How do you slow down the running speed? Is there a reestat for this machine? The motor is a Tacsew Clutch Motor, Model RM1878-1L, 1/2 hp, Sr. # HMC 04051325

How long has it been since it has been serviced?

You don't say if it is a clutch motor or a servo motor. They answer to your question is yes you can slow it down by either motor (usually a knob or switch) and by pulley. If you don't have a servo motor I would strongly urge you got contact Cobra Steve at leather sewing machine company (google it) Tell him you want one of the smart servo motors with needle position-er (needle up and down automatically)
His smart servo motors are also modified so they work very similar to home sewing machines... with graduated speed based on how hard you press the pedal.

But if you must and want to stick with a clutch motor... you might try this fellow's hack.

http://www.adafruit.com/blog/2007/11/08/hacking-an-industrial-sewing-machine-by-adjusting-the-clutch-motor/
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Welcome to the world of industrial machines. As you are discovering they are not like a home machine. Typically these machines have either a clutch motor or if you are lucky a servo motor.

If you have a clutch motor there is typically a knob that allows you to slow it down. Or sometimes the pully needs to be replaced to "step down the speed"

A digital servo motor is a bit more complicated and you'll need to find some instruction on how to use it. But they can be easier to control. I've provided a video of the servo motor I have and recommend. (I sew heavy leather)

I have 1 pdf somewhere for a smart servo motor that might help. If yours is a servo, be careful because I got mine to run backwards once. lol Good luck

try adjusting the foot pedal, there are jam nuts under the foot pedal
to make it go slower or faster.

Hi Stacy.... I is this an industrial machine? If so it sounds as if you have a clutch motor designed for immediate and FAST seams. Most clutch motors have setting where you can at least slow them down some... but it will never work like a home machine with a variable resistance type foot pedal allowing slow stitches.

If I am correct and it is an industrial the best thing you can do for yourself is to buy a servo-motor. I believe if you call Steve at http://leathermachineco.com/ He can fix you up for a reasonable price. Make sure he modifies it for variable speed for you. (he does it free)

Sounds like a failed clutch brake on a clutch motor. I would replace that with a servo motor. You will like it much better.

There is no speed adjust, but 1st you must know the source of the slow down. Remove the belt from the motor pulley and run it, if it goes fast try turning the hand wheel if it is stiff the machine needs clean and oil. If it turns freely you motor may be dirty or just slow. If dirty, sometimes you can without the blet attached run it at full speed for 10 to 15 minutes. It should get faster as you run it. If not motor is weak or slow needs replace. Just replacement motor as genuine probably no longer available.

THE FOOT PEDLE IS NOTHING BUT A REASTAT. A REASTAT IS A WIRE WRAPPED AROUND A CONDUCTOR THE WINDINGS START OUT FAR APART AND GET CLOSER TOGETHER THE FARTHER YOU PUSH THE PEDLE DOWN . THE FAR APART WIRES ARE THE SLOW SPEED THE CLOSER ONES ARE FASTER SPEED
THESE WIRES WILL WEAR OUT OVER TIME OR CORRODE OVER TIME AND CAUSE THIS PROBLEM. TRY SOME ELECTRICAL CONTACT CLEANER AND SPRAY THE FOOT PEDLE INSIDE ANY WHERE YOU CAN AND LET DRY. MOST CONTACT CLEANERS ARE NONE CONDUCTIVE AND DRY IN MINUTES

Generally going to have to be addressing the drive motor, either substituting a lower speed clutch motor for the existing one, ie if you have 3000 rpm motor, then going to a 1725 rpm motor. Alternatively, selling the clutch motor and buying a servo motor instead, which should be available with a variable maximum speed control. The Sewquiet 4000 can select different speed ranges with the flick of a switch. If you already have a servo motor and it does not have a variable max speed, then switching the drive pulley on the servo motor to a smaller size should help
A more convoluted solution would be building a support under the table for a layshaft with different pulley sizes so that the motor could drive one pulley and then a smaller pulley on the shaft would drive the sewing machine. Not a bolt on solution but pretty cheap to do if you are good at fabricating.