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Physics and the art of starting a Bullet: Part 3- Then why doesn't it ?!!!
"To run properly, an engine requires only proper compression, proper mixture and proper spark" !!!!!
Wow. All the wisdom of the motoring world condensed into one sentence! Leaves you breathless, doesn't it? Like the Laws of Thermodynamics, or Einstien's Equation! And, unfortunately, ditto when it comes to
understanding & application!
From that cryptic "high level spec.", let us ponder our way to something tangible. No, fellas, fear not. We won't go the insulting level of "first ensure the
key is in the ON position"! (That we leave for the Executive-whose-driver-is-on-French-leave category) We only list some possibilities and in the process warm up our grey-cells; call it a leader or a refresher.
Even the remedies are left to the reader for now.
C'mon! Your computers are multiprocessor, OS multithreading, code multiplatform; Shouldn't you be a multi-discipline engineer to be worthy of your multi-digit
salary?!! :-)
Proper Compression
The Bullet engine is not TOO sensitive to compression, and it will tolerate a surprising loss of compression before it refuses to start on that count.
Perhaps you remember
the compression when your Bullet was new. With the piston near TDC and kickstarter horizontal, you could stand on it, and it would not budge! Slowly, it has reduced over time, ...and now it is easier to kick! Is
that good or bad? We shall see -
Some causes of low compression:
Adjusting pushrods too tight or when engine is hot.
Decompressor open, stem/cable stuck.
Burning, pitting of valves due to overheating, lean mixture.
Valveseat cracked, Valve warped/stretched due to overheating.
Valve guides wornout.
Head gasket leaking/"blown".
Piston rings broken/wornout.
Bore gouged/wornout.
Hmmm. None of them sound too good, eh? :-)
Later on, we shall link these points to parts of the page detailing the symptoms, causes and remedies
of these situations, but for now let us be content with the listing.
Proper Mixture
We have already dealt with the physics of this. And why a lean mixture has difficulty in burning. But how about a rich mixture? The smoky combustion stops midway from lack of
oxygen! Resulting in a sooty deposit in the cylinder, including the plug which shorts. The same effect is caused by fuel wetting the plug too.
Some causes of mixture problems:
Fuel not reaching the carb
Throttle cable/slide stuck (open)
Carb adj-screws tampered
Water/dirt in petrol
Float chamber flooded
Leak between carb & head
Choked Air-filter (!)
Proper Spark
The spark must be right on time as well be strong enough to be effective. Obviously, right? But what is the right time and adequate strength? In a Bullet, the spark should occur when
the piston is 0.8mm before TDC on compression stroke ...YAAAWN!... Hey! Hang on, we are coming to the interesting bit now: at LOW speeds!!! As the speed increases, the timing is advanced by a centrifugal mechanism
("governor" in the mechanic's parlance) behind the Contact-Breaker ("distributor" in the m. p.). Woke you up, didn't it! Can you guess the reason for this automatic advance? Yes! We have already
discussed the mechanism of combustion. We saw it starts from the plug and propogates thruout the volume, exerting pressure on the piston because the hot products have a much higher volume than the intake mixture.
This takes a finite time. And the time available for the power-stroke decreases as the RPM increases. So the combustion must start earlier if the max pressure is to be exerted on piston during the power-stroke
instead of wasting it in the exaust-stroke.
If you think that was dicey, you haven't read about the opening and closing of valves at speed! On the other hand, if you want to know how the RATE of advance can
be adjusted, hey! Patience. For now, we return to ignition.
We would be quite perplexed by ignition problems if we treated the system like a low voltage one. The joker in the pack is the
High-Voltage-insulation-property of materials. So it is quite common to find a coil that has both, primary and secondary OK, but does not produce a spark! This is also the cause of intermittent spark or "weak
spark" in coils.
It is much easier for a spark to form in atmosphere than under high pressure. Which means, you may see a spark when you test with the plug in air, but it may not be sparking when you
kick the engine! So be sure to note the strength of the spark. A feeble spark has to have it's reason investigated, even if the engine works OK. It could be a "sword hanging by a hair" !
Some causes of ignition problems:
Discharged Battery
Loose contact in wiring
Contact-breaker points burnt
Contact-breaker misadjusted
Spark-plug fouled, shorting
Coil shorted/open
Coil insulator spoilt
Ignition cable frayed
Capacitor spoilt (rare) OK.
So now we are better armed to tackle starting problems intelligently, right? Actually, we will get more specific later on but
don't wait for "algorithms" because I don't plan to give any! Algorithms are for "Blue Streaks". You'd rather be Gary K., right?
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