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Soumds like whoever was working on the engine was guessing and parts changing. Was the powerpack and optical sensor actually bad or what?
Was the carbs actually fouled? The syncronization and jetting is very important on that engine, it has to be correct. If the carbs start to open before the timing is advanced enough then it will stall or give poor acceleration. When you push in on the key you are operating the primer which opens a solenoid and allows additional fuel into the intake manifold. This tells us that there is still a problem in the carbs or at the intake. The throttle bodies and intake are very prone to leaking both the parts and the gaskets. They must be put together properly or they will warp.
So if the carbs were rebuilt what of the intake gaskets? Was the engine synchronized after the carbs rebuilt? Was the engine water tests? I would expect the shop to pick up on the lack of acceleration. Personally it sounds like the shop didn't do a proper job.
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