How to slow down and control clutch motor on industrial sewing machine.

I have tried the internet and all the ideas. This is by far the best idea so far. It's impossible to slow down a clutch motor because the cork grabs and snatches the machine to sew. In order to sew slower you have to slip the clutch just like in a car or motorcycle. Other wise when the cork clutch grabs and engages, its impossible to control. Especially when starting. I've extended arm, changed pully and loosed the belt. non works as efficiently as adding oil to allow the clutch slip and engage smoother. I'm definitely not a pro. found that this works the best. It's got nothing to do with leverage or speed. I find the machine to work best in its standard form. I have said that my spring is farly tight. NOW. I never mentioned this but tighten the spring all the way until it does not move the arm or engages the sewing machine. loosen half turns until machine just starts to spin the sewing machine. Not to tight or you would have to apply lots of pressure on the foot and not to loose or else it will spin up to fast. There is a sweet spot, find it. Good luck.