DT RE Chris Posted August 5, 2017 Share Posted August 5, 2017 50cc Moped, compression tested at 40psi, So, fitted brand new barrel, piston & rings, compression tested 40 psi again?? Base gasket/ head seals are fine with no leaks Is it possible the air is being forced out the crank seals under compression? Link to comment Share on other sites More sharing options...
Variomatic Posted August 8, 2017 Share Posted August 8, 2017 Seems either you goofed up when rebuilding it, or your comp tester isn't working. Bad crank seals would make it run poorly but wouldn't lower compression. Also try fully opening the throttle before testing compression, closed throttle can drastically lower readings sometimes. 1 Link to comment Share on other sites More sharing options...
Moderator Cynic Posted August 8, 2017 Moderator Share Posted August 8, 2017 You cannot do a compression test on a 2stroke like you would on a car engine. Engine doesn't work that way. You need to do a leak down test to check the crank seals and such. Plug the exhaust (at the barrel) and the inlet again at the red block not at the carb. In either of the plugs you need a bike type valve and a pressure gauge, a tyre pressure gauge will do. Pump some pressure into the engine, not massive pressure 10psi tops. Then see how long it holds. Ideally it wont drop at all. Probably it will drop a bit but if the pressure is gone in the time it takes to have a brew you have problems. Link to comment Share on other sites More sharing options...
slice Posted August 9, 2017 Share Posted August 9, 2017 This is just one of dozens of "How to,s" on YOUTUBE, seems an easy thing to do if you have the right kit or can make it. Link to comment Share on other sites More sharing options...
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