Spark advance usually is specified in crankshaft degrees If

Spark advance usually is specified in crankshaft degrees. If the spark advance on a four-cycle engine is set at 15 degrees before HDC and you wish to change it to 20 degrees before HDC, how far must the distributor be rotated?

Solution

suppose spark advance15 degree before HDC disrtibutor speed a rpm.

then spark advance four cycle engine 20 degree before HDC distributor speed b rpm.

15 Degree speed a

1 Degree speed a/15

5 Degree speed a*5/15 = a/3

Due to four cycle=( a/3)/2

=a/6

=100/6

=16.67%

Distributor speed increse 16.67 %. compare to previous(ref:spark advance 15 Degree) distributor speed.