50cc likes really weak springs.
I'm running my very ported pp to cyl reed stock cyl with the stock spring and it's great.
But I'm using the stock tjt plate.
I think hobbits have a bigger prob with overvariation than derbi.
I also noticed my 50cc derbs is very sensitive to gearing. Can't speak to your bike cuz I have 16s which is kinda weird(edited)