SuperRat Instructions

PLEASE READ THESE INSTRUCTIONS BEFORE INSTALLATION These Instructions are Copy Righted: Rev. Sept. 20th, 2015

New; GT2A-SM SuperMouse > 50cc / Internal CDI & 203 crank brgs. Angle Fire Sq. Head;
New: GT5A-SR SuperRat > 70cc / Internal CDI & 203 crank brgs. Angle Fire Sq. Head;


Kit Box Contents:

• 50cc 40x40 or 70cc 47x40 integral CDI 2 cycle gas engine made for V frame mounting:
• Chrome exhaust muffler; / Tear Drop 2.5L Fuel Tank plated inside with shut off valve
• Drive chain; 415 or 410 depending on how was ordered from factory. / Sheet metal chain guard;
• 44 or 48 tooth 9 hole Sprocket with mounting hardware to fit on a 36 spoke wheel;
• Ball bearing chain idler: / Push button clutch lever, / Carburetor: / YD CNS or NT Speed.
• Twist throttle with integral kill switch: / Tool kit with extra parts. / Boxed with WD seller’s name.

Mechanical aptitude and ability is required to perform this DIY installation. Many “do it yourself” mechanics and hobbyist will find this project fun and rewarding. More reference at

STEP #1 Mounting the Engine:


Note: This is a “DIY” Do it Yourself Kit: The End User or the Installer becomes the vehicle manufacture and assumes all laws of the land including any product liability. Do not use or buy this product if you expect otherwise. Mechanical aptitude and working ability is required to perform this installation. Many “do it yourself” backyard mechanics will find this project rewarding. A love of small engines is the only required catalyst for this project. However, installation is sometimes best done by a professional auto or motorcycle mechanic. WARNING: Do not use a donor bike with front and rear fenders as side struts can come loose and lodge in wheel spokes causing danger to rider. Frame size should have a 70 degree included V angle with down-tube of 28 to 42mm dia. and seat tube of 28 to 34mm dia. For sufficient engine clearance select a bike with a seat tube length of at least 12 ½ inches measured between bottom of top tube and top of pedal sprocket tube. Using a bike coaster brake only is not recommended as chain can break leaving no brake at all. Use a bike with front and rear brakes to ensure best stopping ability; Remember, a quality installation and daily maintenance is paramount to safe usage and long term satisfaction. You may find many uses for this engine such as power for a stationary machine or for other off road machines. Have fun and good luck on your motorized project. Happy trails to you from GruBee / China GAS and YD:

STEP #1 Mounting the Engine:

The engine mounts in a 70 degree angle “Vee” tube frame. It is best to make sure all 4 engine studs are securely bottomed out in the engine before mounting. Use a Jam nut procedure to tighten studs. Consider using Duct Tape on the front down-tube & seat tube of your bicycle to protect the paint finish while test fitting the engine to your donor bike. If the distance between the two frame tubes exceeds the engine mounting span then additional spacers or welded brackets may be required. Mount the engine to the seat tube first and then fit to the front tube. If frame tube is smaller than engine clamp dia. use strip shims to fit. NOTE: Down Tube Shim Kits are available at Buy it Now web site.
Depending on how your engine kit was ordered it may have M6 front mount studs or M8 studs;
CDI magneto module is mounted inside the engine with the super size magnetic rotor.
New; GT2A-SM SuperMouse > 50cc / Internal CDI & 203 crank brgs. Angle Fire Sq. Head;
New: GT5A-SR SuperRat > 70cc / Internal CDI & 203 crank brgs. Angle Fire Sq. Head;

Engine Front Mount ideas you can make or buy:

d.) Pedestal front mount with rubber cushion for weld on application;
Rubber wrap should also be used to isolate the rear mount on the seat tube if an alum. frame is used.
Mount your engine as low as possible in the frame with top chain as horizontal as possible:
If engine is mounted at a high angle position the chain will hit the sprocket cover. Keep as low as possible.
Remove front and rear fenders for safe operation: End user assumes all product liability as prime contractor: Beach Cruiser bikes work well as do the classic 70 degree old time European style. Note: Front mount size is different with new 6203 crank brg. engines versus old style 6202 bearing engines.
1. If the rear frame tube from the seat down to the pedal sprocket is too small to fit the rear engine mount, a rubber shim can be made from an old bicycle rubber inner tube. This also helps reduce engine vibration. Engine needs to have the carburetor set in a level position. Too much engine tilt can cause chain to hit the drive cover and the engine to not run correctly due to carb. float angle. It is best to have the drive chain to rear wheel sprocket be as horizontal as possible with no more than 15 degrees max engine tilt. After the desired engine location is determined mount the engine to frame. Loc-Tite thread lock is recommended to avoid loosening due to vibration. Note: All threads are metric.

