Why is the engine bogging under acceleration?
I recently managed to get the moped running properly, albeit for only a short time. Things were looking up: I replaced the spark plug, cleaned out the exhaust, adjusted the idle / fuel screws, and even replaced the brakes (I can stop now!).
So everything was looking good. I was test-riding it after a short hiatus, and the moped was running fine. Near the end of my ride, I stopped at an intersection to make a left turn. I revved the engine while still stationary in anticipation of the light turning green -- I didn't want to hold up traffic, and the 50cc engine takes a while to get going. I pushed off from standstill, the scooter started moving fine, but soon thereafter started lagging. It absolutely refused to go past 20 MPH even though I'd usually go 27-28 on the same stretch of road. It was consistently going slower afterwards, and I scurried back home as best as I could.
I've revved the engine like this in the past (with the moped stationary), but for shorter durations. This time I'd say I was on full throttle ~5-10 seconds while stationary.
A couple of days later I fired it up, but it ran horribly. I managed to get it moving, but it hit 10-12 MPH on mostly flat land (where 27 MPH was common). It felt as if the engine was providing power for a couple of seconds, then stopped the power delivery for a couple of seconds, and it kept doing this over and over. It was so weak that I had to push it up a hill to get it back to my residence.
I thought -- for just a second -- that I smelled a burning clutch when I was riding it back home after the intersection incident. After I got home, I "smelled" around, but couldn't pick up any burnt clutch scent.
I sincerely hope the clutch isn't the culprit, since it's an expensive fix. Please let me know if you have any ideas as to where I can start troubleshooting (aside from the clutch, of course), as the weather's nice and I'm hoping to get it going this weekend -- fingers crossed!
I swear guys, I'm not crazy. There was a problem with the moped.
This morning I fired it up the usual way, hoping to analyze how it runs. It hit 29 MPH on flat land like nobody's business and I rode it around for a good half hour with no problems. I have no idea what changed in the last couple of weeks -- perhaps it had some time to think about its problems and resolved them.
I have a feeling this won't be the last time this problem pops up. I will most likely be coming back to this post sometime in the future :)
Thanks for all the replies!
Always start with checking for strong spark, a bad condenser can put out strong spark when cold, but will quickly fade. Arcing across the points is a good indication of a bad condenser. Assuming that the ignition system is in tip-top shape, that sounds like a fuel related issue.
I would start with checking fuel flow, as well as the fuel level in the tank. With a gravity feed fuel system, a clogged gascap vent, or misrouted/to long of a fuel line are common culprits, especially as the fuel level gets lower in the tank. If the tank is venting properly, and fuel flowing well to the carburetor, I'd clean the carb.
Pay close attention to the small holes in the carburetor jets, just because you can see light through the hole does not mean it's not restricted by residue left from evaporating fuel causing a lean fuel mixture. Poking a small piece of soft copper wire through all the holes, followed by a good blast of air or carb cleaner works well. Do not use steel or anything to hard to clean out the jet, you may accidently enlarge the jet, changing the fuel mixture.
If the bike does not idle smoothly or takes a long time to settle back down to idle speed, and you've already cleaned the carburetor, you may have an airleak. Airleaks can be caused by a bad seal in the engine, or in the intake/carburet
Hope those suggestions help!