Wild engine build thought

[1976RD400C]

Maybe this has been done but I wonder how it would run if you make a "big bang" motor? Assemble the crank so both cylinders fire at the same time. You could use a 2 wire coil to get both plugs to fire at the same time. I remember Yankee did that with their twin Ossa cylinder bikes. Torque monster or catastrophic failure? It would definitely sound different. 

[m in sc]

its been done for sure. you going dirt riding? thats kind of where its useful .

i think on the old forum somebody did it to a GT250 or 350.

[oxford]

i think on the old forum somebody did it to a GT250 or 350.

I remember someone doing them as well.  It's also pretty popular with the xs650's.

[m in sc]

say goodbye to high revs but should be a torque monster. id venture to say this is where a 2-1 carb intake might actually prove useful.

[Dvsrd]

i think on the old forum somebody did it to a GT250 or 350.

I remember someone doing them as well.  It's also pretty popular with the xs650's.

say goodbye to high revs but should be a torque monster. id venture to say this is where a 2-1 carb intake might actually prove useful.

First of all, it would vibrate worse than a stocker. Yes, the rocking couple would go away, but primary and secondary imbalance would double.
The XS 650 has a 360 degree crank as stock, just like old british bikes did. And they vibrate quite a bit. The popular mod for the XS, is a 277 or 270 degree crank, just like the current 700 cc Yamaha twin engine. With the correct crank balanse factors,  vibes are reduced. This concept was pionerer by Phil Irving ages ago. Nourish racing engines (An 8 valve Triumph Bonneville clone, initially developed by Weslake/ Rickman) was ofte delivered with a 270 degree crank. This sort of makes a paralell twin behave like a 90 degree v twin, which gave perfect primary balanse (Ducati, Guzzi)
So if anything, trying a 270 (90) degree crank would be my suggestion, although that would require 2 independent ignition channels.. 

[m in sc]

agreed. however i have seen big bang xs's as well.

I'd never do it. But still. Prob be similar to a 250 single in vibes. there's a bunch of flywheel weight to tone it down a bit.

[1976RD400C]

The crank has 12 splines, so 30 degree increments. A 90 degree gallop may be cool. These little Dyna pick ups are small enough to make  the ignition. I'm just thinking out loud. Tearing the motor back out and pressing the crank apart to fix it would be depressing.

[pidjones]

I would think that a separate crank would be built for such an experiment. You would only need one trigger (point or magnetic), double-ended coil unless not firing exactly together - then use optical trigger.

[Gil Gallad]

it was me that built the suzuki big bang 250  no faster than a normal gt250, and a bit of a pig to kick up until you master the starting technique, but boy does it pick revs up fast? i love it  ;D
cheers, gil.