Now thats a Spark !

Once again a reader has asked for some guidance and stories of the Magic of the Whiz, Bang, Flash
sequence that is the key ingredient of the trials bike’s pie, namely ignition. Using the KISS principle I’ll
try and keep things on a level plane without going into high tech electrics and ohms and other technical

Going back to my youth and first bikes, ignition was one of those things that I was first given instruction
on and all the settings were the same then as they are today when dealing with vintage bikes. Let’s look
at what the ignition system consists of and then see if we can improve it. The following is the basic set
up and goes like this for all the bikes that I usually deal with:

Stator Plate, comprising:
1. Source Coil
2. Lighting Coils
3. Points
4. Condenser
5. Wiring

Flywheel, with magnets to create the field, according to Faraday and other sages

Wiring Loom

HT Coil

Kill Switch

Plug Lead & Cap


An Earth point

Going back in time a lot of trials bikes were also used by their owner’s as everyday street transport and
come the weekend lights were removed ready for the Sunday trial. Getting home on a winter’s evening
could sometimes be problematic in the gathering gloom and sometimes the mustard keen police
would pounce and try and give you a ticket for riding without lights. Clearly it was illegal to be riding
without lights in the dark but the rules were that if lights were fitted then THEY must work.  If you didn’t
have any then you were good as long as it was still daylight. Super Plod would sometimes say that if
the lighting coil was still fitted that constituted “lighting” and therefore you were illegal………. so we took
the coil off just so we could argue the point. Currently I tend to leave them in place just for the balance
of the plate but they can be removed and tossed.

Taking our standard TY motor as it arrives prior to rebuild  it is often worth a quick kick to see if anything
is there but do not despair if it isn’t as all parts of the system are readily available. The Yam system has
a fixed stator which works well and is one less thing to slip or go wrong in the heat of battle. I would
personally replace the three philips retaining screws with allen bolts when you take the stator plate off.
Obviously you will have already removed the flywheel, and don’t forget the TY250 and TY175 pullers are
different so make sure you have the correct one on hand. More often than not the 3 wire harness
running from the stator plate to the back of the engine will have become brittle as the heat of the engine
cracks the plastic sheath. I normally cut all this off and then remove the 2 Yellow colored wires leaving
only the live Black wire.  If these wires are “hard” or “brittle” I would replace the Black wire with a new
one. I like a nice firm ignition wire and protect mine with a piece of color coordinated petrol piping
running all the way from the top of the flywheel case and then up the frame tubes to the HT coil.  I take
the rubber bung out of the ignition case and drill out the 3 wiring holes to make one 1/4 hole in which I
insert a 90 degree rubber vacuum bend. This will protect the Black hot wire and the 5/16 petrol pipe will
slip neatly over the unit. The hard part of this operation is getting the wire through the rubber 90! ….. so
while I’m prepping other parts I shove a small guage allen wrench through the rubber effectively
straightening it out.

The 90 bend with hot black wire on a prepped motor

With any old competition engine you may as well always start with new points and a condenser. Some
of these may require a little soldering. New points, especially some of the later OEM ones arrive with a
dull teflon looking coating over the faces and this MUST be removed before fitting. When you remove the
old points you might as well replace the adjusting screw as these usually have all manner of abuse! I
normally fit the points at the mid point on the adjustment scale and this can be reset if required once the
flywheel is back on. While the flywheel is off I normally give the magnets a clean and before reassembly
make sure you remove all the little bits of metal and filings that are stuck on the ends of the magnets
and between each other.

The protective tubing (in yellow) from source to HT

Running the new wire up the frame tubes to the HT Coil I use a connector with a 5/16 diameter at the
top end. I drill out the top screw in portion to allow for a double thickness of wire so that I can mate the
wire from the coil and a single wire which I will use for the kill switch. On the Yam frame there are plenty
of holes in the frame and the kill switch wire can easily be run forward and up to the headstock and

The 90 bend again with the slip over in red

Protective purple tubing running the other way on Ossamaha

HT coils don’t usually cause too many problems and can always be quickly interchanged if there is any
doubt on their serviceability. A couple of things to check here are the main core wire and the plug cap
itself. A quick inspection of the inside of the cap will tell you whether the screw on metal at the top of the
plug needs to be retained or removed. Also worth a check is the condition of the main wire and whether
that is rusty and corroded. A quick snip back to bright wiring is all that is required.

