Hi Dave,
Good to see you are trying to look after your mower.
To help you out.
First get a decompressor in serviceable condition and install it in the mower as it should be. Without the decompressor you will ruin the starter mechanism fairly quickly as it puts un-necessary strain on the pull cord. Try to keep them as original as possible and you won�t go wrong.
Now with regard to your reving up and down it sounds as if the lifter within the carby (it goes on after the cam,It has three prongs coming out of it) has not been placed into the three slots in the carby correctly. If you don�t slide this piece in correctly it will give the symptoms you identified. You will see the three slots in-side the carby that this piece needs to slide into.
If you need a picture I am happy to put one up for you.
Put some pictures up for us, it makes it easy for others to help out.
Blokes like Bigted and Joe Carroll who are much more experienced will no doubt have some other comments.
