How to Install The 4-Stroke 79cc “Beast”

It’s Time To Unleash The Beast

At the time of this post, the 4-stroke, 79cc engine, dubbed “The Beast,” is Bikeberry’s latest engine kit. It provides the torque and comfort of smaller 4-stroke engines while providing the top speeds that make 2-strokes so popular

Preface

The engine kit comes with everything needed to get it mounted and running, though due of its large size, replacement parts may be needed to fit within most medium and small sized bike frames. Due to the likelihood of modification to the kit or the bike, this engine kit is intended for experienced builders that are familiar with customizing bike frames and engine kits.

The bike needs to be a  diamond or cantilever style frame, such as a men’s beach cruiser, road bike, or hardtail mountain bike. The seat and down tubes should be 25 to 28 millimeter in diameter. The frame should also have at least 13 inches of clearance between the bottom bracket and top tube. The kit will fit most 26 inch or larger wheels with a standard 12 or 14 gauge 36 count spoke.

Bicycle Preparation

If the bike being used has never had an engine installed, there are some need modifications that we’ll first need to address.

  • Wide crank assembly
  • Rear sprocket assembly

 

Wide Crank Assembly

To have enough clearance to effectively use the pedals, a wide crank assembly is commonly needed when adding a center mounted engine to a bike. This is an additional purchase of $55.95 and not at all a necessity, especially if the rider plans to exclusively use the engine as the driving force.

The process of dismantling the existing bottom bracket varies, largely depending on the type of crank set that will be replaced. One piece crank are typically the easiest to dismantle, needing a 30mm spanner, pedal wrench, and a pin spanner. Many mountain bikes and road bikes require hex keys, a bottom bracket tool, and a crank puller tool.

Start by slipping off the chain from the chainring and undoing the main locking mechanism with a hex key and crank puller or a 30mm spanner, taking off pedals or any other parts that may impede the the crank arms from being dismounted. Once the crank arms are off, remove the bottom bracket with the appropriate bottom bracket tool. If dismantling a one piece crank, use a mallet and rod to unseat the cups. After completely dismantling the bottom bracket use a degreaser to clean the inside of the bottom bracket shell.

Begin installing the wide crank assembly and extended spindle, using the American to English Thread bottom bracket adapter if needed. Slide in the axle rod and the bearings, making sure the slot on the spindle is on the right side of the bike. Apply grease to the open bearings and use the cups to secure everything in place. Apply an anti-seize lubricant to the threads of the cup. Slide the chain ring on the spindle and tapping in the lock pin to hold it in place. Mount the crank arms, being sure to use the correct side, indicated by an “R” and an “L”. Tighten everything down and place the dust cover on. New pedals may be needed to fit into the 1/2″ threads of the wide crank arms. Apply thread locker and anti-seize where ever needed.

Rear Sprocket Assembly

The last preparation needed to make the bike engine ready is to attach a sprocket assembly to the rear wheel. Start by uninstalling the rear wheel from the bike by disengaging the rear brakes and loosening the axle nuts or quick release levers. If the bike is equipped with disc brakes, the rear brake and rotor will have to be uninstalled and replaced with a disc brake sprocket or replaced with a non disc brake wheel. If equipping the disc brake sprocket, the wheel can be reinstalled after attaching the sprocket.

For rim or coaster brake wheels, fit one of the rubber gaskets over the left side of the axle followed by the sprocket. On the second gasket make a through cut on one side, in-between two holes, and place it over the axle on the inside of the spokes. Using the three metal brackets, align all the holes from the sprocket, gaskets, and brackets, then secure everything together with the bolts and nuts, making sure to use a thread locker.

Alternatively, the BBR Tuning Sprocket Assembly can be purchased for a more precise and secure mounting option.

Once the sprocket is mounted to the wheel, the wheel can be remounted onto the bike. Remember to reconnect any rear breaks that are available. If using a coaster brake, the brake arm may need modifications to provide clearance from the bolts.

