Re: Hard starting opposed twin

[Anonymous]

What sort of "basic ignition test"?

The laws of Physics (see http://en.wikipedia.org/wiki/Paschen%27s_law) say the spark needs to be able to jump thru the spark gap times the compression ratio in open air.

So if you have, say 80PSI compression at sea level, that’s 80/14.7 = 5.4:1 compression, and with a 0.020 spark gap, that means you need to jump 0.11" (0.020*5.4) in open air.

BUT if your opposed twin has one coil for both cylinders, you need to double that (with one of the ignition wires grounded).

The "rule of thumb" for post-war engines is to jump 0.25" in open air. If the spark gap is 0.020", that means the spark is good for compression up to 12.5:1 (0.25/0.020), or at sea level, 187.5PSI of compression (14.7PSI * 12.5)!!!