Motorized Bicycles - Helpful Hints and Solutions

Extra Items Needed For Bike Engine Build

When you begin adding the engine kit to the bicycle of your choice, it is often necessary to get a few added supplies from your local automotive parts store. Some of these supplies should include Loc-Tite, 3M weatherstrip adhesive, a Paint Pen, Silicone sealant, and Teflon tape. These are essential accessories to have on hand because there are always things that need to be secured in place, marked for cutting, and to insure no leaks! (Teflon Tape not included in picture)

Be sure and use the Loc-Tite for all bolts that you do not want to have vibrated loose, the Teflon tape works well for fuel valve fittings and exhaust bolts, the paint pen is ideal for marking where to locate your chain tensioner and the link or links that may need to be cut out, and either weatherstrip adhesive or silicone to hold gaskets in place and prevent leaks from occurring.

extra items needs for bike engine build




Centrifugal Clutch Not Engaging

There have been a few customers that have experienced the 9 or 10 tooth gear, that attaches the chain to the transmission, slipping and making it seem like the centrifugal clutch is not engaging.

If you have experienced a problem with the engine revving up but the bike does not move, then check to see if the rear transmission gear has come loose. Itís possible that the set screws, located in the jacket of the gear, are not properly tightened.

The constant movement of the gear back and forth may loosen the outer gear bolt and then the key slides out allowing the gear to spin around the jack-shaft.

Before putting the transmission on the engine, it is best to remove the 2.5mm outer gear bolt and the 2mm setscrews and apply thread locker such as Loc-Tite and then reinstall. If you take the time to perform this short preventative maintenance, then you will save yourself time and hassle down the road.

Sprocket Components
Set Screw DetailKey Installed with Sprocket on Jackshaft




Reed Valve Diagnosis

Have you been experiencing problems with oil and fuel coming back through your carburetor on the HuaSheng 49cc engine? The problem may very well be that the reed valve inside the engine is no longer seating properly and closing off the combustion chamber as needed. The purpose of the reed valve is to close off the combustion chamber for the intake side as the piston comes up on either the exhaust or compression stroke. Fortunately, this is a relatively easy diagnosis by checking compression and performing a leak down test.

To check compression, remove the spark plug, and install a compression gauge. Pull the rope starter 5-7 times and record your results. The minimum compression should be 90 psi and we are looking for a value of around 135-145 psi. (Values will differ depending on your area and tool you use.)

To perform the leak down test, place the engine at TDC of the compression stroke, and insert a fitting designed to allow air through it. When the engine is on TDC it means that all valves are closed and we are on the compression stroke therefore, we can add air to the cylinder and see where any leaks may be coming from. Please keep in mind that these are relatively small engines and you should only need around 40 psi of air pressure to hear an air leak. A leak down tester allows you to use a gauge and see whether or not the cylinder will hold air pressure as well as, regulate the amount of air coming into the cylinder. This is the preferred method of leak down testing. If you have air coming through the carburetor and intake, then it is most likely coming from the reed valve or intake valve. Air coming through the exhaust would most likely mean that there is a leak in the exhaust valve and seat. Air coming through the crankcase is usually due to bad or misaligned piston rings. Remember that we carry many of the individual parts that you may need including reed valves, intake and exhaust valves, pistons and piston rings.

Reed Valve




Clutch Gear Cover Fixed Pin Problem

Have you experienced problems with the clutch lever coming out of the clutch gear cover?

We have had a few customers tell us about a problem that is occurring with the clutch lever assembly on the 2-stroke 66cc SkyHawk Angle Fire Engine. It appears that the fixed pin, (Item #ENG2S66-25SSP) part #25 in the diagram, may come loose from its position in the clutch gear cover and therefore, allow the Camshaft Shifting Rod, (Item #ENG2S66-CSRM6) Part # 27 in the diagram, to come free of the transmission. This will require removing the clutch gear cover and checking the fixed pin location for a bent or broken pin.

The aluminum Clutch Gear Cover may be broken where the pin inserts and need to be replaced. We stock the original plain Clutch Gear Cover, (Item # ENG2S66-DC) Located in the Crank and Piston section of our website or a chrome finished Clutch Gear Cover to add some sparkle to your engine, (Item # 2S-CP-DSC#55ACHROME) The fixed pin is already in the Clutch Gear Cover of your choice and allows for easy installation. If your gear cover seems to be in good shape still, you can replace just the pin, (Item# ENG2S66-25SSP) Part#25 in the diagram, just be sure to use a little JB Weld to keep the pin in place.

Cam Pin Damage

Cam Pin without Damage

Cam Pin Damage

Cam Pin without Damage





Restricted Exhaust

One of the most overlooked drivability or idle issues with gas powered bicycles is restricted exhaust. If your bicycle engine attempts to start but will not stay running especially with the throttle wide open, then the odds are good that you have restricted exhaust. One of the best ways to identify this problem is to try and start the engine, when the engine stops, listen for air escaping from the muffler port. The escaping air is an indication of back pressure that is too high and not allowing the engine to run. Simply, remove the muffler from the engine and try and start the engine again! If the engine starts right up then you have diagnosed the problem and need to purchase a new muffler.

This problem can occur on either the 2-stroke engine or the 4-stroke engine however, it is more common with the 4-stroke. The mufflers are not coated with an anti-rust material on the inside and do to manufacturing, shipping, and humidity in your environment rust forms on the inside. The particles of rust break down from the running of the engine and collect in the small outlet tube at the rear of the muffler. After repeated buildup of rust particles the passage becomes blocked and the back pressure is too high for the engine to run. Because the entire muffler is welded shut there is no method for cleaning out the particles and it is better to just replace the muffler.



Clogged Exhaust

Clogged Exhaust

Clogged Exhaust





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