One thing to try is spray a little carb cleaner/brake cleaner around the intake mount to see if you get a raise in rpm, just make sure it isn't near the carb opening. It isn't good for the motor though, no oils in fluid, so don't do it much. You can also use a smoke machine but that is harder to get.
and the motor has to be running
are your hoses good, gas line could be partially blocked or collapsing?