Check if there is an adjustment screw with a small spring around it on the carb. If there is look on the outside of the carb where the screw is, there may be a small hole for air and if the screw is set open enough you will have to cover both the carb opening and this hole for it to suck fuel while priming. I noticed this on my o.s. 46la the screw is on the front of mine so the hole is directly in front of that screw on the outside of the carb. I almost thought I was going to have to remove my carb and replace the oring because mine wouldnt prime without an electric starter 'till I noticed this(should this screw be open that far?I never messed with it.) .</p>