Tacho Drive on Pre-Unit Triumphs

[A65rider]

I have a Triton bike which has a Triumph Tiger T110 engine. At some point a different timing case with a Tacho drive has been fitted (with internal gears etc) along with a Smiths Chrononmetric tacho (RC1307 / CO 4-1). I'm convinced the tacho is reading high, ie maybe 2X or higher. Is there any way that the tacho drive can be mixed up to do this. Regards

[L.A.B.]

At some point a different timing case with a Tacho drive has been fitted (with internal gears etc) along with a Smiths Chrononmetric tacho (RC1307 / CO 4-1).

Could "CO" actually be "00"? (RC1307/00 = 10,000 RPM 4-1 anticlockwise rotation with light)

I'm convinced the tacho is reading high, ie maybe 2X or higher. Is there any way that the tacho drive can be mixed up to do this.

Count how many crank rotations per cable turn.

[A65rider]

Thanks for the response. I think you're correct with the numbering, it certainly has 10000 RPM 4-1. Sorry to be dim but how many crank rotations per cable turn should there be. Rgds

[L.A.B.]

Sorry to be dim but how many crank rotations per cable turn should there be.

Well, if your tacho is 4-1, then 4-1.

[A65rider]

OK I assumed that 4-1 referred to the gearing in the Tacho head but inlight of what you've said I guess the works inside that are just converting the variable rotational speed of the cable into the calibrated pointer movement on the face. So another dim question, should it be one crank rotation for 4 cable turns or the other way around. Were there other tacho drives that had different ratios and could have been installed. Rgds

[Rex]

Hardly, otherwise when the engine was doing say 5000 rpm the cable would be doing 20000 rpm! Anything rotating that fast would be dangerous and short-lived...

[L.A.B.]

So another dim question, should it be one crank rotation for 4 cable turns or the other way around.

Well, I did say crank rotations per cable turn, not cable turns per crank rotation but turning the crank a few times should also have answered your question.

So, for instance, 1000 (crank) RPM would be 250 cable RPM, a 4-1 tacho, if accurate, should, therefore, show a reading of (250 x 4) = 1000 RPM.         

Were there other tacho drives that had different ratios and could have been installed.

Edit: I'm not familiar with pre-unit tacho cable drives, so possibly, as there are also 2-1, 6-1, 8-1, (I doubt it would be 3-1?) tachos, but you could answer your own question by checking the crank to cable ratio. Either it is 4-1 or it isn't. 

Edit: There are also clockwise and anticlockwise drives/tachos. As your tacho does work it is obviously the correct drive rotation but any replacement tacho needs to have the same drive rotation.   

[A65rider]

Thanks to all those who kindly responded to my question. I have at long last got to the bottom of this. As pointed out my tacho is driven from the exhaust camshaft clockwise at half engine speed. I had in fact a 1:1 reversing tacho (the sort normally fitted to Norton ). Therefore my 4-1 anticlockwise tacho was reading 2X. I now need to obtain a 2:1 reversing tacho drive (available Speedograph Richfield, Gaggs etc) and hopefully all will be sorted.