revtothelimit - motorbike forum

Bike Breaker wrote: Isn't it a 'wasted spark' ignition system wcith a single contact breaker firing one plug on compression while the other sparks at the same time on the exhaust stroke and, therefore, no bang from that pot?.

It is...most 2 and 4 cyl petrol engines worked that way until very recently, a lot still do. The only reason they don't any more is because they now use coil on plug a lot where each plug gets a coil of its own and there are no HT leads.

Wasted spark is cheaper, lighter and simpler than separate sparks so why not? Theoretically you get weaker sparks because you're...well, wasting...half of them. For the most part however it doesn't matter.

Whichever way you phrase it it amounts to the same thing...it always fires both plugs simultanously. Of course, they actually fire at the end of the compression stroke (or the exhaust stroke on the other cylinder) because of the spark advance.

Bike Breaker wrote: Wed Aug 19, 2020 6:24 am

Mr. Dazzle wrote: Fri Aug 14, 2020 8:53 pm They're 360 degrees apart, so one fires while the other is on the intake. That's "the norm" for parallel twins AFAIK - so you're basic CB250, CB500 etc. will be the same I think. Twins that are 180/540 degrees apart rock side to side loads. Modern stuff with 270 degree cranks etc. tend to have balance shafts.

I think the original "big bang" engines were Brit twins re-cammed to fire two at the same time, but don't quote me. This bike sparks both cylinders every time and has completely separate carbs and exhaust for each cylinder, so it'd be a pretty easy mod. Literally just change the cam I think.

Isn't it a 'wasted spark' ignition system wcith a single contact breaker firing one plug on compression while the other sparks at the same time on the exhaust stroke and, therefore, no bang from that pot?.

Not with original points system whether magneto, twin coils and CBs or single CB and distributor. Wasted spark came in with Japanese parallel twins & fours and Norton, Triumph etc when fitted with electronic ignition such as Boyer.

The Citroen 2CV had wasted spark ignition, from 1948.

It may well have done but I thought we were talking about Meriden Triumph twins?

iansoady wrote: Wed Aug 19, 2020 11:52 am It may well have done but I thought we were talking about Meriden Triumph twins?

Strange thing to think, after posting this:

iansoady wrote:Wasted spark came in with Japanese parallel twins & fours

Le velos did that in 48 and i think Harley had been doing that for a while. As has been said, it simplifies things ....as a variation the viceroy fires both cylinders to gether but as its a stroker they are both on the firing stroke.

I had a 1974 Kawasaki 125 two stroke with a twin plug head (single coil, two HT leads), that was the opposite of wasted spark.
I know this post is irrelevant but the whole wasted spark discussion is.

It does seem a waste. So i gather the spare sparks up after each ride.
I keep them in an old canteen.


You can always find sparks in the canteen.

Maybe you could find a way to get them into the wiring to replace the smoke.

I want to see the scored piston skirts and stuck rings.

iansoady wrote: Fri Aug 21, 2020 12:06 pm Maybe you could find a way to get them into the wiring to replace the smoke.

that's what the plugs on the top of the battery are for, but use a funnel, and don't put too many in at once or it will overheat.