Best 150-350cc fuel injected engine for small, light, aerodynamic reverse trike
 

I plan a small, light (300 lbs), aerodynamic reverse trike, able to cruise at highway speeds for hours. I'm coming to believe that fuel injection and electronic ignition are best. I've been thinking 250cc, though smaller or somewhat larger would be OK. I want maximum mpg, AT LEAST 125 mpg avg on street and highway. I expect cruising at 60 mph. I imagine using the entire rear of a motorcycle, from engine back and building my own front end and body.

Suggestions for best engine/donor machine?

Thanks,
Scot

It needs to be water cooled, for better efficiency and for practicality of good aero.

Yes. Any suggestions for particular engines? Preferably ones that could be had at a reasonable cost and would be very good for the application.

air cooled isn't a deal breaker, they have aftermarket fuel injection conversion kits for small motorcycle engines as well. One cylinder is probably better than two, but expect to pay a premium for a factory injected cbr300. I would consider a 10hp diesel yanmar clone too, but then you have to sort out a transmission.

I am sure it has been mentioned elsewhere on Ecomodder repeatedly, but be sure to check CraigVetter.com for recommendations. Also, the 2014 Honda PCX150 scooter has a fuel-injected water-cooled engine, and stock it already gets a rated 102 mpg, so if your body shell will be ultra aerodynamic (as Craig says, "round in the front, and pointy in the rear"), and you'll be riding one-up, and only want to go 60 mph, I imagine that that scooter's drivetrain would be up to the job.

Honda CBR250R would be my choice in that range. Single cylinder, water cooled, F.I. and tuned for lower RPM running than most.

CBR250 engine is a good choice. Is there a California spec version or are they all? The reason I ask is that the California version of many of the older fuel injected bikes had an O2 sensor, meaning closed loop, meaning more efficiency.

What I would do is go to bikebandit.com, look up the parts schematic for the bike in question, and see if it has an O2 sensor at all. If it does, then BAM!...that's a good candidate.

Example...check out this CBR250 breakdown. Item #11 is the O2 sensor. BAM!
2011 Honda CBR250R Parts, 2011 Honda CBR250R OEM Parts - BikeBandit.com

I would go for a CBR 125 but I don't know if you get them on your side of the world.

Reason is "there is no substitute for cubic inches" also applies backwards for FE ...

Also, 6 speed and injection from 2007 ...

I'd have a hard time chopping up a cb250r, but no problem with an old $300ish bike. Consider one with a title to make it easier to get a plate. Also check craigslist if you think you can cobble together a fuel injection system (not too bad especially on a thumper, premade for $400-$500 and in reality maybe $60 in actual parts).
wenatchee motorcycles/scooters classifieds - craigslist

Thanks all. Some good ideas and additional research for me to do.