Monday, March 7, 2011

I gotta know.....the RPM

I mentioned previously that I was a little dismayed to discover that there is no factory tachometer on the NX's dash.  I decided to procure a digital tachometer (and hour-o-meter) from Trail Tech.  For just under $40 (incl. shipping), I now have a tach for the bike.  The tach is powered by an internal battery which is supposed to last for about 10 years.  Additionally, it displays the hours the engine has been running when the engine is turned off.  While I prefer an analog tach, this is about as affordable as it can get.  I prefer analog simply because it can be glanced it and your mind will register (approximately) what revolutions the engine is turning, based on the location of the needle in relation to the tach face.  With digital, your mind has to see the numbers and then 'convert' said numbers.  It similar to switching from an analog to digital wrist watch.

To begin, here's an image of an unaltered NX125 dash. To the right of the speedometer, you can see a balck plastic 'box'.  On bigger Hondas, like my TransAlp, this 'box' covers a fuse block.  The NX has no fuses under this cover, there's just an open hole in the underlying dashboard.  This is a perfect mounting location for the tach.

Here's a shot of the tach mounted on the fuse cover.  I used 8-32 x 1" stainless steel button head fasteners with Nyloc nuts.  It's hard to see but the wiring exits the unit in the upper right hand corner so I had to drill a third hole, in the cover, so that the wiring can run thru the dashboard.  The cover is held to the dash by the tab at the lower edge and one Phillips screw.  Unfortunately, the screw is only accessible from behind the dash which means the front and side fairings have to come off the bike to gain access.  Note: the tach has a few functions (refresh rate, etc.) that are set by a small button on the backside of the unit.  It's necessary to use the included mounting sticker to affix the unit to the cover.  Why?  This will ensure the vibrations don't change your settings because the button is being "pushed" by the cover when the fasteners are tightened.

In this image, I've run the wiring thru the dash and followed the throttle and choke cables to the spark plug wire.  The coil is just out of view (upper left) and the cap has been removed from the spark plug (seen exposed just behind and to the right of the coiled red wire).  I used a zip tie to hold the tach wiring to the spark plug wire.  I chose a place where there's a wear sleeve which will provide some cushioning.  The red wire runs within a black sheath.  The red coil is what will pick up the electrical pulses to the spark plug.

I wrapped the red wire tightly and secured it with some electrical tape and another zip tie.  I used additional zip ties to hold the tach wire to the throttle/choke cables.  When the throttle cable branches off, the tach wire continues to follow the choke cable where it goes thru the dashboard and the extra wire is contained within the cover.

Finished and ready to go!

Addendum: I forgot to mention that, with a little help, I finally discerned the red line for the Honda 125cc four stroke SOHC engine; it's 10,500 rpm (!).  Maximum horsepower is made @ 10,000 rpm.  I don't know about you but my ear isn't good enough to "hear" that red line, hence the need for a tach.

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