Hi, just want to add my 2 cents worth because I had a similar thing with mine when I first went to put it back together after replacing the bearings.
When you remove the backplate , you will see that in the rotating disc there is a rectangulr hole which must line up with the end of the crank that sticks out slightly through the conrod, which is also the same shape as the hole in the rotating disc.
It can be assembled incorrectly and still give you some pressure in your tank, but it will not work as intended.
Just rotate the disc to line up with the end of the crank. Reinstall your backplate and away you go.
To check it is correct remove the carby plate as well and you can see if the disc is moving at exactly the same time as you turn the prop shaft.
Hope this helps.