squareye,
First thing before doing all this check bearing wear by holding the blade plate and seeing if you have any movement.
If you have movement then don't bother with the list as you will have to replace bearings first.
1. I would check the fuel flow from the tank to the carby.
2. Check vent hole in petrol tank cap is clear.
If fuel flow is slow (1) then check (2) if still slow replace fuel tap.
Then if you still have the same problem then check this.
1. Check to see if the carby mounting block has movement. If so tighten up the two screws.
Test again and if the problem is still the same check this.
1. Check 'O' ring on neck (replace this one if damaged)and one that goes into cylinder block (replace this one even if not damaged) if they have any damage replace and test again.
Then if problem is still the same do this.
1. Remove starter assy and check 'O'ring for damage but you must replace the 'O' ring and reassemble.
Hope that this is of some help.