell events or venues require a spark arrestor, if you are racing anywhere near dry grass or tinderbox paddocks, or there is a fire ban in place, it's a very good idea to fit one. We recommend that at the least have one in your toolbox just in case.
If your engine came from a farm shop, chances are that it might already have a spark arrestor installed. If you are not sure you can easily check by inserting a thin stick slowly into the exhaust. If the stick stops a short way inside the silencer then it has a spark arrestor fitted. If you can push the stick all the way in then it doesn't.
Exhausts should have silencers fitted. Most venues also have sound level limits. The stock silencer is of course fine, but if you have upgraded your exhaust system, please make sure that the silencer works.
Carburettor
Another common modification to the engine is to change the carburettor out for a higher flowing version. This along with changing the exhaust silencer and removing the govenor are all classified as legal mods and if done together can increase the power output of the engine to about 10hp. NOTE: this is the maximum permissible power increase.
A note on throttle return springs: The guidebook safety requirements state that... "All CycleKarts must be fitted with a minimum of two (2) automatic throttle closing devices." Loosely translated this means that you should fit two return springs on the carburettor throttle arm such that if one spring fails, the second one will still be operational. This is a specific requirement for anyone participating in a CCA event and a common requirement for most motorsports, so it is worthwhile considering adding an extra spring.
A note on slide style carburettors: If you are using a Mikuni or similar type slide carburettor, the slide retaining cap MUST be safety wired in position. This is to stop the slide from being pulled out of the carburettor and the engine subsequently being stuck on full throttle.
Other engine modifications
Modifications such as increasing capacity, changing camshafts or modifying valve-train components are not allowed.
Torque converter
Most commonly the 'TAV-30 series also known as the TAV-2 30' torque converter is used. (Or a clone.) Clone units are not known to have any real issues. They are pretty basic and there's not much to go wrong with them.
The pitch of the torque converter sprocket must match the pitch of the axle sprocket used and vice-versa. The chain pitch must also match both. The Torque converter sprocket must also match the output shaft diameter of your engine, else it will not fit. The most common output shaft size is 19mm (3/4") but always check before ordering your torque converter as there are some other sizes available as well.
The torque converter bolts directly to the engine motor casing and uses a drive belt with a centrifugally operated clutch and pulley to moderate the output shaft speed. The faster the motor spins, the more the drive ratio increases and so the faster the output shaft rotates. The torque converter also disengages the output shaft when the engine is at idle, allowing the CycleKart to idle without moving.
It should be noted that with the typical GX200 and TAV-30 setup, the torque converter can be mounted to the engine in a number of different orientations. There are several sets of mounting holes that allow the torque converter to be pointed in most directions. Changing the torque converter orientation relative to the engine has the knock-on effect of changing the engine position relative to the rear axle, Typically it will place the engine either higher or further back relative to the rear axle. It is also worth paying attention to the orientation of the drive chain relative to the torque converter as the torque converter casing will prevent the drive chain from being connected in some orientations. This largely dictates the orientation that you mount the torque converter to the engine and in turn the engine position relative to the rear axle.
Of course, this may or may not be an issue, but it does largely depend on your body design as the engine location may clash with the bodywork. It is well worth mocking up the engine and torque converter along with a bit of Cardboard Aided Design of your planned body work and trying several different orientations to see what works best for the body style you have chosen.
Engine mounting
Once you are happy with the torque converter orientation it's time to mount the engine. In its simplest form the engine simply bolts to a base plate with four slots to allow for engine adjustment. However, due to the forces involved you would be smart to consider adding some kind of tensioning adjustment to prevent the engine from sliding on its mount. This most often comprises of a nut, or threaded metal flat-bar or angle welded to the chassis so that two bolts can be used to adjust the engine on the mounting plate and provide tension to the drive chain. Once the chain is tensioned the engine mounting bolts can be locked in position.
Drive Chain
Drive chain is available in a variety of different sizes, none of which are interchangeable. This means that the sprocket on the torque converter, the sprocket on your axle and the chain used all have to match else they will skip and throw the chain.