I have seen several posts on installation of aftermarket tachs.  Mostly on cafe builds etc.  The guy I bought my bike from messed with idle speed and I don't trust my ear enough to get it right.  My question is: I've heard conflicting stories that It takes a particular tach to work accurately on single cylinder machines.  Is this true?  Or will any tach that plugs into the ignition coil work?  Thanks for the help everyone.

Also,  I've heard the advice, "if bike runs at 9 mph in first with no throttle then your idle is good."  Well, I'm kind of a heavy hitter, 240lbs if I get the idle up to where it takes my butt 9mph the motor is racing!

Most of the tachs should work also.  The difference being how many cylinders.  But I think it comes down to how many times the coil fires per revolution.  Some fire twice.

If its just to check 1 time couldnt you just use some Kragen dwell meter to read it? They can go pretty cheap.
http://www.ebay.com/itm/like/151275611016?lpid=82

Thanks everyone.  I just hear horror stories about being set too low causing damage.  Don't want to starve the old girl of oil.  Question dave:  Idle is 1100 rpm recommended, why would it sound like 500 bpm?

Awesome, thanks again everyone!

Here's a picture of the Ryca tach on my S40.
I used a piece of aluminum angle, a couple of
u-bolts, and rubber grommets.
As kuri77 said, its easy to hook up and it works well.
I have more detailed pictures of the installation
if you are interested.

I'm on the road right now.
I'll try to get the detail pics of the
installation posted Sunday night.

Thanks all,  I'll look into the Ryca and other options.  Not really interested in making it a cafe, just an option I'd like.  Todd if you could give some advise on install and pictures, that would be tons of help!