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Posted by David Sherman [72.47.9.228] on Thursday, October 07, 2010 at 21:18:28 :
In Reply to: low compression posted by tom kralovic [12.165.43.246] on Thursday, October 07, 2010 at 20:53:03 :
That seals the heat riser pocket. If it failed it would cause an exhaust leak around that area which you could feel with your hand with the engine running. It would not leak into the intake and it would not affect your compression reading. Have you done a wet/dry compression test on your motor? Squirt about a teaspoon of motor oil into #3 and #4 via the spark plug hole. On a flathead, you'll need an oil can with a spout that you can angle back towards the cylinder because the spark plug hole isn't over the cylinder and if you just pour oil down the spark plug hole some of it might or might not find its way to the cylinder. If the compression comes when tested with the rings soaked with oil, then you need a ring job. If it doesn't come up, the problem is probably the head gasket or valves.
A good way to find valve leaks is with a "leakdown tester". You don't really need the special tester, just a way to adapt your air hose to the spark plug hole, such as drilling out an old spark plug and welding an air coupling to it. Turn the engine to a position where both valves on that cylinder should be closed (TDC on the compression stroke) and turn on the air. You may hear it leaking past the valves. If it's not obvious, put your hand over the exhaust pipe and then the carburetor inlet and see if you feel air coming out.
Normally you would do these tests before changing the head gasket, but since it's so easy to change the head gasket on a flathead, it didn't hurt much to just shotgun it.