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Choking; How and Why
You need to think a bit more about this..
Firstly, you need to be aware that your engine is STAGGERINGLY similar to some sort of 'air pump'. Ok, it has the peculiarity of BURNING some fuel in there but broadly, without the ignition system, what it does is takes air IN through the carburettor and pump it OUT through the exhaust!
Seriously, if you spun it over without the ignition and/or fuel, you could use it to pump air!
However, it would NOT work as a pump if you kept the carburettor CLOSED! Think about it: if the round 'butterfly' valve is closed, no air passes through the engine.
("What IS he going on about?" Keep reading and you'll see!)
Now... placing you HAND over the air intake does something odd! As we saw, the engine is a PUMP, it 'sucks' air in the intake and out the exhaust. If you BLOCK the intake, the pressure will DROP behind your 'hand' (the engine will 'SUCK' on your hand). What THAT does is to 'SUCK' extra FUEL in as it CANNOT get any air!
However, as we said, if the butterfly valve is NOT open, N*O*N*E of this HAPPENS!
So......... to GET the engine to 'suck' more fuel in when your hand is over the intake, you MUST (must, must, must, MUST!) apply FULL throttle!
In reality, this is what you do:-
* Arrive at the end of the straight
* Shut the throttle (lift off)
* Put your hand over the intake
* Press FULL throttle..... (you'll feel the 'suck'!)
* Release the throttle
* remove your hand!
* RE-APPLY THE THROTTLE!
That last bit is CRUCIAL! You WILL have partially FLOODED the engine with fuel! When you press the throttle, the engine will simply NOT 'accelerate' as it's flooded! However, given a few moments of throttle, the engine will start again, produce a HUGE cloud of smoke, and then start pulling properly again! So, if you want the engine ready to PULL again as you leave the corner at the end of the straight, you need to re-apply the throttle EARLY just to 'clean' the engine before you NEED the power!
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So, why do we do it:-
Well, in the old days of air-cooled F*A*S*T 100cc engines, they were run very NEARLY on the limit! They were VERY hot and VERY 'tight' and would be VERY prone to seizing. A touch of 'choke' at the end of a straight did VERY useful things:
1) It delivered a DUMP of cold and EVAPORATING (which makes it VASTLY 'cooler') fuel into the engine cooling critical components DRAMATICALLY!
2) The fuel would evaporate but leave a LARGE 'dollop' of two-stroke-oil all over the engine components making a seize LESS likely.
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Being brutal: the TKM and Rotax are such SLACK old DOGs that they are nowhere NEAR as critically near their limit as the engines of old. The Water Cooling of the KF and the reduction in Revs being used today tend to make the engines remain MUCH closer to their correct working temperature than the FABULOUS air-cooled SCREAMERS of old!
It therefore is MUCH less critical to know what to do than it used to be...... BUT..... if you ARE going to do it, you should know HOW to do it and WHY to do it and WHAT it REALLY DOES and, most importantly, WHEN to do it!!
Kind acknowledg(e)ment to Ian Turner.
If you feel the need for choking, then try getting him to practice Ian's way in the seat.
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Why choke Comer W60's? by 'KevinZac' (29 Nov 2010 @ 8:23)
