How do I set the speed of the car on powerup?

Hi,

I am building a simple racing game.

i have a power up which on collision, is supposed to boost the car to double speed for five seconds, and then return to the original max-speed.

I tried this:

and this:

but neither seem to affect the car's speed at all!

I replaced it with 'stop' and that works fine.

Any idea what I'm doing wrong?

Thanks

Andy