Were the points adjusted recently? The first suspect would be the ignitor, especially if it's the newer electronic type.

You might want to think about thinking about getting in a
qualified tech. that hopefully knows what he or she is doing. Left unattended your furnace could possibly put new
meaning to the big bang theory. Oil fired furnaces are not
something a novice should even attempt to play with. Ever
seen an oil furnace explode ? It's pretty impressive what
that little box can do. If the oil line ruptures during the explosion it gets even more exciting. call someone in.
I've had an oil burner licence for 20 years and thought i
saw it all untill i saw someone had connected their oil
tank to the furnace with a garden hose. Yes the tank was in the basement.

A weak transformer can delay ignition.
Also a set of sooted electrodes can do it.
bay and K are both right. Delayed ignition is serious.
Have it repaired professionally.
-br