If your pull unit does not appear to catch to pull start the engine, it is usually a sticky "dog" latch. This is very easy to check and fix.

To check this: :

1) Remove the 4 phillips screws that hold on the pull unit and pull off the pull unit. Check that it recoils properly.

2) Under the pull unit is a thin metal spacer "gasket". You will also see the "dog" catch flange that is connected to the engine crankshaft. Turn or rotate the flange with your fingers and make sure that your engine turns over (you will feel some resistance from compression at certain spots). On the flange there is a movable white or metal lever on a spring. Check that the lever is able to move freely and returns to the original position when it is rotated against the spring. Occasionally these can get stuck if there is debris caught between the lever and the flange, preventing it from rotating back to catch against the pull unit when the chord is pulled. You can also try a little grease under the lever. 

3) When replacing the pull unit make sure that the holes in the spacer gasket line up with the holes in the engine housing face and the pins on the pull unit. There should be no space between engine housing, gasket and pull unit. All covers and other parts should be on top of the pull unit including the exhaust grill tab and the gas tank brace. Check that the engine "catches" when you pull the pull chord.