I recently restored an old OS FS60 engine, the type with the machined aluminium carburettor & open valve rockers. this was bought from a friend for £60.00. It had been in a box of junk for 20 years & was so gummed up, it would not turn over at all. I carefully dismantled it & dropped all the components into a pan of neat methanol for a couple of days, agitating the methanol frequently & changing the resulting contaminated mixture 3 times for new methanol. The amount of sludge that came out of the engine was surprising. After using an old toothbrush dipped in the methanol, I ended up with a clean engine - inside & out. Warning: If you do this, wear rubber gloves when handling methanol - I didn't & temporarily desiccated my hands, which is not a good idea! I dried off, polished up the cylinder head, lubricated & re-assembled the engine.
The bearings were in remarkably good order, plenty of compression as well. It started easily & runs very well & is reliable at idle for long periods. It was fitted in a "Majestic Major" which is a vintage style plane, ideal for flying on pleasant summer afternoons. The engine's performance was not up to modern OS fourstrokes. I had an idea: As the fuel air mixture goes into the cylinder, past the inlet valve stem, this mixture is very cold, causing the inlet valve stem to contract. With the exhaust valve, the opposite happens - the products of combustion leaving the cylinder are very hot - causing the exhaust valve to expand. I decided to set the valve clearance on the inlet valve to zero clearance. I did this with the engine cold. so ....... no clearance at all on the inlet valve rocker, normal clearance on the exhaust valve rocker. Result: Same fuel, same prop, same day = 1000 RPM more than with the normal valve rocker clearance!
I can go into a coma much quicker now, while flying my "Majestic Major" ! It flies itself really, all I do is occasionally steer it about & eventually land using much "down elevator". Stress free flying when I have had a busy week at work! This setup just opens the inlet valve fractionally earlier than normal & closes it fractionally later than normal. A bit more fuel/air mixture in the cylinder gives a bit more power. I feel that this may only be effective on early fourstrokes, As I am sure that the manufacturers have since altered the cam profiles to give this valve "lap & lead" effect - a bit like on a steam engine maybe. Having said that, I haven't tried it on any other fourstrokes. What's next? "variable valve timing"?