Magnetos are timed to the engine so as to provide the spark at 20° to 25° before top dead center of piston compression stroke. This timing is designed to provide engine efficiency at normal operating speeds. Since starting occurs at very low engine turning speeds, magneto timing must occur closer to top dead center to push the piston down. The objective of the impulse coupling is to delay magneto firing from 20° before top-dead-center until about 5° before Top-Dead-Center by building in a 15° lag angle into the magneto impulse coupling. This happens only in theory.
The degree of impulse coupling retard is directly proportional to crank speed. Take for example the Lycoming O-235-L2C engine. The original starter cranked the engine too fast. Lycoming Service Instruction 1362 changed the lag angle of the impulse coupling from 15° to 5° to compensate for the fast turning starter and greatly improve starting on this engine. (with the 5° lag angle the engine may have a tendency to kick-back during hand propping. Don't hand prop!)
A later starter (Prestolite MMU-4001R) was specifically designed to turn the engine slower. But be careful now, this slower starter works with the original 15° impulse coupling so the magneto impulse coupling lag angle should remain at 15°.
Now lets say you purchase a starter or magnetos for your O-235L2C engine. To get optimum starting performance you need to match the starter with the impulse coupling.
The same hard starting problem may occur if you switch to a faster turning light-weight starter and don't change the impulse coupling on any impulse coupling magneto.