Just brought a 2006 big wheel Rev 80. Parked the bike next to my 4rt and noticed that the seat height on the beta is 50mm higher.

Is this correct?

Should I fit the standard yellow shock which came with the bike? If so, how do I check that it's ok?

The bike is for a 10 year old.

Thanks

 Yellow shock or spring? At the same time your 4rt could be sagging pretty good. I was noticing that yesterday with 3 older 4rt`s next to 2 brand new 2016`s. 

Yellow Spring. How can you check that the shock/Spring is any good?

This video shows testing shock with spring removed.you should be looking for pressure both ways compression and rebound without knowing length of original spring, hard to say if the spring is any good maybe they had adjusted it up.

.https://www.youtube.com/watch?v=1irem3oC4xw

Fit a smaller seat then!!!!!   Just kidding, The Beta seat area is quite tall for a bike of it's size.

 

Beta Big wheel 80 varys on specs but seems to be around 660mm.

 

The Montesa should be 650mm.