All pulsa-jets have diaphragms, with the possible exception of the one fitted on the tiny fuel tank, more often seen on 2.5hp engines. As the engine runs on choke it canít be too blocked. If you have fiddled with the mixture screw, turn in until it stops (donít over-tighten), out 1.75 turns and the engine should run. Final adjustment is, when warm and revving hard, screw the adjuster out until the revs fall, in until the revs fall, then back to halfway in between.
Almost all spares are available for the 3hp pulsa-jet engine. If briggs-bits wonít help either your description was too vague or they canít be bothered. Your local B & S agent can get almost any spare you need, take the broken part and the model, type, code, details to ensure you get the right part.