In my experience, most VW fires are caused either by the feed tube popping
out of the carb float bowl as /martha describes; or by ancient fuel lines
disintegrating and leaking. In some cases these fuel lines don't even have
clamps! Was this a policy with brand-new VW bugs and buses? If so, it may
be fine when they're new. But when they get old, they loosen up, eventually
pop off, and spray fuel all over your nice, sparky generator and/or
distributor.