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Motorcycle Repair/Compression Testing??


Hello. I own a 2010 Yamaha Majesty (YP400). When I checked the spark plug, I decided to check the compression. As the "Scooter" only has about 7200 miles. I figured it would be nice and high. Using my Mityvac brand compression gauge with 10mm adapter(Made by Lincoln). On a cold engine. Not started before checking. I got a Low reading of 110 PSI. The engine has a "Decompression Start" feature. Did I do something wrong here?? The Yamaha manual just instructs to use their tester and Extension.


Chris, Thats still an acceptable amount. The decomp mechanism is lowering the the pressure so it will start easier. Once the cylinder fires the engine speed picks up and the mechanism pulls back by a weighted centrifgal cam and the compression goes up.

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Gary Blake


I have built from ground up race winning motocross bikes and atvs as well as atv,s for flat track and TT racing. I am starting to take on the utv challenges now for racing. This is a new area for me so Im in a learning curve.


I have extensive experience in off road motorcycles and atv,s.I also work on street bikes and personal watercraft, Some experience was self taught and some was factory trained. I have been working in the powersports industry since the mid 70,s I hope I can help. Thanks Gary

Was a sponsor for K.O.R.R.(kansas off road riders) and now I am working with ROMEO,s (retired old motorcyclists eating out)and was in charge of the Quad Wars for the monster truck series in Central Kansas

Factory trained Kawasaki Heavy Industries. Factory trained GasGas motorcycles and atvs worked as a technician and was as a dealership owner.Kaw.Suz,Honda,Aprilia,Arctic cat,See Doo,Can Am, Bombardier,Yamaha,GasGas,Qingi,TM,Moto Guzzi,Harley,s

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