I forget if 510's have a voltage regulator on the back of the guages, but by the sounds of it with the fuel guage playing up also,it tells me it has a regulator and it is playing up.
Small box looking thing on the back of the guages. Simple to change.

Take the wire off the sender unit & earth out on the on the engine with the ignition on. If the gauge moves up it is ok. Replace the sender unit. if the gauge doesnt move there is a problem with the gauge or the wiring to it. Hope this info helps

first of all i would like to thank everyone for input about installing a 5 speed in my 1971 510 wagon. I do live in the u.s. as surmised by most everyone.
My next question is about the water temp sensor.
Does anyone know how to trouble shoot?I am thinking that there must be a voltage reading thay i can take. the gauge was intermittent, but rarely works now. this non operating gage is really bugging me.suggestions would be appreciated.my fuel gage seems to be "slow" also.
thanks
hak