Old Mercury 2 stroke 5HP problems?

I've got a Mercury 2 stroke 5HP, and I've got a problem with it: when I try to start it up on low power, it won't start, when I try it with higher (just over medium) power, It starts up and runs without problems, but when I put it back under medium, it turns off, anyone a solution? I guess the low power nozzle is clogged, but I'm not sure.