Tacho Options for Enfield Bullet It has always been a regret that RE 350 does not have a tachometer unlike the thunderbird. And fitting a tacho is more of understanding what it needs. You can go for a tacho used in any other motorcycle like a Pulsar (old models) / Thunderbird / Fiero, etc. You get the tacho with a case for a Thunderbird & Fiero. For Thunderbird fuel guage is a bonus.
If you don't find those, but you can manage to procure a car tachometer for 4/6/8 cylinders still it is possible without any major modifications.
1) Option 1 - Connecting a motorcycle tacho to a bullet (FOR TCI IGNITION ONLY).
These tachometers need 1 pulse per revolution of crankshaft for showing the right reading. Refer to the first figure (Tacho I/p from TCI). A multimeter comes very handy now (if you don't have access to the wiring diagram). One of the wires carries +12V as soon as you key on. You don't need that, or use it to power the tacho with +12V. The other wire goes to kill switch and also to the TCI. This is the signaling port for a single cylinder tahco. Splice this wire a little, tap an extra wire and connect it to the signaling point in the new tacho.
2) Option 2 - A petrol car tacho for bullet.
These tachometers need 2 or 3 or 4 pulses per crank revolution for 4 / 6 / 8 cylinder options. Here you need a little of soldering expertise. Refer to Figure 2. Prepare a single phase bridge rectifier across any of the 2 yellow wires from the stator. Do not add a capacitor after the bridge. The output pulses will have the same frequency as the number of magnetic poles in your maget rotor.
For example you magnet for 4 pole pieces (2 North & 2 South), The output from the diode bridge will have 4 pulses per crank revolution. So set the tachometer for the 8 cylidner option.
What if your generator has 6 poles? The bridge rectified output will have 6 pulses per revolution which corresponds to a 12 cylinder option, which you dont normally find in a low cost car tacho. So, just follow what you see in figure 3 and set the tacho in 3 cylinder mode.
The above explanation is common for any 2 wheeler. You don't need the bridge rectifier if there is already an AC output from your alternator like in a Pulsar UG3 & below. Set the tacho to as many pole-pairs (NOT POLES) as you see in your Magnet.
3) Option 3. I tried this yesterday, but again I will have to wait till the engine is bolted on to the chassis frame and started.
As I had the stator plate in hand, I was just curious to know how well option 3 can work. With this, there is no need for diodes and rectifiers.
Purchased 10 meters of 27 gauge varnished copper wire and started winding around over and above the existing stator coils. The Electra has a 12 pole – 3 phase alternator. Which means every 3rd coil is of the same phase. One can simple start winding from any convenient place. Just note that the all windngs need to be of the same orientation – means if you had started clockwise, then retain the same convention for other 3 coils also. Solder 1 end of the new copper wire to body ground at the ting clamp that holds the bunch of yellow wires exiting the startor. Luckily there was a small hole in this clamp, into which I had inserted 1 end of copper wire and soldered.
Again, Luck was on my side. The rubber grommet had an unused extra hole through which I had inserted this thin blue wire (as seen in the picture) and routed it into the black sleeve. This was soldered to the free end of the new copper wire, and secured it with a rubber sleeve. Some Araldite was poured over the coils and the sleeve so that they don’t move or dislodged while dipped in oil and the magneto spins.
Time for some physics… As the 6 pole generator of the engine spins, there will be 3 complete waves appearing at the blue wire. I plan to feed this into the tachometer while setting it to 6 cylinder mode. Also a parallel wire will be tapped from here and passed on to the Bajaj Pulsar BCU, so that it does not turn off the hedlamp after 5 – 10 seconds. |