Attaining maximum performance with a centrifugal clutch on a backyard fun kart.
Just what is a centrifugal clutch? The clutch is an automatic transmission that is activated by the increased rpm of the engine. The clutch should not engage when the kart is at idle. The engine manufacturer sets the idle speed of an engine at the factory. The idle speed of the engine is normally around 1,650 rpm +/- 200 rpm. There are several things that can cause the clutch to engage at idle. Because the kart manufacturer does not start each kart after assembly, it is important that the dealer or the kart owner take a couple of minutes to read the owner’s manual and review the recommended procedure when starting or servicing a new kart.
1. Throttle linkage can bend while riding or in transit to the dealer. It is highly recommended to always start the go-kart with the rider sitting in the driver’s seat with his or her foot on the brake pedal. MAKE SURE THEY KNOW WHICH PEDAL (the one on the left) IS THE BRAKE VERSUS THE GAS PEDAL!!! If the driver has to start his/her own kart then place the front end of the kart against a stationary object like a tree or fence post. Once in the kart, you can roll it backwards with your hand or foot to get the kart in a position to drive forward.
2. During assembly at the kart factory it is possible that too much tension was placed on the throttle linkage causing the engine to idle above 2,000 rpm (the starting speed of the clutch for engagement). THINK & STRESS SAFETY!! When you start the go-kart, be in a position where you can shut it off if the clutch wants to engage by itself. Most karts have two shut off switches on them—one on the steering column and the other on the engine itself. A third possibility is to pull off the spark plug wire, which might give you a slight shock. If the clutch wants to engage by itself, turn off the motor, and verify what is causing the problem or bring it to a dealer that can correct the problem.
3. The dealer where you bought the go-kart or a good lawnmower shop can put a tachometer on the engine to verify just where the engine idle speed is set. You cannot verify the correct rpm by listening to the engine and concluding it is too high or too low.
4. Some of the driving habits that can ruin a clutch are:
a. Driving too slowly. A clutch starts to engage around 2,000 rpm and will lock up around 2,600 rpm. Driving at full throttle gives the clutch a chance to cool down. Full throttle locks the shoes in the clutch against the drum. When the clutch is not locked up, the shoes slip against the drum causing tremendous heat which dries up the lubrication in the oil impregnated bushing. The tremendous amount of heat generated can also anneal the spring which is the heart of the clutch. If the shoes turn purple from excessive heat more than likely the clutch is ruined and the spring no longer has the ability to pull back the shoes at idle.
b. Short stop and go driving. The longer you drive at full throttle the better off it is for the clutch, because it gives it a chance to cool off before the next engagement.
c. Driving with the foot on the brake. This is a problem with new drivers that are unsure of themselves. When riding a go-kart for the first time, try to find an open area that has no obstacles that you have to maneuver around until you get use to the brake and gas pedals. The driver must first be able to build up his or her confidence on the kart before putting obstacles in their path. A small back yard is not a good place to learn to drive a go-kart. In my opinion, a minimum of ¾ of an acre is the yard needed for a go kart. Teach your child to drive with one foot on the pedal—be it the brake or the gas but not to press on both pedals at the same time. You either want to go or to stop but you cannot do them both at the same time.
d. Changing the size of the tires. Putting on tires that are larger than what came with the kart will result in clutch problems. Tires exceeding 12″ in diameter stress out a clutch if the kart is not properly geared for the larger tires. If you are driving on smooth flat terrain than you may be able to get by with 13″ tires but once you get on hilly terrain, deep grass, then you are putting additional strain on the clutch, which will cause premature wear. Big tires look neat on a go-kart but you create your own problem changing to a bigger tire when the clutch isn’t designed to handle them.
e. Weight. A centrifugal clutch is designed to be able to move a certain amount of weight. Once the weight limit is exceeded, then the life of the clutch will be shortened. A good rule of thumb is the kart and driver (and passenger when it is a two seat kart) should not exceed 400 pounds. The kart weighs around 150 pounds add to this the driver (and passenger weight, if it is a two seater kart). If you know ahead of time that you will exceed the 400 pounds then buy a torque converter go-kart and avoid the problems of burning up the clutch asking it to do more than it was designed for.