Chain Wheel Sprocket Installation:

    The Drive Chain Sprocket has a 36.9 mm dia. center hole and mounts on axel hub on the left side of the rear wheel against the spokes dish side in. The sprocket must fit over the hub in a perpendicular plane with the axle. This insures that your rear chain sprocket spins true with the rear bike wheel. *NOTE: On some older bike axle hubs like on coaster brake models it may be required to slightly enlarge the sprocket center hole to obtain a flush and concentric fit next to the spokes. This is best done on an engine lathe by a professional machinist.. . It is also recommended that the rear wheel be re-spooked to 12 ga. spoke wires to insure long life. Most any Bike shop can do this operation for you. Applying thread adhesive and equal tightening of the sprocket bolts. This keeps the chain sprocket true with axle and free from wobble while spinning. With bike upside down spin wheel and check sprocket for wobble. The chain can jump off the sprocket if the sprocket installation is done incorrectly
1. For kit sprocket installation, locate sprocket on axel hub with curved side next to spokes, shinny side in.
If not pre sliced, cut the rubber isolator ring between holes in order to fit INSIDE the spokes and around the axle. Install the split steel retainer plates next to the rubber isolator and insert 9 bolts.
2. Secure with 9 bolts compressing the chain sprocket to the spokes. Note: Rubber isolators may be needed on both sides of sprocket for chain alignment.
3. The Chain Sprocket on the Wheel must align within 1/2 cm to the Chain Sprocket on the Engine.
4. The wheel chain sprocket is mounted with teeth-out and dish-in next to spokes. SEE FIG. #2

Place Sprocket’s bright chrome side inward next to spokes with dished side outward:

The drive chain can be easily shortened to the correct length. Special tools are required to remove and replace the master link when shortening the chain by removing links. Ideally, both your pedal drive chain and your engine drive chain should have the same tension.
A. Remove left rear cover plate from engine. This is the plate next to and under the clutch swing arm.
B Your engine may come with a standard bike chain or with a Heavy Duty 415 chain depending on how it was ordered by your dealer. Engine drive sprockets are different depending on chain size. The 415 chain uses a wide drive sprocket and the std. bike chain uses a narrow one. A 415 chain will work with a narrow sprocket but a std. 1/2”-1/8” bike chain will not go over a wide drive sprocket. Note: Install chain with master link clip on outboard side of the primary drive sprocket teeth. ( Note: wide tires larger than 2.125 may rub on a wide 415HD chain: )
C. Use supplied spark-plug wrench to turn engine crankshaft sprocket to feed chain around it. Do not pry
on the sprocket with a screwdriver or similar object.
D. Fit chain, measure and remove excess links to assure proper length. Be sure master link connection rides on the outboard side of the primary drive sprocket or interference of link and sprocket can occur. Proper chain length is when top chain has ¼ inch to ½” deflection with the bottom side of the chain loop tight.
E. Chain tension adjustments can be made by pulling rear wheel back if frame has straight slot wheel drop out. If both chains can be adjusted equally then installing chain idler on the wheel strut may not be necessary. At installer’s discretion the chain idler can be installed on either the pedal chain or engine drive chain.
F. Install supplied chain safety guard by attaching to engine and wheel axle struts.

Engine Kill Switch installation

A) Install Engine Kill Switch Wire on throttle to spade clip on the CDI located inside the magneto box. Install the other wire with eyelet to a good frame ground not on paint. This will ground ignition and stop the engine when the kill button switch is activated.
B) Route all wires away from engine heat. Secure wires with a plastic tie straps.
*!WARNING! Engine operation without the stop or kill switch installed could result in personal injury if an emergency stop is required! The only alternate way of killing the engine is by releasing the clutch lever with bike brakes on and the engine at slowest idle rpm.
Carburetor Choices available at buy it now web site:
Std. Push button Clutch lever: Opt. Dual lever Clutch & Brake Opt. Dual brake cable throttle

Clutch cable installation and adjustment:

