Oil fired hot air furnace. When unit is off for an extended time it will have delayed ignition on start up. Pump has been replaced because it failed a vacuum test. New nozzle and electrodes, new filter. If thermostat is set at normal (68*) it cycles fine. Problem is only when thermostat is turned way down and the unit totally cools.?

Sounds like there is a fuel accumulation after a prolonged shutdown. Have you explained this to your repairman? Are you sure it's delayed ignition?

Bad internal pump check valve can cause this. It allows nozzel drip when unit is off and when restarted the excess fuel lights off with a rumble. A delay selonoid valve in the pump to burner line solves this problem.