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I have a 2 stroke Mercury 2.5 hp that has recently been dropped in the water. The motor used to run fine but now it won't start. I have dried the motor out and cleaned the carby, it's got plenty of spark, clean fuel and compression but it won't start.

One of things I've noticed is a lack of vacuum at the intake manifold, there is some there but it's not nearly are strong as the compression at the spark plug.

How strong should the vacuum be compared to the compression?

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Vacuum will range from very little (1-2 inches of Hg) when wide open throttle to 10-12 in Hg when choked and throttle closes at medium RPM. What's much more important is how steady the vacuum is. At constant throttle it shouldn't change more than 2-3 in Hg. If it does, look for a bad reed valve or leaks in the crank case.

Those little tiny motors often develop crank case leaks around the carb base, the reed base and internal to the carb. If there is a priming bulb on the carb check it's base as well.

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