What is the best and safest way to remove carbon from the head and piston tops (in car)?
Head still on and engine runable?
W/the engine running at a good fast idle, mist water into the carb intake. You'll probably have to work the throttle to keep it running while you do this.
Pull the head and scrape. Put rags in the cylinder where the pistons are down. Of course some of the valves will be lifted off the seats and will also need to be scraped. Make sure to do the combustion chambers in the head. Avoid compressed air as much as possible to avoid blowing chunks of carbon down into the rings. As long as you're doing maintenance flush the block out with water to help your cooling. Try to find the video Steve Jelf posted on YouTube there's a few hints there that'll help you decide what not to do and a whole bunch that'll help you figure out what you might want to do.