You'll destroy the exhaust and catalyst if you continue to do this.
I don't know why handbooks still peddle this myth. If too much fuel and air enter a hot catalyst, the heat generated from the oxidation can be enough to melt the matrix-I know, I've done it! We're talking of taking the cat to sustained temps above 900C.
But during a cold start there is no catalytic activity, so there is no damage. Otherwise catalytic convertors wouldn't last 5 minutes in Canada, Scandanavia or northern US states, where every winter cold start is a vicious one requiring horribly rich mixturess for at least the first 30 seconds of starting. And they do.
There are various reasons advanced for lawn mower syndrome. Fouled plugs is one. Loss of compression on the exhaust valve is another. The theory is that charred deposits on the exhaust valve stem act as a sponge for water and unburned fuel. As the deposits absorb moisture, they expand and reduce valve guide clearance causing the valve to fail to seat properly.
You ned a couple of successive short cold starts for this to be a problem. Normally the moisture is evaporated out after about 30 seconds of operation and the deposit shrinks again.
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