The tach is cable driven. First order of business would be to inspect the cable inside the sheathing. Disconnect it from the motor, and clamp the metal spade in an electric drill. Spin it and verify it turns the gauge. Then, lube the cable well.
If you're riding in 4th gear at 30MPH you're not doing your engine any favors. These motors prefer to be revved well above 5,000 RPMS, mostly between 6,500-8,500. Many ride well above 8,500-9,500 constantly. Even on a "new" motor, revving the engine is not a bad thing, just vary your engine speeds and avoid sustained RPMS for the first 50-100 miles.