25 June 2013

EtekChopper: Making a 250lb Engine Disappear

TL;DR: We tried to start it. We failed. Now the Engine is gone. 
One of the first steps in getting my CB750 ready for electrification is removing all unwanted crap related to the gas engine, and essentially be left with only wheels, suspension, brakes, lights, etc. However, I want to check whether or not the engine will even remotely start before I go ahead and throw away a perfectly good hunk of steel. I was assisted by MIT alum, former CB750 owner, and my supervisor at my summer job at Aurora Flight Sciences Terrence McKenna, by fellow intern and MIT student Thomas Villalon, and two housemates of mine looking to eventually build their own personal EVs Adam Rodriguez and Miranda Gavrin. 

Because most of the cables were straight-up cut off of the "spinal cord" of electrical wiring, it involved a lot of multimeter continuity checks and squinting at the manual's electrical diagram to figure out what went to what. There is both an electrical starter motor and an alternator (I thought the alternator was the starter for a while, but I was mistaken). The starter is attached to a relay with an integrated circuit tying a handlebar-mounted starter switch to the motor, as well as preventing the system from working unless the keyswitch in the bike is turned to "On". Because none of these switches were spared from the "spinal tap", we had to strip the wires and just electrical-tape them together. We attached a trusty A123 12v7 battery to the starter circuit. 

No dice. Comatose. Not one remote sign of movement, where a coughing engine would have been something. After double checking all the connections and verifying we weren't being idiots, we declared this engine dead, or too borked to warrant spending the time and money necessary to get it back running again. 

The first step in engine removal according to the manual was to unbolt and remove the carburetor from the frame. In order to remove that, though, the bulky air filter unit needed to be moved fully back. In order to do THAT, however, the battery holder framing had to be removed. Even after doing all that, the carbs would not budge from the rubber hose output connection the the engine combustion chambers. 

That's when we pulled out the Sawzall and tore that sucker out. With the hoses permanently cut, there was no going back on our decision to junk the engine. 

The carburetors weren't in bad shape, though. I'm going to take these apart, clean and oil it and maybe sell it on ebay for a half-decent amount.

Next was decoupling the rear wheel from the rest of the transmission. After removing the chain cover and loosening the rear wheel placement adjuster/chain tensioner, we were able to get the massive #530HT (High Tensile) chain off the rear sprocket.

Then came removing the chain from the small 15-tooth transmission output sprocket in order to free the transmission assembly from the rest of the bike.

This involved removing the sprocket itself. I decided to keep everything that I could remotely reuse, chain drive components included.

We then lifted the engine/transmission assembly with a jack and hammered out the attachment shoulderscrew/pins.

A hefty kick...

And the motor comes out! Note the lack of an oil pan in the transmission (we are now looking at the underside of the engine/transmission). Poor poor engine... I called the local scrapyard, Atlas Metals, to pick it up the next day free of charge. 

Here Adam gets a feel for the bike. It looks insanely high (like you can't touch the ground with your feet), but that's because it is up on the large kickstand. Still, I think it would be worth it to lower the front fork and possibly replace the rusty rear suspension with new, shorter shocks, so my stocky 5'8" build can comfortably and safely sit in the driver's seat.

Without the ~250lb engine, the ~500lb motorcycle was now SOO MUCH MORE MANEUVERABLE. Lowering the bike (and thus lowering the center of gravity) will help the bike feel lighter by decreasing its tendency to tip over. 

Goodnight, sweet prince. Soon you will live again, kissed by the power of electricity. 
Next step: drive and power train design!


  1. About the bike being high... don't you think it's that high because you're missing at least 150lb on the front suspension since the motor is gone? In other words, do you even have to shorten anything, considering your battery packs and stuff will push the fork down a certain amount?