It's probably a setting in the admin options. Keeps the search index small.... and useless! A forum admin should be able to tweak this, but may not want to. Meanwhile, you can use Google, which indexes these forums pretty well. Simply add site:forums.ybw.com to your query, like so: site:forums.ybw.com tmc 40 oil leak - Google Search
The idea is to exclude common short words like "and" "of" "the". But I agree it's a crude tool when you're looking for very specific uncommon short words. A common feature of forum software is that not enough effort goes into searches.
Meanwhile, do as Yngmar suggests. Google is good at searching.
Works the same with the DuckDuckGo.com search engine but they aren’t harvesting every piece of data they can about you and monetising it. Unlike Google. ?