how do you have the tank plumbed? is there a break in the supply line (other than at the tank stopper)? You would be surprised at where these things will pull air from sometimes instead of fuel. The only diaphragm that would affect the pumping ability would be the pump diaphragm, It's been 4 months since I put that carb together but I thought it was in decent shape.

I encourage trying converting one, the only special part is the prop adapter, the rest is just labor making it look the way you want. On some you have to have an engineering mind to figure out how much of the front you can cut off and not make it weak, like the Homelite where the coil is mounted off the front shroud you can't just go chopping it all up. A lot of people get hung up on chopping a lot of aluminum off of the things, most of the weight is in the crankshaft... steel is a lot more dense than aluminum. Feel free to steal some of the ideas I used on those engines, I borrowed them from others, like the studs out the back of the backplate, I got that idea from av8tor and it works great!