When I got a bunch of engine parts cleaned they were nice and shiny, since then I've had a few...let's say....mishaps. So the intake got dirty from leaving dirty tools on it and touching it with dirty hands. Just dirty....
The wife and I have jumped on the natural wagon the past year and I saw a few recipes for vinegar uses and one of them was for cleaning metal. Thought I'd give it a whirl.
Spray white/distilled vinegar on it and it will lift dirt off of metal. Since the intake on an M20 has a texture to it it makes it hard to clean. While it's not a miracle worker it made a considerable difference. Thought I'd share.
The right side had already been cleaned when I decided to take a picture, but it looked like the left side
If you do this make sure to rinse off with water after as vinegar is acidic.