1973 Solex 3800 - replaced spark plug, now no longer running

Daniel Woodard /
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My son and I bought a 1973 Solex 3800, and the owner demonstrated it starting, running and driving. I had never seen one of these things with the engine up front, on top of the wheel, and I was intrigued, so I bought it. We drove about 3 hours round trip to go pick it up and come back home.

Once we got home, I noticed that it seemed to run well for 15-20 seconds, then run with less power for 15-20 seconds, almost like a repeated cycle.

I thought that maybe it had really old gas in there, or perhaps even ethanol, so I dumped out the tank, got some alcohol free gas, and added 3 ounces of synthetic blend oil to a gallon.

After filling the tank back up, I noticed two things. First, the pattern of good power and lesser power was much less pronounced, so I guess the 3800 had more even operation with the new fuel. However, I also noticed that even after running it for a mile, I was no longer able to open up the choke like I could before. Now I had to keep it over nearer the start setting, or it would die. I was curious about that change - I wouldn't have thought to see that just from new fuel.

After this, I thought it would be a good idea as well to replace the spark plug, since I had no idea how old it was. I purchased a ngk B6hs, which turned out the be exactly the same as the old one that was already in there. My son and I are used to wires that just pop on and off of spark plugs, so I ended up turning the metal ring back and forth, trying to get it to go up off the top of the plug. Instead, the ring popped out of the rubber wire enclosure end. It was only then that we realized you could unscrew the top of the spark plug, then pull the ring up and off. I also noticed that the ring was really an eye with a screw, and that it had screwed into the end of the spark plug wire. That would have been good to know.

Anyway, we installed the new spark plug, screwed the copper eyelet back into the end of the spark plug wire, put that on the top of the spark plug, and got things back together.

Since then, we have tried starting the moped more than a dozen times. It seems to start up, but always dies now. The only exception was when I got on and pedaled hard while it was running - I was able to keep it running, barely, but only by pedaling as hard as I could. It barely kept going forward and running. Or at least it sounded like it was running, until I stopped pedaling.

We are complete newbies, and were hoping all it might need was a new tank of fuel and a new spark plug. Now after the new fuel and new spark plug, it actually runs much more poorly than before, (that is to say, barely, or not at all now) which is leaving me scratching my head. I'm wondering if we might need to replace the spark plug wire? Does anyone have any other hints on what we could try next?

Thanks for any suggestions!

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