Limit to Category search problem

I having problems when I try to search for a product and I select a category in Limit to Category:

Zen Cart 1.3.8a

Added "whole word search" by Nirmal Natarajan.

I've created a very simple search suggest on the homepage of my site, by assisting on the spelling of products on my site I hope I can help people find what they want more easily.

the search works fine if you put in, for example Aprilia RSV1000, it returns 69 products, but if you restrict the search to one category, "Arrow" for example, which I know has at least 4 product for this bike nothing is returned

the issue seem to be how the search query string is put or read in the browser,

when you don't "limit to Category" there is a plus in the query string between the two search terms "Aprilia+RSV1000"

but when you do "Limit to Category" the + is missing "Aprilia RSV1000"

could this be why the search is failing?
how can I amend this so it works?

I have tried restoring modified files like the header_php.php from Pages/advanced_search_result, which is modified by the Whole word search, but this has not had any effect.

https://www.motorcycleheaven.co.uk

any help gratefully received