Timer problem

Ref: the attached schematic. I have a problem. The pot has a value of 220Kohms. The capacitor shows to be 4.7uf but is a 47uf actually then this this gives timings of 218 millisecs and 2.4 secs for the zener to avalanche at 5.6volts and switch on the transistor with the pot at zero and 220Kohm respectively. The sealer is specc'd at 1 to 8 seconds hence something must slow down the charge into the capacitor. If I insert a 100Kohm in series with the pot and 20Kohm then the transistor conducts with further delay but the max/min output timings do not equal to 1 to 8 seconds. Can anyone might be the problem solution? regards.