A number of fourstroke engines have been fitted with two sparkplugs per cylinder to increase power, usually big two-valve heads with relatively poor combustion chambers (many american V8 fits that description) causing a slow burn if only a single sparkplug is used. With two plugs the fire is started in two places at the same time and the cumbustion is completed in shorter time, leading to increased performance, less ignition advance needed etc.

Twostrokes has a better spark plug placement than the two-valve fourstroke, but has anyone tested using multiple plugs on a twostroke head? Either all in the same cumbustion chamber, or with one small cumbustionchamber around each plug, and "squish" in between. Yes, I know, crazy idea. But I'm curious if it has ever been tried.

Some older snowmobiles had twin plugs per cylinder.....I cant seem to think of who...but I think it was either yamaha or mecury, Twice the problems if you ask me

This has been tried again and again, with positive results depending on the application. I think the main reason people run dual plugs on 2 strokes are hillclimb bikes that are burning alcohol. These bikes are using alcohol I think for the cooler burning characteristics do to very high compression ratios, but must severely increase the jetting to get enough btu's to make up for switching to alcohol.
I am not even sure how effective this is, just know because I bought a CR125 that had a plugged hole where some amateur tried to mimmick the results of a pro hillclimber. From various readings I think this will actually cause you to loose power on a traditionally built stock 2 strock for MX or woods, but the FMR gurus know best on this.
Stan

I believe KTM did this on their 550 in in the nationals in 93.AMA allowed the extra displacement.(Last year of Open class)

Okay, so it does exist on hillclimb engines today and has been used on other engines in the past. I just remembered, I have seen some old Jawa 250/350 with a double plug head.

The conclusion seems to be that most engines don't benefit from multiple plugs today (not enough to make it worth the complexity/cost anyway), but there may be times when it can be useful.

I think Honda used dual plugs on their oval cylinder engines. I can't remember if those were two or four stroke.

Those were fourstrokes. 8 valves and 2 sparkplugs per cylinder.

Not sure about old Jawas, but quite a few old two strokes used to have two plugs in the head. However, only one would be connected to the plug wire. The 2nd plug was there in case the first plug fouled - just swap the plug wire to the fresh plug (for some reason the plug that was not firing would not be fouled).

The ZABEL engines (700 cc 2stroke) are using two plugs. Current world championship winning engine in the sidecar cross.