f. Gear ratio. The sprocket on the engine and the one on the rear axle should have a ratio close to 1:6. This means for every tooth on the clutch there are six teeth on the rear sprocket. So if you count 10 teeth on the clutch there should be 60 teeth on the rear sprocket. (12 teeth on the clutch means a 72 teeth rear sprocket). A torque converter has a variable speed system between the driver and the driven pulleys so it can improve upon this ratio, which gives the torque converter an advantage over a straight clutch system. It improves the ratio by approximately 3:1 and can turn larger tires and drive at slower speeds without doing any damage to the clutch system. A torque converter is a more expensive system initially but it will last longer and is more trouble free when maintained.
g. Restrictions. Don’t try and reduce a go-kart speed by limiting the travel of the gas pedal or by putting a restrictor in the carburetor to limit full rpm. Either of these methods will result in the clutch slipping, which is self-destructive. A clutch needs full rpm to lock up as quickly as possible to be able to start to cool down. To slow down a go-kart you would need to purchase a gear reduction unit, which is like training wheels on a go-kart until the driver gets use to how the go-kart handles. A gear reduction unit can reduce the speed of the go-kart in half.
Now that you understand the don’ts of go-kart, what must you do to prolong the life of the clutch? That is simple, do some preventive maintenance. Just what must the homeowner do so the kart can be driven rather than be in the repair shop all the time??
1. Oil the bushing that is located in the sprocket at least every two (2) hours of driving time. Where do you oil it? Right where you see the snap ring on the sprocket at the end of the clutch. There is a bronze bushing behind the snap ring that needs lubrication. With the engine off, squirt several drops of oil while the bushing is warm.. At the same time oil the chain. What oil should I use? A good automotive oil like 10w 30 or a straight 30 weight oil is good. Once the chain gets kinky from lack of oil it should be replaced before it does damage to the clutch sprocket. A new chain is a lot cheaper than a new clutch. If you keep the clutch and chain lubricated you should get years out of both. Oiling the chain after it gets dried out is a waste of time. Remember it is preventive maintenance, which means before the items get ruined.
2. Don’t put oil inside the clutch. The holes on the drum of the clutch are to let the hot air out of the clutch. DON’T SQUIRT OIL IN THESE HOLES!! When oil gets in the drum portion of the clutch it will cause excessive slippage, which means undue heat will be generated.
3. If your kart has a grease fitting in the crankshaft for lubrication then a shot of grease once a month should be sufficient lubrication. The grease is coming from the inside of the sprocket outward, which is a better way to lubricate the clutch. The keyway in the crankshaft acts as a reservoir for the grease, which cuts down on the frequency needed for maintenance compared to oiling the snap ring area. The chain still needs to be oiled frequently to keep it from kinking up. There are some excellent chain sprays available that are waxy and will not let sand and dirt adhere to them. Dirt and sand will wear the chain as well as the sprockets.
4. All chains stretch over time. When the chain starts falling off the sprocket then it is time to move the engine forward or backward to take up the slack in the stretched chain. There are four bolts that hold the engine on the motor mount plate. Take an open-end wrench and a socket wrench (most likely a 9/16″) and loosen the four bolts- pull back the engine until you take up the slop in the chain. Tighten the bolts back again but make sure the two sprockets are perfectly aligned with each other. You don’t want the sprockets out of alignment because this will cause the chain to come off or wear the sprockets unevenly. Leave about a ½” of play between the top of the chain and the bottom—don’t pull the engine back where the chain is a tight as you can get it. A tight chain can cause the kart to move in neutral with no one in the driver’s seat. A tight chain puts friction on the bushing or roller bearing, which could cause it to engage when the engine is idling.
Teach your child how to do the maintenance on the kart and accept responsibility for its upkeep. Every second tank of gas in the kart is about the interval for oiling the chain and the bushing. If your child learns to do this preventive maintenance the go-kart should be problem free for many years.
Clutch warranty. How long will the clutch last? If put on the crankshaft properly, without using the “magic hammer” to pound it on the shaft, using the right length key (1/2″), and observing all the hints that are pointed out in this article, a clutch should last many years. But if it is improperly installed, driven in abusive conditions (sand, steep terrain, excessive weight in the kart), the clutch might only last 20 minutes. I will tell people to send us the clutch for review but if the shoes are blued don’t bother sending the clutch. We didn’t sell it with blued shoes and we know one of the hints given above caused the premature failure of the clutch. If the clutch is abused, the tell-tale signs tell us what possibly caused the problem. Call the clutch manufacturer if you have a legitimate problem and they will repair or replace the clutch.