[tsubaki]

Yes you are correct about the warmup lever.
Taking one apart is scary the first couple of times.
Pay close attention to the parts locations and what they do, even take pictures.
The culprit is most likely any locations that have to ride on metal parts.
It wouldn't be surprising to have to spray penetrating oil and sit for a while on corroded bolts and fittings on the control box.
The thumb bolt that is near the ignition switch is a friction adjustment for the throttle handles, it is almost never the culprit (unless kids got into the boat and started turning stuff).