This is an educated guess, from what I know about injected and carburated cars. I'll follow the "F1Technical forum method": you make a wild guess and some professional reading the forum will be so upset, or will laugh so much, that he will lend a hand...
First: when you close the throttle, the engine manifold vacuum increases. I think this is the same thing that happens when you block the pipe of a vacuum cleaner with your hand, for example.
On carburated cars, like NASCAR (ehem...), the 'idle port', (below the throttle plates), is then subjected to high intake manifold vacuum throwing extra fuel into the combustion chamber.
On some cars, injectors are located between the throttle plate and the intake valve. The intake engine vacuum on deceleration pulls fuel from these injectors and send it to the cylinders. In modern cars the injectors have a tight shut-off on deceleration, specially at normal, highway speeds. I believe F1 cars have the injectors above the throttle plates, so this theory, that explains Montoya's flames, doesn't work in F-1, unless I'm wrong.
Second: I wonder how the spark behaves. I imagine a lower voltage at lower RPMs, at least on the old american cars whose spark timing I've adjusted
. When you check a spark plug, the spark towards ground seems more brilliant when you accelerate, but I can be imagining things.
So, up to this point, we have an engine that was going at 19.000 rpms when suddenly the entrance is blocked and a high vacuum sucks extra fuel: Birel99 explains that Renault uses a lean mixture in this situation. However, we have a third problem:
Third: The intake/exhaust valves of a high RPM engine have two "valve overlaps": in the first overlap, the intake begins way before TDC and exhaust goes beyond TDC, thus, the engine sucks fuel directly into the exhaust at low RPMs. In the second overlap, the exhaust begins well before BDC and any unburnt fuel can go out. The lower the RPMs, the more deleterious are these two overlaps on the amount of fuel that simply passes through the engine without being touched by the spark. Check at
http://auto.howstuffworks.com/camshaft.htm for some cool animations, including the 3D camshaft by Ferrari.
Anyway, what Birel99 wants to know, it's why the fuel does not burn:
when you close the throttle the mixture lacks oxygen, and the (weaker?) spark won't be able to ignite it. The fuel won't burn until it gets enough oxygen, at the end of the tail pipe. It's the same trick used by people that throw flames from the mouth.
When you have an engine that works at extremely high RPMs, you have to use a mixture as lean as possible, as Birel99 explains, because that unburnt fuel has effects on the engine: it washes oil from the cylinders (it should cool them, too, probably making even more difficult for the mixture to ignite).
You can buy a "flamethrower kit", in case your engine's miserable cams aren't that race-worthy to spit flames. It completely cuts the spark and ignites the fuel in the pipe with an extra-coil: your car will throw 5 meters flames. As James Bond used to say: "what could go wrong, Q?"
http://www.gizmodo.com/gadgets/car/sema ... 212339.php
).
