Monday, September 22, 2014

Honda Express Moped Engine Soft-Seize Cylinder Jug Piston Seizure : Woops!

Honda Express moped... 

"Houston, we've had a problem here"

Back on Mar 28th 2014 I had taken out my newly "upgraded" honda express nc50 noped for some test riding. The prior weekend I had take in out as well and experienced some technical difficulties... ie. It died on me. Once fully assembled with the following parts, I was able to start the honda noped first-shot. It was great. Now, keep in mind, I used the following stock express moped Mikuni carb settings on pilot run:

  • Idle : 22.5 
  • Main : 185 
  • Needle setting (clip position) : default 
The "low end" throttle was very poor, being sputtery and slooooow on the go. But, when I was in the 15-20 mph range... the moped FLEW! Because I was breaking in the new setup and honda carb, I never had the throttle more than 1/2 and kept the speed to 30-35 mph (measured by the Trail Tech vapor speedometer).

The moped ran excellent for 30+ miles and unfortunately my Trail Tech vapor temperature gauge is not installed correctly, so I was running without and monitoring of engine temp. On my way home there was a half mile or so of flat open back-roads and I decided to test max honda nc50 acceleration. I was probably between 35-38 mph and the honda noped began to "buck" (like I was jamming on the breaks) and then finally the engine died and I cruised to the edge of the road.

At the time, I didn't understand the problem. I thought it might be electrical or perhaps something went wrong with the carburetor and the engine lost fuel. As see in this forum post, I experienced a Honda Express Cylinder Jug Soft-Seize. As you can see for the honda express parts photo at the beginning of this post, I had soft-seize the engine a few times 5-10. After the piston and piston rings had melted to beyond functionable, I was unable to start the moped because I lost all compression I had.

I thought the cylinder and piston were shot and I would need to drop at least $150+ for a new engine kit. Some suggestions I received included:

Of those options I choose the Muriatic Acid Wash technique, because is was the least expensive. $60 for the Honda Dr 70cc 46mm upgrade piston. $7.29 for the Muriatic Acid. Some chemical resistant gloves, baking soda, Rubbermaid tub, goggles, air mask, paper towels... all things I already owned.

Next post reveals the video documentation of the process.

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