Author Topic: leakdown test-can hear air thru crankcase vent hose  (Read 1223 times)

Offline bigbird337

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leakdown test-can hear air thru crankcase vent hose
« on: January 02, 2021, 03:39:37 pm »
So I performed a leak down test on my 87 lt500r. I sprayed the entire outside of the motor no leaks. I can hear air thru the crankcase vent. I removed the clutch cover and listened and sprayed but there is no leak. I also removed the power valve because I couldnt get it to stop leaking and plugged it with a rubber plug. looking for directions on how to find the air leak.

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Offline MotorGeek - Jerry Hall

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Be creative and make a plug that can be inserted into the exhaust port between the exhaust port window and the power valve hole in the floor of the exhaust port.  The LT 500 and LT 250 s are very difficult to get a meaningful pressure test until a perfect seal at this point is obtained.  Doing this will eliminate the usual leaks in the power valve components and resonating chamber in the front of the engine cases.  Small air leaks in these two areas will not affect the jetting or cause lean mixtures contrary to many arm chair tuners.   

Once you are absolutely sure the exhaust port plug is 100% leak proof, perform your leak test again after  inserting the transmission vent line into a bottle of water.  If the vent line blows bubbles, then you have a problem with the right crank seal, the case halves or a crack in the cases.  The OEM right side crank seal seldom fails unless the right main bearing fails and the bearing fragments ruin the  seal.

If the engine has experienced a broken piston skirt or lower connecting rod bearing failure sometime during it's life, the leak is probably in the case halves sealing surface or a crack somewhere in the engine cases.

Other common leak areas are at the rear of the cylinder next to the base nuts and the base gasket under the reed valve. 


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