The spin cycle does not always spin clothes dry. how do I tighten belt?

I suggest going on the Whirlpool manufacturers website and check their forums for a similar problem. It could very well be something very easy to fix.

387780 does not seem to be a valid model number.
If you can locate the model number enter it here at sears Parts Direct.
Parts for Brands Craftsman GE Kenmore Whirlpool and more

Link to videos about washer not spinning.
top loader kenmore washer won spin YouTube

drain or drain pipes blocked
bearings gone

First thought is that the cycles are off.we have the same problem with ours right now. That slow spin speed concerns me. The motor could be going bad. If this washer is 10-12 years old, you may want to look at replacing it. Does the machine ever start agitating before it finishes filling. If so, the cycle mechanism is going bad. This has a whole host of potential problems. Ave a rep look at it. He can narrow it down quickly.

Try to check the swith that operates when you close the top door. It may be broken or loose.

This problem is almost always a broken belt. Yes you can do it yourself.
Go to repairclinic. com and put in your model number for the exploded view of your dryer. Locate the belt part number and belt orientation.

as with basic spin faults -a number of reasons usually 1 motor brushes, 2 motor 3 motor control pcb or the obvious one - the belt has come off.

it sounds like a worn clutch.

There is no belt. This is a direct drive unit. There is a motor coupler located between the motor and the transmission.

Possible your belt is loose; you have to unscrew the back after disconnecting electrical connection from wall socket. If the belt "gives" more than half an inch, you have to loosen the motor bolts and push motor upwards, then tighten the bolts very well. Try spin only to check, before closing the behind. It shd spin very fast.