A) Install clutch lever to left side of handlebar and attach cable end barrel into lever arm slot hole.
B) Squirt oil down the cable sleeve: Route clutch cable through the ball-mount on the engine with the big spring around the cable jacket and ahead of the ball mount. The big spring serves as a cable heat shield.
C) Insert cable wire through small spring and route through clutch arm and attach brass cable-end and screw. Adjust cable tension to allow very slight play in arm. Handlebar clutch lever must be in the released or outward position to complete this operation.
D) Activate lever a few times, and check clutch arm for slight free play: About 1/16” engine clutch arm free play is required with the handle bar lever in the released in what is called the clutch engaged position or the engine will fail to start if cable is too tight. Re-adjust as required.
E) Basics of clutch operation: The handlebar lever pulls the cable that moves the engine clutch arm. In turn the clutch arm pushes a rod through the motor that pushes the clutch plate out. ( similar to a car clutch.) Releasing the handle bar lever engages the clutch and provides engine torque to the drive chain or to start the engine. The friction of 15 pads on the clutch plate allows the engine to start, and also transmits engine torque to the drive chain. When the bike is in pedal mode the handle bar clutch lever is locked inward held by the push button. In pedal mode the bike then operates in default as it would without any engine. Periodic clutch adjustment is necessary to maintain efficient operation *NOTE: Cut off excess cable extending from clutch arm, before operation, to avoid possible interference with pedals, chain, your legs, etc. See Figure #4
OFF SET Z Use this optional long Z intake tube if bike seat tube has a clearance problem.

Carburetor and Throttle Installation

3 Intake Tubes for GT5A-SR / GT2A-SM engines. Note: 40mm stud hole spacing.
( NOTE: If carb hits the bike seat tube use short stubby steel or you can use the long Z steel tube. )

SuperRat standard NT Speed RED AC Carburetors

Use this procedure for attaching throttle cable to carburetor slide valve:

The small stop on the cable wire end slides through the long groove on the slide valve. Early slide valves were made of brass and later ones are made of black plastic. Beware that there are 2 sizes of black plastic slide valves. The one best to use with the 66cc is 14.95mm in dia. but has to have an appropriate carb. housing to accommodate the larger valve.
Be sure to check carb. air cleaner attach screws for tightness before installing engine:
Air cleaner screws coming loose and entering engine is not covered by warranty;
( Note component positions in pictures; Needle clip is factory set in second slot: )
The spring is placed inside the cylinder slide and is compressed when the throttle is twisted. Be sure it is seated all the way inside the cylinder. The spring then forces the throttle to return. For this to work properly the throttle must twist freely on the handle bar in both directions prior to the cable being installed.
A) Install twist grip throttle on right side of handlebar end. On some handle bars it may be necessary to ream out the handle ID to fit the bar so that the throttle will twist freely. It never hurts to add a few drops of light wt. oil to let trickle down the cable inside the full length of sheathing.
B) After installing cable inside the carburetor you are ready to mount it on engine intake tube and tighten clamp screw. Mount engine so carburetor sets as level as possible.
1. Note: Factory Sealed Air /Fuel ratio screw. Factory set by turning adjustment screw to closed position and then opened ¾ turns outward or less than 1 full turn before it was sealed with epoxy.
2. Jet Needle: Carb. comes with clip set in position #2 from the top. Different positions may be required to make your engine run correctly depending on altitude above sea level and fuel economy desired. If a more rich gas mixture is required you can move the jet pin “pac man” clip to the next lower position notch.

Fuel Tank installation

A) Attach fuel petcock to tank. Use Teflon tape to seal threads. Careful not to strip threads.
B) Mount tank on bike top crossover frame with two supplied brackets and nuts.
C) Attach fuel line from tank to carburetor. Best to use USA made fuel line like GoodYear SAE 30-7 4.8mm obtained from local automotive stores. Factory supplied clear plastic line gets hard over a period of time. *NOTE: Filters are contained in the petcock and in the carburetor inlet. If engine runs poorly clean the valve filter as residue from the tank may have clogged it.. It is highly recommend that a tank liner coating be applied inside the tank before installation. This product is called Kreme and is available from motorcycle dealers;
Your engine is a 2 cycle design, therefore, a gasoline/oil mixture is necessary. During the break-in period (1st gallon of fuel ), the ratio is 16 to 18 parts gasoline to 1 part oil. After the break-in period, the ratio can increased to 25 parts gasoline to 1 part oil. *NOTE: Synthetic 2 Stroke Oil can also be used to insure proper engine lubrication. Fuel with a lot of ethanol mixed in is not recommended for these engines.
!WARNING! Remember safety first: Wipe up any spilled fuel. NEVER fuel a hot engine or smoke while fueling. This could result in sudden fire, personal injury. Always move your motorized bike at least 10 feet from any fueling area before attempting to start it. Never leave the tank fuel cap off after fueling as rain water will contaminate the fuel and cause engine failure.


