mauiSRT/10 said:
Very logical post. What causes a cylinder or two to misfire?
patrick
Several things can cause misfire: bad plugs, bad wires, bad coils, improper air/fuel mix, etc.
Here is something you guys might find interesting: back in college when I worked at an engine shop, we had a distributor machine that was used to calibrate ignition curves on mechanical distributors and magnetos. It would mechanically drive the distributor with an electric motor, and you could turn a knob to increase or decrease the drive speed (up to 9000 rpm or so). You also could connect all the plug wires up to a set of spark plugs that were mounted onto the machine and watch all the spark plugs fire through a clear chamber.
What was interesting was that the plugs would fire very easily at standard atmospheric pressure. But, the machine had a way to pressurize the clear chamber where the plugs resided with compressed air. At high pressures (above 150 psi or so), the plugs would start to misfire if the gap was too wide, or the coil was too weak, or if the magneto had a weak generator, etc. So with this machine, you could see and hear the plugs misfire at a given rpm. This would allow you to measure the actual rpm where your ignition system would start to misfire. On a stock system from the 70's, it would start to misfire around 4500 rpm with the equivalent of 10:1 compression.
The reason that the misfire occurs is that the energy needed to fire across the gap in the plug goes up exponentially with air pressure between the gap in the spark plug. This is why with forced induction (which raises cylinder pressure immediately prior to spark), you are more likely to have a misfire and need to narrow the plug gap accordingly.
Also, even if the plug does fire correctly, if your air/fuel ratio too far off, the flame front can get extinguished before the flame travels from the plug to the cylinder wall across the top of the piston. This allows unburned fuel and unused oxygen to be dumped into the exhaust, which then burns on it's way out of the ehaust pipe as it is mixed with other hot gases and hits the catalyst. This is why a lean mixture raises your exhaust temperature so much: it is mostly caused by incomplete combustion within the power stroke and the subsequent continuing of the burning process throughout the exhaust stroke. (This only applies to spark ignition motors, as the exact opposite is true in a diesel engine, which are stratified charge motors, a topic for another time).
