Honda izy ticks over but hardly any revs ?

Hello all , my Honda izy hrg465 has been working fine until today when half way through cutting the lawn the revs died off , it still starts fine and ticks over but I can only slightly increase the engine speed, I've had a quick look and the throttle cable pulls the lever on the carburettor tight up to the stop and any further engages the choke , any ideas would be welcome ,thanks