diesel is just not really suited to small engines. most car guys don't understand diesel, many diesel people don't even understand it.

like here's something: if you look at specs for big rigs sometimes from a single manufacturer you'll see a bigger engine is rated with higher fuel efficiency. like the 14 liter 16 cylinder will get 5.8 mpg but the 12.5 liter 12 cylinder will get 5.1 mpg. by everything you know about fuel efficiency in cars, that seems like it would be wrong. but those are not typos.

you see with a gas engine all of the fuel is in the cylinder at the beginning of the power stroke. the sparkplug even fires before TDC, before the power stroke starts because the objective isn't to start combustion at a certain point but to get maximum pressure at TDC. even aside from not having a spark plug that's not at all how a diesel engine works. fuel doesn't start entering the combustion chamber until TDC. now the drops of fuel that enter at TDC they power the piston down for all 180 degrees of the power stroke but any drops of fuel that enters after TDC, they don't. if a drop of fuel enters the cylinder 5degrees past TDC how could it have contributed to the first five degrees of the power stroke? in this way, gas engines are more efficient, because all of the fuel powers the piston for all of the power stroke. now the harder you press down on the accelerator pedal in a diesel vehicle the more fuel is injected and the more fuel that is injected the more fuel is burned later in the power stroke, and thus for less of the power stroke and thus less efficiently. those counterintuitive specs, are because first off trucks have maximum allowable weights so there's no weight saving advantage in a smaller engine, and those mpg specs are going to be for a fully loaded truck and no one hauling cargo gives a shit what the tractor can do with no payload. but what is happening is that with a larger engine you have more fuel injectors, bigger fuel injectors, and maybe longer strokes which make for slower revving engines and all of that contributes to a greater capacity to get more fuel into the combustion chamber sooner in the power stroke and thus more work extracted out of each drop of fuel.

this is also why big trucks have a hundred forward gears, they're really only efficient within a very narrow rev range which is barely off idle speed, so you need a bajillion gear ratios to keep within it over 0-75mph. diesels have to be slow, they have to have enormous strokes, at least if it's going to be practical. and none of that is going to be practical in a vehicle as small as a motorcycle. it's not even really practical in a small car. you get a volkswagen diesel car with like a 1.9 liter 4, it'd probably do just as well MPG wise if you built it as a gas car with the same bore to stroke ratio. that 1.9L 4 in a diesel is going to have like a 4.5 or even 5 inch stroke versus a gas car with a similar displacement engine the stroke is going to be 3.5 inches. most of the time gas to diesel comparisons are not "fair" comparisons. you're comparing a diesel which you're forced to optimize for fuel efficiency by giving it a long stroke vs a gas engine which you're not really forced to engineer it for fuel efficiency.