Electronic ignition for the Yamaha Tx750

Selfbuilt Electronic ignition for the Yamaha Tx750
( Info : ledo(add)kabelfoon.nl ) add=@


I found a plan for the electronic ignition while I was reading about maintenance
of contact breakers on the German TX750 site and the website of Roger Cole (USA).

The original plan serves as basis for my plan.
I still have the small prints and plans for a magnetic ignition system, but the
integrated circuit used, the MC3333, is difficult to obtain, so I discovered
that the plan on Roger Cole's TX-site is better suited to use.
A small expansion with a resistor and an optocoupler gives an
excellent electronic ignition without contact breakers.
Those contact breakers will now be replaced by an interrupted
disk that rotates between the sensors of an optocoupler and in this it way
it provides for a spark
about this later.)

With this prototype I can move a small screwdriver in between the
optocouplers sensor, then I always get a spark no matter how slow that I do that.
Thus it is perfect for starting an engine like the TX750.

Mechanical Part:
Now, what should we do about the mechanical part? The electronic plan is
very simple and provides no spark advance. With contact breakers, spark
advance is done mechanically, and we will simply re-use the mechanical
spark advance. We place the rotor over the contact breakers'
rubbing-block/shaft, where normally the breakers would rub against.
Because we are still using the spring-loaded advance mechanism, the
rubbing block/shaft will turn a little when the motor is required to go
faster, thus the rotor, which we have mounted on the axle, accelerates to
advance the spark. The spark advance problem is solved.

So, we need a rotor cylinder and I think you or someone else has to make
this on a lathe, after that we must also solder
on the rotor cylinder.
Now the rotor is complete, but not yet ready for duty.
After that we have to cut out 2 equal parts of the rotor plate, this has
to be done very precisely.
(see the photo, drawing and the "Important" note later
We then put this assembly onto the rubbing-block/shaft so the rotor passes
in between the sensor of the optocoupler. See photos for more detail.
A drawing of the rotor. ( side view )

Keeping in mind that the rotor assembly must later be placed over the
rubbing-block/shaft and perfectly aligned in relation to the groundplate
so that the rotor blades pass evenly between the optocouplers sensor,
height measurements are important.

IMPORTANT: divide the rotor plate accurately in 4 pieces, carefully
removing every other section, leaving 2 parts directly opposite each
other. ( 180 degrees
must be fired at exactly the same time. Some of the photos included on
this site should make these instructions more clear.

So, with this system we will have 1 spark with one rotation of the
crankshaft, thus we now let both coils spark at the same time, then there
is always 1 of the cylinders that
spark is not needed because that cylinder is now busy with exhaust.
This may possibly cause a little more sparkplug wear and may require a little
cooler heat range on the spark plugs.

Why did I design it this way? I have tried it with 2 optocouplers, using
the electronic plan from the TX750 Interest Site, and it went well as long
as the motor was running at idle. The rotorplate had only one
interruption, and because of this there was an unbalance and the spark
advance couldn't do its work very well any more.

The manner that we now use gives a balanced rotorplate and so it works
well with the mechanical spark advance. (Note: Don't make the rotor too
heavy. I used brass.)

The rotor exactly as it looks.

1 Optocoupler for both cylinders (sparking at the same time).

Assembling of the optocoupler:
A very sturdy tab must be made wherewith the optocoupler can be fastened
to the ground plate. Be sure to place the mounting tab so that the
optocoupler is in correct distance from the rotor shaft
(rubbing-block/shaft) to avoid having the rotor touch the optocoupler.
Each blade of the rotor should pass through the optocoupler smoothly, with
no runout.

I shall place here the drawing of the rotor
see what I used for measurements.

Now then, first solder all wires on the optocoupler and make it as smooth
as possible, using shrink sleeve to keep it all together. You should first
test the ignition on the workbench to see if it is finished properly and
producing sparks. After that, place it in the TX750.

The way to test the ignition is to connect all wires as in the plan
( electronic schema
part
Supply a 12 V battery with a fuse
when moving a small screwdriver between the optocouplers sensor.
When there is nothing in between of the sensor, the current should
be low, when a screwdriver is in between the sensor, the current
will increase to about 4 Amp.
And then of course

That's all

Now mount the rotor. If I now tell you that as soon as the rotor blade
ends its pass through the optocoupler, a spark is given, that should be
enough, right? When the suction begins on the compression stroke, look at
the timing marks on the dynamo and mount the rotor assembly on the
rubbing-block/shaft so that the rotor is just exiting the sensor of the
optocoupler. In other words, just leaving the middle of the
optocouplers sensor.
Of course, the ground plate should already be in the correct location.

The rotor has to fit very tight, so it should be possible to gently give a
light tap with a hammer on the rotor to get it in a good rotation position
(for timing purposes) and also on top so it will evenly rotate in
between the sensor of the optocoupler. But not to far, because then you
would need a small puller to bring it back, which could damage the rotor.

Keep in mind that the rubbing-block/shaft has a little play in it. The
plate naturally should never come in contact with the optocoupler, because
then you have to walk home.

The electronics:

The working electronics is suited in a box, sealed at the bottom with
silicon stuff, it will fit somewhere under the buddyseat.
Put this in silicon rubber ..... or use something else to keep it dry.

The electronics is quite changed, because of the use of 1 optocoupler. My
first change in the electronics was simply 1 resistor with use of a BMW
coil. This coil gives a double spark, handy to use for the TX750, but I
wanted the 2 original TX coils, because the place under the petrol tank
is a little cramped, thus I have enlarged the plan with some extra
components until what it becomes now, and it works very well. The current
of both coils is now doubled, because they
both give a spark at the same time; this is a change compared to the
original method. So, you'll see the free standing green lamp flickering a
little bit when running at idle. The current will be about 8 amp max when
the engine is running.

It is also not recommended that the contact key be turned on to long
without running the engine, because that will discharge your battery
twice as fast as with contact breakers, when using both original ignition coils.
The plan with 1 optocoupler, both sparkplug do get a spark at the same time.
If you don't want to much ignition current, you should use a 2 spark-cable ignition coil
for example from a Yamaha FJ1200 or a Benelli Quattro.
The primairy value has to be 3 a 4 ohm, don't use a BMW coil, it's resistor value is to low.
The plan with 1 optocoupler, 1 ignition coil with 2 cables, both sparkplug do get a spark at the same time.