Yep, the tach is working fine. The engine is certainly idling too low - it shuts off going around turns from time to time.

Can you help me find the adjustment screw at the base of the pump? When I find it, which way do I turn it to increase the idle? Are there other potential issues/side-effects from simply increasing the idle speed, especially if there is some other issue at play?

Where is the OVP (overvoltage protection relay) you mentioned located? It could be a broken wire/loos plug but I can't say I know well enough - I know quite a bit about my car, but I'm definitely not a mechanic.

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Autos:
1991 350SD 276,000 miles
2001 VW Beetle TDI 115,000 miles

Horns:
1955 Selmer Mark VI Alto (55,xxx)
1958 Selmer Mark VI Tenor (85,xxx)
1964 Selmer Mark VI Tenor (125,xxx)
1967 King Super-20 Tenor (430,xxx)
2002 Selmer Series III Soprano

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