Are you picking up input pulses to the tach.?
I found that these pulses, on my Harley Sportster, are greatly disfigured by the ignition coil. (they are the same as the electronic ignition output) I tried using a parallax basic stamp to count them but the counts were very innaccurate, in other words the ignition output pulses were very difficult to work with. I know that the ignition pulses feed the tach circuit, where a ckt. similar to a lm2917 converts the pulses to a varying dc.
I found two other sources on my bike that provide clean pulses. One is the speed sensor, and the other is a sensor that feeds the electronic ignition.(can't remember the name of that one). I also changed to an arduino set up, which works much better, for me, and is less expensive, than the basic stamp. I prefer to use the speed sensor, since it won't mess with the ignition and is easier to get at. However, the speed sensor pulses come out at 12v, but I use a volt divider to reduce them to approx. 4v.
Presently, I'm simply counting and timing the pulses and storing that data on an SD card. (SD/MMC FROM THE GROUND UP, by sirmorris, arduino forum exhibition)
By your picture, it looks like you have your ckt. attached to the gas tank cap.