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[jefrs]

That does not read right, lets try, "it mapping point is determined by ..." i.e. whereabouts on its map.

Charles' Law and the Universal Gas Laws tell us that air pressure is proportional to mass for a known volume (500cc)

The carburettor needle has a curved taper for good reason, to match mix to load, the ECU emulates that, the mix is not or should not be constant with throttle opening and revs.

[jefrs]

Is the Keihin ECU sophisticated enough to increase the advance when the engine is accelerating (revs increasing, the CPS) as distributor bob weights do?

[ric]

It gets any ignition information off a preloaded map similar to this NON enfield map

[jefrs]

The Power Commander mapping is viewable in the software. It has two axes, throttle position and engine speed. It alters two parameters, fuel and ignition timing. The operator input is the throttle, the TPS. Enrichment starts at the 2% throttle input point, this is regressed back down to zero i.e. a curve through the input points is used. The other input is engine speed from the CPS, this affects the fuelling too and can be affected by 'user input' e.g. the bistarter. The Keihin also receives inputs from the other sensors, MAS and Temperature but the Lambda/O2 is bypassed with the PCv in play. Anything that increases or decreases engine speed, like the bistarter, moves it around the mapping chart, and yes from the Power Commander chart that does appear to alter the injector pulse duration. Advancing the timing also requires altering the fuel mix, and the a/r comes off the bottom with the TPS opening too.

No, the operator cannot directly alter the enrichment but what they do with the controls can. The only way to alter the mix is to delve into the ECU map, and it is not a straight line.



Unfortunately the Power Commander software does not support cut 'n paste of numbers so it's not easy to put them into a graph.

It's in my nature as a scientist to find out how stuff works, to need to know the ins and outs of a donkey's earhole, and to do empirical work too.

[Thack]

The main reason the needle is tapered is not because the mixture needs to be richer at large throttle openings, but because if you let more air in you have to let more fuel in to maintain the same mixture.



Of course, jefrs it totally right to say that you don't want the same mixture everywhere - generally a weak mixture is great for light running, as it maximises fuel economy, and for high power outputs you want a rich mixture for maximum power and cooling.



The most disappointing thing is that the modern three-way cat-con actually requires a stoichiometric mixture to work, so sadly for a lot of the time our bikes are not running at the ideal mixture for the conditions. We lose power and we lose economy.



By the way, jefrs, I haven't monitored the ignition timing yet, but providing a varying degree of ignition advance with engine speed would be trivial. In fact, I'm sure it will do that, plus varying it with engine load, too. I'll be able to report more fully during this year.

ric: that's a great post - thanks. Interestingly, it's only a two-dimensional map - the manufacturer's original may well be multi-dimensional (another dimension for each sensor, potentially).

[ric]

Losing power and economy - that explains why I'm only getting a rubbish 90mpg

The only problem I've found with adding more power to pull a higher gear sooner to improve fuel economy is that once you've flashed the ECU to gain the extra power with the best will in the world you inevitably start using it and then the extra economy you would have gained is lost.

[Thack]

jefrs seems determined to prove that the bi-starter causes enrichment!



We both agree that its function is to increase the engine speed. Whether the increase in speed causes a richer mixture, the same, or a weaker mixture is entirely down to the mapping chosen by the manufacturer. The important thing to realise is that the ECU has no way to distinguish between air flowing through the bi-starter path, the adjustable idle speed path, or under the throttle plate (unless the throttle is opened enough to take the TPS above 0.8V)



It is essential to appreciate that increasing the injector duration when the engine speeds up does NOT necessarily mean a richer mixture. Some increase is necessary simply to allow for the extra air going in - in other words, to keep the mixture the same rather than leaning out.



On the Enfield, I've noticed that - when the engine is warm - the mixture gets richer when the engine speed decreases below idle. Opening the bi-starter actually makes the mixture leaner as the engine speed increases. The same is true if you crack the throttle.



In other words, every bit of evidence I can find supports the same conclusion: the bi-starter plays no role in providing cold start enrichment, either directly or indirectly. This is supported by my measurements of the fuel injection pulse durations. Nor is there any need for it to cause enrichment - the ECU already has enough data to know how much enrichment is required for cold starting and running.