Water, dirt and oil ingress will shorten the life of the timing belt. Imagine driving down a dusty road, then u go thru puddles of mud, I dont think the timing belt would like that, esp if some large piece of stone chip gets lodged between a timing belt tooth and gear. Not to mention the timing belt might slip off the top cam gear. Other than that, I think its fine to run without a timing belt cover, why dont u go ahead and try it? :lol: