Low compression is a prime cause of hard starting. After the bike has been sitting overnight, you are relying on cold compression to start. Once started, oil is splashed up on the cylinder walls and helps bring up low cold compression due to poor conditions of the bore, rings and piston which cause excess blow-by of compression pressure. Experience shows that an engine in good condition can develop, say, 120 psi cranking compression when cold. A bad cylinder may read only 60 psi, yet with injection of oil in the cylinder, come up to 125 or higher. When warm the same cylinder which tested 60 psi may come up to 95 psi. All this means is a worn engine may start and operate okay when warm, yet fail compression when cold and be a mysteriously hard starter. The above does not take into consideration leaking valves. Generally, if low compression does not come up when oil is introduced into the cylinder for a retest, either valves are leaking or a cylinder head gasket is blown. For differential diagnosis of these faults, you can use a "poor man's leakdown tester" as follows: Take an old spark plug and gut it of its porcelein insulator. Then, braze-weld a quick-disconnect air hose fitting to the plug. Remove the plugs from the engine and crank the engine until the cylinder to be tested is at TDC on its compression stroke. Screw in the tester, attach an air hose and subject the cylinder to 60 psi. If the engine turns, its not on TDC. Try again and with the bike in gear, apply the rear brake. With air pressure in the cylinder, listen for escaping air at the carburetor (intake valve leakage), exhaust pipe (exhaust valve leakage) and for good measure, at the crankcase breather (piston leakage). Remember, test a cold engine for meaningful evaluation, contrary to popular belief and even text-book recommendations.
