After doing a little more research maybe I just didn't know how to tune my vibratory feeder correctly. I don't know why they don't give you instructions when you get them. After years of service I now know that the lowered vibration is caused by the spring not being stiff enough. For everyone interested out there here are the instructions for tuning your vibratory feeder.
TUNING INSTRUCTIONS for VIBRATORY FEEDER BOWLS
When purchased without a feeder bowl, it is necessary to "tune" the unit after the feeder bowl has been installed to obtain proper performance.
To determine the tuning condition of a feeder bowl, the following steps should be follows:
1. Gain access to the springs.
2. Check the air gap. It should not be less than .020" for this test.
3. Set the control knob at 100 dial setting.
4. Energize the feeder bowl and observe the vibration and feeding characteristics.
5. While observing the feeder, loosen slightly one of the spring clamp bolts on the spring rack. Note the change of vibration and feeding characteristics as you loosen the bolt.
If a tuning correction is required, proceed as follows:
UNDERTUNED UNIT: VIBRATORY FEEDER BOWLS
An undertuned unit is evident by a reduction of feeder bowl vibration when a material load is placed in the feeder bowl. The spring system is not stiff enough. The condition can be corrected either by adding springs to the racks or replacing the present springs with thicker springs. A small change to the spring racks will show a significant change in vibration. Any spring alterations should be in small increments. When changing or adding springs, ensure that a symmetrical pattern is maintained on the spring racks at all times. After each alteration to the spring arrangement, the air gap should be re-checked and re-adjusted if required.
OVERTUNED UNIT: VIBRATORY FEEDER BOWLS
If the current exceeds the name plate rating, the air gap should be checked and adjusted per the air gap instructions in this manual. With the air gap properly adjusted and the current within the limitations, check the feeder vibration. If the bowl speeds up excessively when bolt is loosened - it is overtuned. Simply remove springs or replace with thinner springs, keeping a symmetrical pattern on the spring racks at all times. After each alteration to the spring arrangement, the air gap should be re-checked and re-adjusted if required