Topic: 78A, install a tachometer from an 80s honda?

[78unknownA]

hello, i recently got a cb750a which is in pretty good shape, question being, i got a tach from a 80s honda, not sure exact year for free and was wondering what wire would i hook up to the bike? would it be a positive, ground and one to a coil?, it doesnt have a tach cable, only brown/whitestripe, green/bluestripe, black/brownstripe, yellow, and two green wires. ( gauge has a mini temp gauge on it) so figured two wires would be to that, possible could even get a sensor for the gauge, but any help would be really good, tried searching everywhere and couldnt find anything, dont want to bring it somewhere, cause nowadays people just rip you off. but yea any help would be much a appreciated.

[raymond10078]

Based on reviewing Honda bikes of the 80's here's what I think (best guess).

The Black / Brown wire goes to a switched hot.
The Green goes to ground.
The Yellow goes to the point side of a coil.

I know you said two green wires.  One green is for the tach bulb, the other is for the tach itself.  Per the 85 750 Sabre schematic I'm looking at, the two greens are connected together out side of the tach (going into the main harness) so I would bond them both the ground.

Per that same schematic, the Brown / White goes to the tach bulb; and the Green / Blue is the temp sensor wire.

Please realize that it may not work.  The 80's bikes were CDI - meaning that the pulse going to the coil differed from a points bike.

P.S.  You could mount up a normal (non-auto CB750 cable driven) tach if you wanted to . . . .

[78unknownA]

thanks much for the replay, im going to have to try this later tonite. i do have possibly the idea it wont work but figured id give it a try since it is a free tach... any coil it shouldnt matter? cause the shop that my dad brought it too ( he tried to make the bike a birthday gift for me, but a shop #$%*ed it all up) installed a dyna-s on it and was going to connect it to that wire since its already a splice connector. just figured install a tach would be able to help me with idle issues/timing  and fixing my vacuum leaks

[78unknownA]

by the way, how would i be able to use an tach cable, i tried looking into it but couldnt find anything on the autos, didnt know if they would be in the same place.

[chickenman_26]

There's a fly in the ointment. If you remove the pilot box to install a tach, you also remove the speed warning device that operates the idle up control valve. You'll also lose the oil pressure light, gear indicator, and fuel gauge. 

Stu

[raymond10078]

The speed warning device is in the speedo.  But your point is sound - you'd lose some functions.

[chickenman_26]

The speed warning device is in the speedo.  But your point is sound - you'd lose some functions.

Yeah, sorry. Brain fart...

Stu