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I see hundreds of cylinders and crankshafts every year and It's my experience that one of the most common causes of engine failures are air leaks . Air leaks will introduce unwanted air into the fuel charge and make your machine run lean. Think of it like this . If you build a fire, it will burn at a steady pace . If you blow on that fire , it will burn much faster and hotter . Your engine works the same way . An engine is nothing more than a fancy air pump . A lean engine is a ticking time bomb . It won't fix itself and it will fail . Excessive engine temperatures will melt and burn holes in pistons , seize lower-end bearings , peel the coatings off cylinders and sometimes throw the connecting rod through the cases.
1 . When you rev up your engine , does it wind back down to an idle slowly ?
2 . When you start your bike , does the engine race and have an unsteady idle ?
3 . Does the engine smoke excessively and does the smoke smell like gear oil ?
4 . Is the engine hard to start ?
5 . Does the engine make a loud banging sound at full throttle ?
6 . When you start the engine , does it run for a few minutes and then die ?
7.Does oil come out of the gear case over-flow tube ?
These are all symptoms of a vacuum leak!!!
The only way to tell for sure is to do a vacuum test . Remove the pipe and carb . Plug the exhaust flange with an expandable rubber freeze plug . You can get this at any automotive parts store . Next , plug the intake boot with a steel freeze plug with a hole in the center . Tighten the intake clamp . Insert the vacuum pump into the hole on the intake plug and apply 9 inches of vacuum . If it leaks down to less than 7 inches in a few minutes , you have a vacuum leak .
The most common place for an air leak to start is the crankshaft seals . Remove the ignition cover and take a look inside . Oil inside the ignition housing is evidence of a crank seal leak . Did the engine smoke and smell like gear oil ? Is oil coming out of the gear case vent tube ? If so , you have a leaking seal on the clutch side or a leak at the case between the crankcase and gear case . Other places to look for a vacuum leak are the cylinder base gasket , reed gasket , carb boot or the carb itself . The important thing to remember here , is to fix the leak as soon as you find it . Seals and gaskets are cheap . Pistons , crankshafts and sleeves are expensive . It's a good idea to change the crank seals every 2 years or so . The constant exposure to fuel , heat , oil and high RPMs has a tendency to dry them out .