mcculloch Runs at Very high Revs?

Hi I've just repaired mower but now it runs at very high revs! There is no throttle or adjustment screw and only automatic choke. Is this a fault in carb jet? as I can't see why it revs so high. Plus not having owned one with-out a throttle I'm not sure how fast it should rev .(but this seems too high, about to take off). Also it is a pig to start, I will replace plug tommorow.
Any help much appreciated.