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If you have a head gasket leak (due to uneven head or cylinder top surface) or a crankcase leak then you will have power loss. Using sandpaper on top of glass you can easily plane the head and cylinder top.
How to check for crankshaft seal leaks
Remove the spark plug and magneto. Put water and about 10-20% dishwashing liquid soap in a spray bottle. Spray the left crank seal and push the bike forward and rearward so that the descending piston creates a momentary crank pressure. If there is a seal leak then it will blow soap bubbles. If the leak is big then it wont blow a bubble and you will have to listen for escaping air.
Put the magneto back on and remove the stator coil. Remove the primary drive gear. Spray the right crankshaft seal and turn the engine over by turning the magneto. It is possible also to put an electric drill onto the left crankshaft to turn the engine over. Turning it by hand is hard to do fast enough.
Both seals can be replaced by prying them out with a jewelers screwdriver and pressing the new ones in by hand after putting engine oil on the inner parts of the seal and also the exterior of it where it will be contacting the crankcase. You should have someone move the crankshaft CW and CCW by moving the magneto while installing the right seal. That is because there is a lip on the shaft that the seal sometimes will not go over. Best seals are available from Rock Solid Engines in Australia.
AS an alternative to turning the engine over while looking for bubbles you can have more internal pressure in a continuous way by sealing the exhaust and intake (only if no reed valve is installed) while the piston is at or near dead bottom. The exhaust port plate should have a motorcycle inner tube stem in the middle of it so you can use your air pump to pressurize the crank. I did this for a better idea of how much the seal was leaking after seeing some small bubbles doing it the easy way. I made the exhaust plate out of wood and I sealed it with silicone sealant. Here is a picture of it;
How to check for crankshaft seal leaks
Remove the spark plug and magneto. Put water and about 10-20% dishwashing liquid soap in a spray bottle. Spray the left crank seal and push the bike forward and rearward so that the descending piston creates a momentary crank pressure. If there is a seal leak then it will blow soap bubbles. If the leak is big then it wont blow a bubble and you will have to listen for escaping air.
Put the magneto back on and remove the stator coil. Remove the primary drive gear. Spray the right crankshaft seal and turn the engine over by turning the magneto. It is possible also to put an electric drill onto the left crankshaft to turn the engine over. Turning it by hand is hard to do fast enough.
Both seals can be replaced by prying them out with a jewelers screwdriver and pressing the new ones in by hand after putting engine oil on the inner parts of the seal and also the exterior of it where it will be contacting the crankcase. You should have someone move the crankshaft CW and CCW by moving the magneto while installing the right seal. That is because there is a lip on the shaft that the seal sometimes will not go over. Best seals are available from Rock Solid Engines in Australia.
AS an alternative to turning the engine over while looking for bubbles you can have more internal pressure in a continuous way by sealing the exhaust and intake (only if no reed valve is installed) while the piston is at or near dead bottom. The exhaust port plate should have a motorcycle inner tube stem in the middle of it so you can use your air pump to pressurize the crank. I did this for a better idea of how much the seal was leaking after seeing some small bubbles doing it the easy way. I made the exhaust plate out of wood and I sealed it with silicone sealant. Here is a picture of it;