If you clean out the tank and maybe blow out any fuel remaining in the system, it should start ok (it'll take a while, jump it off a car with a big batt/alternator). The problem could have been caused by a faulty glow plug circuit (i.e. glow plugs not working/not working to their full potential) Check this out if you can. other than that, dirty injectors can cause slow starts. A bottle of injector cleaner through the system may fix this, of you might have to pop out the injectors and get them cleaned by an expert. Hope this helps.