 

 

Engine Modification

The engine comes ready with a fuel tank, exhaust muffler, and carburetor. Though, due to its large size some modifications are required before we can mount the engine to the bike frame.

  • Remove the stock gas tank
  • Remove the stock exhaust muffler
  • Remove the stock air filter
  • Remove the base plate

Removing the stock components will allow adequate clearance for mounting and will be replaced with the provided 2 liter fuel tank, High Performance air filter, and chrome exhaust muffler.

The components are easily dissembled by removing 3 bolts for the fuel tank, 2 buts for the exhaust muffler, 2 thumb screws on the outside of the air filter and 2 nuts on the inside, and 4 bolts for the base plate.

Engine Installation

After stripping down the engine, do a dry with mounting plate and engine. Make any adjustments to the mounting plate to have the engine seated securely and clear from the drive chain. If clearance is limited, the mounting plate may need to be attached to the engine before mounting onto the bike frame. Additionally, a short head can be purchased to give an extra 1 inch of clearance. After finding the optimum mounting position, bolt everything in place, using a thread locker to prevent loosening. We can then start to build up the engine.

  • Install the transmission
  • Install the high performance air filter
  • Install the exhaust muffler
  • Install the fuel tank

The transmission is a modular assembly that bolts onto the left side of the engine. Position the transmission plate with the sprockets towards the back of the bike and the larger sprocket facing out. Leave the bolts loose enough to allow for adjustments and remove the key stock from the drive shaft. Dry fit the clutch and loosen the set screws on the sprockets, using the spacers to make adjustments. Measure out the proper length for both chains and use a chain breaker tool to cut them to sizes. Seat the chains over their respective sprockets and make adjustments to the primary drive chain by adjusting the transmission plate’s position. Remove the primary chain and clutch once the optimum position is found and bolt all transmission parts in place. Reassemble the transmission assembly and mount the idle chain tensioner to the chainstay, taking up any slack on the secondary chain. Attach the chain guard by removing heat shield and bolting the guard in its place. Finish attaching the chain guard with the provided bolt and spacer.

The high performance air filter can be installed by bolting the adapter in place with a thread locker and slipping the air filter over the adapter. Tighten the clamp to secure the air filter in place.

To install the muffler, you first need to install an adapter. Be sure to place the gasket on before installing the adapter. Align the second gasket and the muffler to the adapter, then bolt them on.

If the bike has a larger diameter top tube, a  wide bracket may be needed to mount the fuel tank. This process may require some modification to the bracket. After mounting the tank, screw in the fuel valve, applying thread locker to the threads, and attach a fuel line. Attach a fuel filter, positioning the arrow in the direction of fuel flow, and connect it to the fuel line leading to the engine. Secure the hoses with hose clamps or cable ties.

Throttle Controls

Slide on the throttle and mark the position for the guide pin. Measure the guide pin to find the proper drill size for the hole that will accept the pin. Remove the grip and use a spot drill or center punch to start the drilling process. Connect the throttle cable to the throttle grip and push the cable through the throttle eyelet, laying it inside the guides. Use the two bolts to complete the throttle grip assembly.

Slide the throttle cable through the guide and connect it to the throttle lever. If your cable does not reach the lever, use a cable with a shorter housing, or modify an existing cable, if it is long enough. slide the cable through the throttle lever. Cap the cable with a ferrule or solder end to keep the cable from fraying. Finish the cable by tightening its lock screw and adjust the cable tension. To improve performance and longevity of the cable, apply a synthetic lubricant into the cable housing.

Kill Switch

The bike is almost done, but there’s still the consideration of using the preinstalled kill switch from the engine, or using the kill switch on the throttle grip. If the throttle kill switch is preferred, disconnect the wires on the preinstalled kill switch, and using the bullet connectors, connect the similar wires together. Connect black to black and color to color, then secure all loose cables.

Congratulations!

At this point the engine kit is fully installed. Fill the motor with .35 liters of four-stroke oil and 87 octane unleaded gas. Use the pull start and have some fun.

For more help on installing this engine, check out our installation video

Leave a Comment