Hi, Karatejohnny there are few things you need to look at when trying to troubleshoot your scooter for power loss. How does it sound? If you run it wide open throttle when warmed up, does it peak out? or stutter? is it loud, or very quite the symptoms can help decide what to try. When you are going full throttle, the engine should get to a high RPM and sound smooth here are a few things you should look for:
1. Plugging air intake- will start well cold, but be hard to warm up and not reach top speed. The engine will stutter at high RPM.
2. Open air intake- to much air or air cleaner is disconnected- engine will not have power at high speed, make vacuum cleaner sound. Acts like it's running out of gas. ( sounds almost the same as a dirty carburetor.)
3. Plugged exhaust port the engine sounds and runs good, but has no power. Slow to take off. Same as plugged muffler, but if the muffler is plugged you get a very smooth exhaust note at idle. Humming instead of pop pop pop pop.
4. Fouled spark plug- backfire. If you get backfire, pretty good chance the plug is fouled.
5. On models with belts, the gear ratio drops as the belt gets thinner (most hondas have belts).
6. On a moped with high mileage, the belt will cause slow top end speed if worn thin.
7. If you have a bike with chain drive in an oil bath and you "OVERFILL" the oil amount in the chaincase, that makes for slow running as it requires too much power to turn the oil.
8. If your engine does not rev up well and stutters or sputters, like when it is cold with the choke on, the engine is running too rich. (too much fuel).
9. Is the air filter soaked with oil? Clean with soap and water!
10. Is the air opening in the airbox plugged? Very common for a spider or mud wasp to build a nest in one of the intake holes.
11. On some scooters it is common to get a leaf stuck in the intake port of the airbox, causing a 10mph drop in top speed.
12. Don't try to adjust the screws on the side of the carb to get lean or rich, as they are for the idle circuit only.
13. The idle air screw should be about 1 1/2 turns out on most scooters.
14. Say your engine revs but kind of wheezes or acts like it runs out of gas it is common to have a buildup of hard carbon on the inside diameter of the hole in the main and pilot jets, and it must be carefully removed.
15. If your moped does not idle the pilot jet is likely plugged.
16. Anytime a moped has not been used for 1 year or more, and still has the same fuel in it, good chance you will need to clean the jets.
17. The poor quality of today's gasoline has made this a rather steady bet.
18. Mopeds are affected more than most vehicles because the small size hole the jet requires is easily plugged, even by thick oily fuel, or water.
19. Drain your carb once in awhile! Most have a screw of some type at the bottom (where the drain needs to be) of the carb.
20. Make sure the air boot to the airbox is not on wrong, kinked, hard and shrunken.
21. Don't oil the air filter if you got the stutters.
22. On some scooters, you can leave one snap loose on the airbox lid to allow it to crack open, to get full rpm.
23. "WARNING" too much lean out on the gas, and since the fuel cools the piston, the piston can "MELT"
24. Still slow? Check the muffler tip. Make sure no carbon buildup is inside the tip, stick something in as far as it will go.
25. Common on some scooters to have the tip bent over on a curb, then carbon up solid. common of all types to have the whole muffler plug.
26. When you have the muffler off, is it heavy?
27. Can you get the baffle out to see if the holes are plugged?
28. Check the exhaust port on the cylinder.
29. If you can't see all the way into the piston, use a mirror and a flashlight to make sure the piston is ok and the port is not plugged where it meets the piston.
30. Common on all models to plug here, Just scrape it out so the hole is full size again.
31. Don't worry about the small film on the inside of the port, I am talking about a very thin film that forms a wall in the exhaust port right next to the piston.
32. A 1 1/2 inch hole can be reduced to the size of a pea...... With a thin film of carbon.
33. If you can see the piston ok, it's not the problem.
34. If you are in doubt regarding your exhaust pipe being plugged, put your hand behind the muffler to see how much pressure is expelled. compare to another bike.
35. It should blow pretty good and have a distinct pop pop pop pop sound while idling, not a steady humming.
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