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Physics and the art of starting a Bullet : Part 1 - The starting drill
Yes, a laughably simple proposition for us veterans. Except... when, with the traffic lights having gone green and the bus behind you inching up impatiently, you have muffed one attempt and are struggling with the
decompressor to "initialize" your engine again! A very rare but a highly memorable nightmare!
So what is the reason for this elaborate "starting drill" in Bullets, compared to the simple
"just kick it till it starts" system on other two-wheelers? The Bullet engine rotates through just about one cycle per kick, while other engines go through several. This means you have only one spark per
kick, and if you don't have the correct mixture-strength, the correct spark-strength, and sufficient compression during that, well, better luck next kick! Not so in other engines, where the the mixture and spark
have several "chances" to reach correct levels during each kick.
What is the reason for this difference? The Bullet engine is approximately 3 times the capacity of others. As a crude approximation,
it would require thrice the effort to kick like a 2-stroke. Not something you would enjoy unless you take after Arnold S.!
Then about the positioning of piston before kicking: "The book" tells you
that before you kick, the engine should be in the position where the ammeter has JUST COME BACK TO ZERO.
Nandan tells you what it means: You start your kick JUST after the sparking position, so that you have
the power-stroke, the exhaust-stroke, and the intake-stroke in which to accelerate the piston (and the flywheels) which start decelerating on the compression-stroke and yet have enough momentum to go thru the TDC
-Top Dead Centre- when the engine will start -and keep running by itself. Yes, if you can imitate Bruce L., or if your engine has slightly less compression (but still enough to run properly) then you can start it
from other positions as well. (An idea about what these strokes mean can be found in our feature on the four stroke car engine, since the principle is the same for cars or bikes)
And then we come to the
infamous Back-Kick! Even the illiterate mechanic's apprentice knows that it is due to "advance", but....sometimes there are doubts! Such as when the back-fire is felt only when the throttle is open, and
not if you kick with throttle closed! Or, if on retarding the timing, the exhaust bent-pipe begins to become blue! The i.m.'s a. is stumped, but you can analyse what is happening!
It's just this:
Your
kick builds up the momentum of the flywheel+piston initially, and part of that energy is used to compress the mixture during the compression stroke, slowing the piston proportionally. So, if the throttle was opened,
allowing more air into the engine, the piston slows down more, and the combustion-front hits the piston BEFORE it passes the TDC, resulting in the reversal of direction of rotation --the Back-Kick! So either you
kick harder, or you retard the ignition timing.
So, OK, you retard the timing. And soon you notice the exhaust pipe is beginning to go blue! If this is the case (It's not always so, of course) it may mean the
carburettor has been set "lean" --leaner mixture needs longer to burn. So it is still "burning" when it exits from the engine and is going down the exhaust tube. Various adulterants also may
alter the burning-rates, causing much inconsistency and mystery. If you know what must be happening, you can take the right decision as to whether it needs a mechanic's attention or a temporary change in ajdustment,
or is safe to be ignored.
The differences in fuel characteristics & carb setting, ignition-timing & strength, compression-tightness, affect all engines, but are noticed more easily in the Bullet
because of the reasons just discussed. That is also why each Bullet seems to require a different "starting drill" particularly with regard to usage of choke. The "choke" has some very wide-spread
myths associated, but that is a long story to which we will devote an entire session soon.
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