Biggest issue with the Weber carbs are vacuum leaks due to the adapter plates not being sanded flat and.or lack of brown goo Permatex aviation sealant on the gasket surfaces. Did you plug ALL the vacuum ports - there are a lot !!! Check and re-check your spark plug wire orientation too.
Also: you had the cylinder head off. The distributor's shaft can be inserted about 12 ways, only one is correct. When you manually turn the engine over to TDC for #1, the distributor rotor should point to the 9 o'clock position as you look at it. And did you set the timing with a timing light and the distributor vacuum line unplugged and capped? If vacuum leak, real tough to set timing with that line connected. Did you get the timing belt installed correctly?
See
http://www.mazdatrucking.com/B2200/B22TimeCheck.html
See
http://www.mazdatrucking.com/B2200//B2200_Timing_Belt.html