Vespa smallframe engine stripdown and rebuild.
14. Setting the ignition timing
you have your engine completed you need to set the timing. The
timing of a cylinder is directly related to the cylinder and
its compression. For example if you are using an ET3 cylinder
on a 50 special you need to use the timing for an ET3. If you
are using a Polini cylinder on anything you need to set it to
Polini's recommended timing.
we go any further here are the correct timing angles for most
50cc : 19 degrees BTDC
90cc : 19 degrees BTDC
100cc 17 degrees BTDC
125cc (VMA1 and
Primavera) : 24 degrees BTDC
125cc Primavera ET3 and PK : 20 degrees BTDC
130cc Polini : 16 degrees BTDC
135cc Malossi : 17 degrees BTDC
If you have any additions or corrections please let me know at firstname.lastname@example.org
are two basic methods of setting your timing, depending on whether
you have points ignition or electronic ignition. Points physically
open when they send the spark to the cylinder, so you can use
the "cigarette paper method" to time a points engine,
but electronic engines can only be timed with a strobe light.
Luckily Piaggio recognised that most people do not have a timing
strobe light so they marked out the cases, stator and flywheel
Let's assume you are setting the stock timing
on an ET3. The basic method is as follows. Take the flywheel
off and look at the stator
plate. you should see a raised mark on the stator plate that lines
up with a similar raised mark on the case. Line these up and you
have your stoc ET3 timing of 20 degrees before top dead centre
You'll notice that these marks are not perfectly
lined up. That's because the production tolerances allow for 2
degrees either way, so if you set the timing with a strobe light
you may find you need to move the stator slightly to get it perfect.
In all probablility you can leave it at that, but what if you kit
the engine and need a new angle? The stock marks are now useless.
Here's how to do it properly:-
Step 1 - Find top dead centre.
You can find top dead centre (that point at which
the piston has reached the limit of its travel - it has stopped
going up the cylinder but has yet to go down) very accurately with
the cylinder head removed and a micrometer secured to the cylinder
face, but that's overkill. All you really need to do is put something
like a screwdriver in the spark plug hole and, keeping it accurately
positioned slowly turn the flywheel clockwise until the screwdriver
stops it turning (because it has hit the piston). At this point
mark both the flywheel and the engine case with a small dot of
white paint and then rotate the flywheel anti-clockwise (keeping
the screwdriver very steady) until the screwdriver hits the
piston again. Mark this point on the case so it lines up with the
mark on the flywheel. Now mark more prominently a point exactly
halfway between these two marks on the case. That is top dead centre.
Now use a degree wheel (you can buy one or print
one off, or use a protractor) to measure the required distance
anti-clockwise (the Vespa engine runs clockwise). So 16 degrees
BTDC for a Polini top end would be 16 degrees before the TDC mark
you made on the case. Mark this point on the case prominently and
also mark the flywheel if necessary to make the previous mark you
made more visible. The idea is that these two marks will line up
at the exact point the spark fires. On an ET3 or other electronic
ignition engine you have one more aid before you use the timing
light. Remove the rubber bung from the flywheel and you will see
two marks by the hole. When these two marks
line up exactly with a white line painted on the pickup on the
stator the spark is fired.
the flywheel and (using the pre-cast marks as a 20 degree "base"
if necessary) rotate the stator (it is loosened
by 3 screws) so that this white line will match up with the flywheel
marks at the same time as the white paint marks you made on the
flywheel and case also line up. now tighten everything up and start
the engine. It should run pretty well already if you measured everything
right. I'm lazy so that is as far as I bothered to go. It runs
and that's enough for me. However to get the timing spot on hire
or buy a timing strobe light. these come with a sensor that you
clamp on the spark plug lead, or actually connect in between the
HT lead and the plug, which tells the light to ignite when the
engine sparks. They usually run off 12 volts DC, so you need a
handy car or car battery, or an old PC power supply to provide
the 12 volts. If all is well with the electronics the White line
on the pickup will line up with the 2 marks in
the flywheel every time, and if your measuring is accurate your
will line up
as well. Production tolerances being what they are you may well
find that they do not line up exactly, in which case stop the engine,
remove the flywheel and rotate the stator plate just enough so
they will line up next time. Repeat until it's perfect (or you
cannot be bothered to adjust it any more - remember 2% either way
is OK on a stock engine)
For a points engine the procedure is pretty much the same, but
instead of the handy white stripe and flywheel marks you need to
find the point at which the points open and mark that. Use a cigarette
paper (e.g. Rizla or Swan) in between the points and pull it gently.
Rotate the flywheel and mark both the flywheel and the case (and
the points too if you like) and the point at which the paper is
just released. That is when the spark will fire. Piaggio do provide
similar help (on "Plastiform" flywheels at least)- there is a small
arrowhead cast into the hole in the flywheel which should align
this point. Use the strobe light and use these marks as above,
rotating the stator
so that both these marks and your white timing marks line up at
the same time. Points often need adjusting. For best performance
you need to have a points gap of between 0.35mm to 0.45mm on a
flywheel or 0.4mm to 0.5mm with a Primavera aluminium flywheel.
Note- depending on your accuracy, or that of Piaggio, the "sparking"
marks may not line up exactly. This is not important - what is
important is that the timing marks you made line up.