Although it should be obvious plugs come in many types both in heat ranges and reach. NEVER use a
long reach plug in a short reach head!!! Know the plug that is the right one for your engine and wherever
you go on a trials bike ALWAYS carry a spare. The plug gap for bikes of the 60’s and 70’s is usually
18-22 thou and of course modern day plugs have many uses and invariably come with a much wider
gap anywhere between 25-35 thou so do remember to reset yours to the design spec.

The latest in plugs

Back downstairs to the points and with the flywheel carefully reinstalled time to check the points open
when required at the desired 15 thou. I usually put the flywheel on using both hands to stop the
magnets grabbing the stator and knocking the woodruff key out of place. It might seem obvious but start
with the key in the vertical 12 o’clock position. Push the flywheel in place and rotate to where the points
are just opening.  There are holes in the flywheel where you can insert feeler guages and screwdrivers
but sometimes this can test your patience as the magnets are pretty strong! You can use a timing tool
at this stage in the plug hole to determine TDC if you so wish. With the points set correctly time to
tighten that screw! …… and with the new plug suitably grounded on the fins a test kick or two should give
a nice blue flash. Time to recheck that woodruff key is in place and then tighten up the flywheel. If the
motor has been taken down starting will normally take 3 kicks as you are establishing crankcase
compression before it will start. Finally seal up the flywheel cover to stop dirt and water ingress and
the job is done.

Another question that some are interested in is the effect the flywheel has on performance? Well,
a Trials engine on a Vintage machine is designed to have a “steam roller” type flywheel where the
engine will just keep ploughing on thereby providing grip and traction at idle and good pick up
without violent wheel spinning when the power is reapplied. A light flywheel will most likely give many
“feet up 5’s” as the motor will stall unexpectedly when power is reduced to idle. Most modern machines
have a much lighter flywheel and are designed to be ridden using the clutch whereas our vintage
bikes are not! In my opinion removing flywheel weight from an older bike will make it rev quicker
but it will also stall a LOT easier and most likely INCREASE your score for the day. Overall opinion,
leave the system as it is!

Size is important!

Now for those engines with movable stator plates you will need a timing tool and a simple light that
can be fixed on the wire. A machine where timing was absolutely critical was the early Bultaco and I
used to spend a fair amount of time getting the ignition set to exactly the right point which if memory
serves me right was 4 degrees BTDC. A simple enough task with a bit of practice and with the correct
tools. With the bike on a stand you can select top gear and rotate the rear wheel to find TDC using the
tool in the plughole.

Simple TDC tool

Depending on what lighting system you use as you rotate the wheel the light will either DIM or go out
completely as the points open. So knowing where TDC is and where the manufacturer suggests the
timing should be you can see exactly where you are or need to be. After all the flywheel on and offs
remember that woodruff key! as it can easily be dislodged. We often used to use a cigarette paper
between the points and with a light pressure you could see the point at which the paper came free.
When you have the ignition system set precisely to your liking worth just scoring the plate and housing
such that if it does slip for whatever reason then you have a “datum” to go back to without going through
the whole procedure every time.

Electronic ignitions? When they are working great, but some of them can be fiddly to set up. Some
offer all sorts of auto advance/retard devices and may indeed give some advantages but for my money
the simple systems when correctly set up and maintained are equally good and for a lot of riders I
doubt that anyone would notice the difference. Of course having a perfect ignition system is one thing
but if your carburation doesn’t match it then you still have work to do, but that again is another story!


What did you think of this article?

  • No trackbacks exist for this post.

  • 5/27/2011 7:18 AM Jon Dearie wrote:
    Hi Tony, when running this single wire conversion, do I need to ground the wire somewhere?
    Thanks, Jon.
    Reply to this
Leave a comment


Submitted comments are subject to moderation before being displayed.

reCAPTCHA challenge image

Get a new challenge Get an audio challengeGet a visual challengeHelp



 Email (will not be published)


Your comment is 0 characters limited to 3000 characters.



Leave a Reply

Your email address will not be published. Required fields are marked *