One-piece question: Kymco 125

Does anyone have experience with the vacuum-controlled 125cc Kymco engines (air-cooled)? It's supposed to be a replica of a Honda engine from the 1980s. This thing has been sitting unused for four years. Of course, the old fuel has been replaced with fresh gasoline from the gas station (E10).

Problem description: The engine starts perfectly, idles normally, and runs smoothly with acceptable emissions. The initial settings were adjusted using a measuring device. However, when the engine is revved, it only responds to the throttle once, and then almost stalls. To me, it sounds like a fuel shortage. The vacuum-operated fuel valve seems to be working, and the new fuel filter shows that fuel is being drawn through it. A little engine doesn't really need much.

We had replaced the original carburetor, suspected of having a leak (vacuum leak), with a (new) Chinese aftermarket replica. With this carburetor, the engine at least starts and the idle speed can be adjusted. The replica matches the original in appearance and has identical functionality.

The spark plug is new (same type), and all oil changes have been done. Now, all the vacuum lines are going to be replaced to rule out that possibility. At that point, I'm at a loss. Where is the flaw in my reasoning, or what have I overlooked?

Hutfahrer wrote:

... It should be a replica of a Honda engine from the 1980s.

What does it mean?

It is!



The Honda engine, originally used in the Dax and similar models, is still being manufactured in China under a license agreement. Even my LigLanLu (Health) Rallox 50-qt13 scooter has one like that (although it only has 50cc... but there's a 125cc kit available http://www.gy6-motor.de/).
If you search the internet for "Honda Dax," you'll actually find information about everything related to it.

Bingo! Still, I can't get this thing to run. I disassembled the original carburetor today, found it to be in good condition, and reassembled it. Tomorrow, I'll put it back in the Kymco. However, I still swapped it out for the new Chinese carburetor, which should have worked. They don't build complete garbage, do they?

What came to mind: Someone definitely hasn't adjusted the valves on that thing yet, and I don't know anyone who wants to do it. Is that supposed to be?!

Valve clearance is of utmost importance at Honda Motor.

My scooter also had that problem with the battery dying. He was over-fueled, and the valves were too open.

Just ignore the fuel tap and hang a can of fuel next to it.
At least you can use this to check if the vacuum-operated fuel valve is involved.
Does the electric choke work?
(If he has one.)
'By the way, what does the compression reading indicate when the engine is warm versus when it's cold?'
Best regards, Frank.

The fuel valve is working; it has already been checked. Even providing a small amount of fuel through a funnel attached to a hose didn't change the engine's behavior.

Apparently, the electric starter is working fine, as the starting behavior is, as mentioned, flawless. It also seems to close properly later on. I re-measured the CO level in the warm idle state and got 1.5%, which is within the tolerance range (up to 2.5%).

I haven't been able to perform a compression test so far due to the lack of a measuring device. However, adjusting the valves remains a top priority, as no one has worked on them in the approximately 20,000 km since they were last serviced. That will therefore be the very next point that will be targeted.

Check if the nozzle needle is properly seated; I once made a mistake on a Piaggio when it had slipped down two notches.

Resolution:

You were very close! The needle valve was stuck due to hardened residue from a cleaner or preservative that was not washed out during the carburetor disassembly. The funny thing was that we had already opened it and blown it out with brake cleaner. The Chinese carburetor is still installed and was adjusted to a CO (carbon monoxide) level of 1.5%.

"With this setup, the engine starts easily, and the idle speed is maintained. However, the compression is at the lower end of the tolerance range, which means reduced power at low RPM. If you don't accelerate with full throttle (which is practically impossible), it's reasonably drivable, and the issue is barely noticeable from the mid-RPM range onwards. Even the specified top speed is achievable. However, due to its design, it's not really suitable for being ridden on the small wheels with the short wheelbase." Speeds of 70 km/h are acceptable, but anything above that is not enjoyable.

The valve clearance is within specification, but it's possible that the valve seats are carbonized. In addition to that, there are several small holes in the exhaust pipe, but I'm not really worried about them for now.