# Crankcase air leak testing methods?



## doggman (May 5, 2008)

Do any of you sage mechanics have pratical tips or methods for locating crankcase air leaks? 

I'm trying to find a little one in an old Sears 51.7 cc ported Brushwacker Model 636.796243.

The engine runs high speed and idles with the choke closed or slightly open. It dies with it open. I've been through a rebuilt carb and then bought a new one thinking I screwed up the rebuild or that ethanol killed the diaphragm. Checked the intake gaskets and adapter block and they're fine. 

The only trick I knew was to spray some WD-40, starting fluid, etc. around the crankcase and see if the engine would speed up - which it does a little - but it might be pulling it through the carb. 

Thanks...


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## geogrubb (Jul 28, 2006)

Try using carb cleaner along all mating surfaces at idle, it is not as volatile as starting fluid, to see if the engine tempo changes. Have a good one. Geo


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## dawgpile (Apr 28, 2008)

Keep in mind that a scored cylinder wall & piston can cause the poor performance as well. Hopefully that's not what you've got. You can check to see if the 'jug' bolts are all tight. Sounds like you've checked the intake side from the adapter plate to the carb.

Did you check the fuel filter in the tank when you did the carb work? Take it off and make sure you can get air through it easily. (You should be checking the fuel line as well to ensure it's not dried up and cracked causing an air leak on the intake side) As the filter begins to clog or if the line is deteriorated to the point of letting some air in, it can cause the same symptom you describe. The engine is starved for fuel and you have to choke it more. If you don't fancy the taste of fuel, just leave the filter off and start up the machine(making sure you have enough fuel in the tank so the hose is in it!). That is a simple indication of whether the filter is contributing to the problem. The line is checked through just a visual inspection.

Finally, an 'official' crankcase leak test is usually done by fashioning cover plates for both the intake and exhaust side of the jug and alternately testing both under pressure and vacuum to a prescribed pressure/vacuum for a prescribed period of time and noting leakdown time. The pressure/vacuum is induced through one of the cover plates or through the spark plug hole. Unfortunately, the average shade tree mechanic doesn't have the resources for this elaborate of a test so we resort to these other more practical(and perfectly acceptable!) methods to determine the problem.

Good luck!


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