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In part one, Jeff and I pulled the entire top end off my wife’s 1981 Honda CM400 Custom to replace a blown head gasket. With both main engine gaskets replaced, Jeff and I had the CM back together and ready to run. Key in, fuel on, a dash of choke and the bike fired right up. Well, sort of.
While plenty snarly, the CM was only running on the left cylinder. To my leisure mechanic’s shame, I couldn’t actually tell. Sometimes you need a real mechanic. Thankfully, Jeff was only about three feet away. Time to check the triangle: fuel, spark and compression. A quick turn of the carburetor drain bolts found fuel in the bowls. Spark was strong, and a quick check on the compression gauge showed nice tight cylinders. That meant the most likely culprit was somewhere between the bowls and intake. It was time for a carb rebuild.
I joked with Jeff that I’ve had these CM carbs off enough times that I could probably do it blindfolded. Luckily my mechanical sensei didn’t make me prove it. We laid out a pair of shop rags and began tearing into the CM’s twin carbs. First the slides, then the bowls, the floats, and on and on until we had two neat groups of not-so-shiny carburetor parts spread out on the lift in front of us. Not surprisingly, there were issues. We found lots of flattened, rotten o-rings, a worn out accelerator pump, and some pieces were missing all together. With the situation fully measured, it was time to wash the empty carb bodies and the jets in Jeff’s caustic washing machine of doom. “It’s pretty nasty stuff in there. If we just left everything in there, the jets would be completely dissolved in a few hours.”
With their ill-tempered bath complete, we rinsed the parts in the wash bin using a solution Jeff described as “somewhat less corrosive shit.” Now it was time to grab a can of carb cleaner (“just slightly corrosive shit”) and spray out all fuel and air circuits. With Jeff’s guidance, I worked my way around the carbs — spraying pressurized carb cleaner through each orifice until it came spilling out the end of its corresponding circuit. It’s a messy process, but seeing the circuit openings sputter, fight, then finally come clean was very telling. No wonder it wasn’t running right.
With fresh rubber bits, missing pieces replaced and clean passages, we screwed everything back together and bench tested the accelerator pump. Now squirting happily, it was finally time for reassembly. Screwing the carbs back onto the motor, I crossed my fingers as Jeff hit the starter. The CM fired right up, and now I could really appreciate the difference clean carbs make. To my further astonishment, Jeff pushed the choke back in immediately and the CM just sat there, idling away even though it was still cold. A few quick twists of the throttle and the bike growled angrily. Pausing the warm up, we hooked up Jeff’s carb sync tool and got the two carbs pulsing fuel in unison. Jeff whispered to the bike and deftly angled the longest screwdriver I’ve ever seen. The snarly old Honda twin calmed down and smoothed out, healthy at last. The moral of the story? You can do a lot yourself, but sometimes you need a real mechanic.
“Sweet! Now what?” I asked Jeff. The mechanicals were solid and the CM was running like a brand new Honda, but it’s still missing some much needed character.
Jeff took a long look at the CM, now down off the lift. “You know what I’d do with this bike?”
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