The reason it's not working right is because the engine is raised up about a 1/4 inch to accomidate the pull start backplate. The wheel is then just hitting the chassis before it hits the flywheel. In order for the starter box to work, you need to get a regular backplate. http://www.amainhobbies.com/product_...oducts_id/4331
Thats the one you'll need.
Good luck
If you don't want to lose the pull start, you can grind the flywheel hole wider, but that's tough, and it can weaken the chassis some.