I bought my first moped this past August (a 1977 Puch Newport), and I've been slowing working my way through its various quirks and issues. At this point, it starts pretty consistently and runs all right; the only issue is that, if I give it too much throttle too quickly while riding, it starts to bog down quite a bit. The engine's sound becomes more muffled, and the pitch of it goes down. The power decreases dramatically. I've tried moving the needle to all of the positions offered by the needle clip, but the issue still remains. I've found that taking the air filter off appears to improve the situation, and when I took a look at the spark plug, it was very dark – essentially black.
Now I'm new to mopeds and working on engines in general, but to me it seems that the engine is running quite a bit too rich. It doesn't four-stroke, but I figure the air filter experiment and the condition of the spark plug still point to this conclusion. Does this seem correct? And if so, what's the proper remedy? Down-jetting so that the fuel-air ratio is leaner? I know the first goal when fixing up a moped is to get it running well with a stock setup, so I want to ensure I evaluate any other possible contributors to this issue prior to changing out the jet in the hopes of resolving it.
In case it's of any help, here's a summary of the work I've done so far:
Cleaned the gas tank
Disassembled, cleaned, and reassembled the petcock
Replaced the fuel line and in-line fuel filter
Disassembled, cleaned, and reassembled the carburetor
Added a stock air filter (it had no air filter when purchased)
Replaced the throttle cable
Disassembled the top end, cleaned the piston and cylinder, and reassembled
Replaced the spark plug
Replaced the clutch, and replaced the transmission fluid
Cleaned the points and adjusted the timing to stock
Disassembled, cleaned, and reassembled the exhaust
Replaced all gaskets