Wings is correct, in most cases you want the velocity tube to be angled to the rear, you do not want ram air into the carb. If the tube is withing the cowl, you need to experiment as to which way is best as the internal circulation will effect it. As to your problems where it first worked fine then failed after taxie tests, maybe the small screen in the carb is plugged up. This is the little fine mesh screen under the metal plate held on with the big screen right in the center. If the plane sat unused for awhile, this could have collecte a bit of trash. Remove it and scrub it off in fresh gasoline or alcohol then replace it. You might also need a carb kit (available at most lawn mower repair shops) which includes new flapper valves, screen etc.