[steveproracing]

Here's a simple one to try
Check you have not tightened your wheel nuts up to tight.
This is really easy to do on the car with plastic hexes.

Best thing to do is as said earlier
Remove pinion and check everything is free.
Gearbox should spin for several seconds with wheels on!

If all is ok
Here's a simple way to set diff and slipper

First. Tighten slipper nut fully up
Then hold the spur gear and a wheel with 1 hand
Then try to turn the other wheel. It should take a fair bit of force to do this!!

If its too easy tighten the diff screw and repeat the testing until it is hard to turn the wheel

Then you are ok to move on to the slipper
Back the slipper nut off til you can only see about 2mm of thread showing on the screw after the nut
Put the car on the ground and hold it flat to the floor with one hand
With the other try to move the spur gear forward with just your thumb
It should be fairly hard but possible without any pain

Now as a final bench test.
Try the diff test again but don't tighten the slipper nut anymore
Ie hold the wheel and spur with one hand and rotate the other wheel with the other
You should be able to rotate the wheel. There will be a reasonable amount of tension but nowhere near as much as when testing the diff

Whilst rotating the wheel watch the slipper nut
It should rotate. If it doesn't then your slipper is still tighter than your diff

Lastly final settings to slipper can only be doe trackside
Set your car off full throttle. Listen to that slight squeeling /slipping sound
Set the slipper so you can hear this for about 10-15feet if
You hear nothing slipping or the car wheelies then it's too tight
If it travels for ages before stopping its too slack

Once you are in the 10-15feet range then that is roughly the sweet spot for that surface but play about with a 1/4 turn of the slipper either way to find your own favoured setting

Sorry for the long winded reply
Hth