BLUEMOMENT

If you can hand-start your iron tops'l read no further. (obviously)
After a long idle period or during a fuel air-bleed session, cranking the engine may sap the available battery power. Lower winding revs is an indication - if it doesn't start soon we're phoqued... On an engine with no decompression facility to assist with higher cranking revs, one solution is simply to block the air input. On my perkins 4108, I have to remove the air cleaner and put the palm of my hand over the intake. I can then crank the engine until it reaches a high speed. (Due to no air needing compressed, this does not take so much power from the batteries, which may otherwise not have been up to the job) . Injector pipes can now be tightened once spurting (if that was the job in hand). Once the revs are up as far as they'll go simply remove the air blockage and the engine should burst into life. If you can't reach the ign key from the engine bay, a momentary switch can be wired between the two relevant tags on the starter solenoid and mounted in a handy place. SAFETY FIRST ! if your engine likes to start at full throttle, MAKE SURE IT'S NOT IN GEAR and also that you can get to the throttle quickly. You can also use this method to prime the oil system after a filter/oil change.

Interesting idea and it obviously works or you wouldn't have posted but my first thought was that creating a partial vacuum on the intake stroke would require almost as much energy as the compression stroke.

I'll now confess to the discovery of this method...
While motoring out of Troon towards Lamlash, The engine died about two miles out. Trying to restart it brought real fears of something badly wrong. The engine spun very quickly - no compression, and the oil pressure guage did not come up to it's usual cranking level. Also it sounded different, and wouldn't give the slightest cough. We sailed back with me analysing the problem in my mind. On the mooring (how we got there is another story as the rib died as well!) I took off the tappet cover, spun the engine, and all looked good. Had to get the air cleaner off to get at the tappet cover so looked at it....... and hey presto - all the airborne debris from refurbishing the wheelhouse woodwork was jammed inside.
Clean out - and all was well. The oil guage was a poisson rouge, as the oil was hotter than normal. Panic over and lessons learned. ( ie clean up properly !)