# 1. How to Adjust Clutch if signs of slipping or squealing are encountered :
A) Disengage the clutch by pulling handle bar clutch cable lever inward and push the lock button in.
B) The bike should now roll easily. Release the clutch lever and bike should slide the rear wheel.
C) If the rear wheel does not slide when the lever is released this means the clutch needs adjusting.
D) Remove the clutch cover and then remove the locking screw that holds the Clutch Adjusting Nut.
E) Pull the clutch lever arm on left side of engine inward. Back off the Clutch Adjust Nut ½ turn CCW.
F) Release clutch lever arm and check for 1/16” slight clutch arm free-play.
G) Readjust *Clutch Adjust Nut as required to get the required 1/6” clutch arm free play.
H) Now Repeat the B) roll test. Rear wheel should now slide when the handlebar lever is released.
I) Be sure to re-install the small locking screw in outer edge of *Clutch Adjust Nut.
G) Good idea to place a small gob of grease at gear mesh area. Use grease sparingly! Then replace cover.
H) Squirt light grade oil down clutch cable sheathing to reduce friction and make for easy lever pull.

Instructions for Drive Sprocket & Crankshaft Gear Puller.

To remove primary 10T drive sprocket secure sprocket from moving and remove nut. Screw in puller tool
to the threads in sprocket internal ID.
To remove engine B5 gear; Secure clutch gear and remove slot head nut; Remove star washer;
Reverse bolt in puller tool; Secure tool in output gear; Hold tool with end wrench and ratchet bolt CW.


Depending on dusty riding conditions, clean air filter every 5 to 20 hours of operation by removing the filter cover to access the screen and element. Wash element with a degreasing agent such as Simple Green™ or Purple Stuff™. Be sure element is completely dry before re-assembly. IMPORTANT: If engine runs poorly clean tank shut off value filter. If engine needs more fuel to have more power move the pac-man clip on the needle down to the next notch position.
3 pt. Spark Plug
Remove spark plug and inspect for excess carbon build up. Clean, re-gap to .0.036” of an inch if necessary. Check plug after every 20 hours of operation. New spark plugs are available from your selling dealer. Be careful using aftermarket spark plugs as heat range and threads differ greatly. An extra plug is included:
Exhaust system
After 50 hours of operation check exhaust pipe for excessive oil and carbon build-up. If muffler is clogged your dealer has replacements. Make sure attaching nuts are tight and no exhaust leaks are occurring. Be sure to use supplied support strap to secure exhaust muffler to a solid anchor point on bike frame or engine.
A) To remove inside exhaust insert loosen the retaining screw on end cap and remove.
B) Pull cap and baffle out of pipe. If you need a replacement muffler contact your dealer. Clean with degreaser, rinse and dry. Re-assemble: File muffler attach flange to have smooth flat surface.
C) Always use a new exhaust gasket and good idea to use double nuts on muffler attach studs;
*NOTE: Excessive periods of low speed operation, idling or leaving fuel petcock in the “on” position during shut down periods may cause the muffler to become clogged with unburned fuel.
#5. Every time bike is ridden check the tension of the drive chain by:
A) Rolling to bicycle forward to remove slack from the bottom of the chain.
B) Find the center and push downward on the top of chain while measuring the deflection.
C) Tighten chain if deflection is more than ½ inch.
Head Bolts
Tighten all fasteners after each five hours of operation. Most important to check Cylinder head bolts : Tighten in a X pattern to 10 ft/lb using a torque wrench. A two piece cylinder and head design engine requires head bolts be kept tight. Important: Check head bolts before each and every long ride, vibration can cause them to loosen and blow a head gasket. Caution: Do not over torque or head bolts may break off. ( Twisted or broken head bolts due to over tightening is not covered by warranty. )

Right side gears:

Remove cover plate and keep small amount of heavy grease on gear train. Do not over grease as leaks will occur and also may adversely affect clutch operation. Regular greasing if required will help reduce gear wear and keep gear train quiet.
General Information
Obey all traffic regulations. Always wear a helmet while riding. Remember that you are riding a motorized bicycle and other traffic may not be able to see you. Never operate your motorized bicycle on a pedestrian through way or sidewalk while the engine is operating. Never operate your motorized bicycle in an unsafe manner. Check local and state laws before riding on streets & wear a helmet.


