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Got a technical problem? Can't understand a piece of jargon or some technical principle? Drop us a line and we'll answer your question. Write to: Ask Silicon Chip, PO Box 139, Collaroy Beach, NSW 2097 or send an email to

Air/fuel mixture display

I found the article on the “Wideband Engine Air-Fuel Mixture Sensor Display” in the November 2008 fantastic, as well as the article on how an oxygen sensor works on page 27 of the same issue.

My question is this: is there a stand-alone kit that can be used to tune a carburettor on a motorbike? If there is, do you know where it can be purchased or could you do an article? The unit could also be used for go-karts, motor mowers, whipper snippers, garden blowers, etc. I have seen units for sale around the $500 mark which isn’t cost-effective for a DIY’er.

Looking at your kit, my guess would be to add in an oxygen sensor that attached to the end of the exhaust pipe? Keep up the good work. (V. P, Attwood, Vic).

• You could use a narrow-band oxygen sensor in conjunction with the Oxygen Sensor display (set to the narrow-band “S” curve setting) to enable mixture tuning at stoichiometric. For accurate tuning beyond the stoichiometric point you need to use a wideband sensor in conjunction with a wideband controller. An article on this is coming up in a future issue as we are still working on this project.

Note that an oxygen sensor must not be inserted into a tail pipe. It must be mounted so that the sensor is screwed into the tail pipe (or a temporary extension piece) so that the main body of the sensor is external to the pipe and only the sensor tip end is exposed to the exhaust. This is so the sensor can compare the oxygen content of the air against the exhaust gases.

Note also that the sensor must be mounted so that it sits more than 10° above horizontal, to allow water to run out of it.

Amplifier overheats with 4-ohm speakers

I have run into problems with the output on the 20-Watt Class-A Amplifier described in SILICON CHIP during 2007. I use a pair of homebuilt “Kappelmeister” speakers which have 4-ohm drivers fitted.

The amplifier overheats after one hour. Can I manipulate or change the circuit so I can use the 4-ohm speakers without sacrificing the sound quality? The sound reproduction is unbelievably clear; it’s the best amplifier ever. (A. H., via email).

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