Starter or high resistance in the connections would be the first reason for one motor spinning faster than the other. After that the next reason is the non starting motor is loaded up with fuel and is partially hydrolocking. After that we don't want to talk about the reasons one motor could be harder to turn than the other.

A few voltage drop tests will tell you if the issue is electrical.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician