>Spark-plug air-compressors have been around for a long time but they don't
>work the way you probably think.  The spark-plug air-compressor is in fact a
>separate one-cylinder compressor, bore & stroke about 1x2.  It uses the
>compressed gasses in the combustion chamber to power its own piston, which
>pumps ambient air.

>If you try inflating a tire with a fuel/air mixture from your engine you'll
>be in for a noisy surprise.

However with a different cam, and manifolds you could take a bug
engine, and use two cylinders for the engine, and use two cylinders
for the compressor. I don't know if the bore/stroke/valve sizes would make a 
good compressor, but you could make a compressor.
The compressor cylinders would need double lobed cams so it pumped every 
revolution. Theres no need for a four stroke compressor.

On a related subject, anyone ever heard of a J Brake for a bug;-)BBRRAAaaaappp!
J Brake being a engine brake that large trucks use. Would you need
a diffent cam, or just head modifications? How do they work? 

Bill Crick
76 Type II.IV.VII
Ottawa Canada