What is happening is this, when an exhaust valve is burnt, it is no longer sealing against the valve seat. When idling, there is usually a great deal of vacuum created during the intake cycle. If the valve is burnt and leaking, you'll get reversion. Reversion is where the exhaust flow changes direction back toward the cylinder, which should not be happening. By placing the bill on the exhaust tip, the bill will rattle. The noise will be very noticeable. Please note, in order for this test to work correctly, you need to have a good exhaust system (without leaks). The bill also has to have a good fit over the exhaust tip. If it's too small, the effect wont occur. I don't know how well it will work with exhausts which have cross-over or h-pipes built into the system. The engine has to be idling as the vacuum is much less and the exhaust flow is much greater when the engine is off-idle.
What is happening is this, when an exhaust valve is burnt, it is no longer sealing against the valve seat. When idling, there is usually a great deal of vacuum created during the intake cycle. If the valve is burnt and leaking, you'll get reversion. Reversion is where the exhaust flow changes direction back toward the cylinder, which should not be happening. By placing the bill on the exhaust tip, the bill will rattle. The noise will be very noticeable.
Please note, in order for this test to work correctly, you need to have a good exhaust system (without leaks). The bill also has to have a good fit over the exhaust tip. If it's too small, the effect wont occur. I don't know how well it will work with exhausts which have cross-over or h-pipes built into the system. The engine has to be idling as the vacuum is much less and the exhaust flow is much greater when the engine is off-idle.