IMPORTANT: PLEASE READ THIS: Gas and Oil Mixture for Fuel ratio
The engine is a 2 cycle design, therefore, a gasoline/oil mixture is necessary. During the break-in period (1st gallon of fuel), the ratio is 20 parts gasoline to 1 part 2 cycle oil. A good quality synthetic 2 cycle oil is OK. DO NOT USE ETHANOL OR E-85 FUEL. After the break-in period, the ratio is increased to 25 to 30 parts gasoline to 1 part oil. The engine crankshaft bearings are lubricated from the oil in the gas mix. A rich break in mixture ensures bearings and rings will not cease and will wear in smoothly. Engine power will increase after break in.
!WARNING! Remember safety first: Wipe up any spilled fuel. NEVER fuel a hot engine or light a cigarette while fueling. This could result in sudden fire, personal injury. Always move your motorized bike at least 10 feet from any fueling area before attempting to start it. Never leave the tank fuel cap off after fueling as rain water will contaminate the fuel and cause engine failure.
Step #1. After filling tank with oil/gas mix open the tank fuel valve. Fuel line is in the open position when the small lever is pointed down. Move choke lever to the on position. Move progressively to the off position during engine warm up period.
Engine Starting procedure:
1. Pull the handlebar clutch lever inward, to disengage the engine from the rear wheel.
2. Pedal; (down hill if possible for first start)
3. A mid frame or rear wheel bike stand is helpful to start the engine in place.
4. Let out the clutch lever all the way out and continuing to pedal. The result is a direct engine hook up via the friction clutch with the rear wheel via chain and sprocket. The engine will now start spinning, Pedal until motor starts. Accelerate slowly at first..
5. Twist throttle to increase speed, reverse twist throttle to decrease speed. To stop,
disengage clutch and apply brakes. To accelerate, pedal and release clutch while opening throttle.
6. Adjust choke to the smoothest engine running position.
7. After warm up push choke lever all the off. If engine races too fast, or too slow, pull clutch lever and lock in the notched catch, stop and adjust engine rpm.
8. If the rpm needs adjusting, turn the idle adjust screw (left side of carburetor) in or out slowly to obtain the proper idle speed of about 1400 rpm +/- 100 rpm
To correctly break the engine in, Do not exceed 15 mph or 30 min. continual running for the first 50 miles during engine brake in. Engine will develop more power after break in.
9. To stop the engine, push Kill switch and turn off gas valve at tank. Turning off the gas will prevent fuel from being siphoned from tank. Warning Note: Never leave the tank gas valve in “open” position” when engine is not running or the bike is in storage.
10. After or before each ride check all mounting fasteners, including hd. Bolts, axle and brakes.
11. Warning Note: Engine lock up or piston seizure due to improper gas / oil mixture will not be covered by factory warranty. This the responsibility of the owner / operator to make sure the gas and oil is mixed correctly. Do not use ethanol if over 10% in fuel ratio.
Proper use and maintenance is required for the continued enjoyment of your Bike Engine. This product has been manufactured to strict quality control standards. For product warranty policy contact your selling dealer. Warranty approval is subject to factory inspection and only the defective part or parts will be replaced, not the complete kit or engine. Only the defective part or parts should be returned to the selling dealer for warranty replacement consideration. Your dealer may require you to obtain his authorization first before returning defective parts. Include description and picture of failure with as many details as possible. Note: Seized pistons due to improper gas / oil mix or shipping damage due to carrier neglect is not warranty. When replacing the spark plug in an Angle Fire head it’s best to use a 3 point electrode spark plug P/N Z4JC to ensure total combustion. ( NKG B6HS plug can also be used. )
Chrome parts for engine dress up are available from your selling dealer;
Ask your dealer for the China GAS Alum. frame w/ built in gas tank & pedestal mount. Some WD distributors in the USA and Australia have complete rear 26” wheels with the Model #2 HD axle pre-installed. This is the pro-shop installation route. Double wall HD rims and 12 ga. spokes.
GruBee HD axles avoid clamping the engine driven sprocket to spokes. Instead said sprocket is bolted to a screw on CCW hub and lock ring that assures true alignment and enhances long lasting durability.
For a ProShop installation ask your dealer for the China GAS Model #2 HD axle kit;
( Complete assembled HD axle Rear Wheels for single speed or Multi speed 26” bikes are available at