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One of the issues you mentioned was the throttle shaft ambling in the carb body. If it’s excessive, first check the shaft for wear & replace if necessary. If there is wear on the carb body, it may need to be replaced. The throttle disc or butterfly is designed that way so it will idle. Racers use full butterflies to shut down the motor when the remote throttle is released. The Crankcase is machined as it is as it was also used on MK 20s which use a card with the mating face oblong as well.
one final thought. From personal experience if your OMC coils come from China they can cause ignition fluctuation as you are experiencing. You must use OEM or those made in Germany,