Gradual power decrease after a 1/2 mile

[ljohnson]

I purchased a 2007 AC Super 9 a few weeks ago. The previous owner had neglected it for the majority of it's lifespan but a new spark plug, thoroughly cleaned carburetor, and unclogged exhaust got it running up to approximately 40 mph (restricted, as far as I can tell). The problem is that it gradually bogs down after a half mile or so, which continues to decrease at about 5 mph per mile. Initial acceleration is good, but that dwindles quickly when the engine bogs. However, two minutes of waiting restarts the cycle, as it goes back to the initial speed. The moped currently has 9700 km on it, but starts well, and it sped up by 10 mph after cleaning the muffler.
Has anyone had this problem, or know what the cause might be?

[zombie]

Couple of maybe's

Clogged vent in the fuel cap.  (run with the cap off, and a rag over the hole)

The CDI or Stator can be failing, and return to norm upon cooling

The cylinder could be worn

There could be a vacuum leak that develops as the bike gets warm

Do a compression test first. it should read at least 120 psi. If you bring the tester with you I would test it again when the bike fails.

This will sound stupid but try blowing thru the pipe. It may be full of carbon from cheap 2t oil, and expanding/blocking the exhaust as it heats.

My primary guesses are clogged fuel vent (in the cap) or bad electronics.

[ljohnson]

Checked out the fuel vent, no issues. Compression is 120+ psi. Exhaust is still clear. Small air leak in the carb, which sealant is taking care of. The CDI might be the issue, but is there any way of checking? Also, do you know where I could get an unrestricted replacement? Thanks for the help.

[DerekG]

a little off topic, but how do you clean the muffler?

[Vivo]

a little off topic, but how do you clean the muffler?

hot water, steam, kerosene, thinner... blow torch...

[zombie]

There is one on Ebay for the LC version... http://www.ebay.com/itm/UNRESTRICTED-CDi-for-KYMCO-Spacer-Yager-Cobra-50cc-People-Super-9-LC-Topboy-/181264909918?pt=Motorcycles_Parts_Accessories&hash=item2a343b065e&vxp=mtr

You could also go to Stadium Yamaha for the OEM (15% off if you mention the forum when ordering)
http://www.stadiumyamaha.com

Or call SPI, and sign up as a dealer for 20-40% off  http://www.spi-parts.com

I always change the CDI, and Stator as a set. It always ends up being the one I didn't order, and meter testing usually shows them as good. For another 50 bucks you get new parts, and alot more peace of mind.  I meant piece of mind obviously...

[zombie]

BE CAREFUL! I use a torch to get it going, and the heat gun on NO HEAT. It is only supplying air. You can actually watch a red ring moving up the pipe as the carbon burns.
A pipe w/ baffles will burn violently until the carbon/oil are gone.

DO THIS AT YOUR OWN RISK! My name is Axy, and I live in Croatia!