I just posted on the scooter forum:
http://www.urbanscootin.com/forum/viewtopic.php?f=2&t=10783
This is what I said:
"Hello,
I've browsed all of the threads (yes, I spent like 5 hours doing it) in search of an answer to my problem. I imagine it's pretty simple, but first, some background:
I own and have repaired a bunch of bikes. Most recently an 85 VMX12, 79 XS1100, 71 CL350, 74 XL350, and an 82 XL500r. I am no stranger to old bikes and how to fix them. I said "old" bikes. Manual choke, predictable carbs, etc.
Well, the in-laws dumped their 03 CHF-50P Metropolitan off in my garage because it would no longer start. They ran the battery down each time they tried. The scooter was purchased used and ran fine for a season. Then they complained that is started getting harder and harder to start and finally, it would not start at all. That's the only clue I have.
Knowing nothing about it, I tried to start it and found that the starter and/or swing relays would rapidly click but the engine would not turn over. I initially purchased relays thinking that they were failing to stay closed (which they were) but later sent them back unused after discovering that this thing won't turn over if the battery is under 12.5. So I charged the batt and tried again, but the engine just turns over and won't fire. The battery will turn the ACG as long as it can and as long as you are holding the brake and pressing the start button. The minute the battery voltage goes too low, the relays won't stay closed. So then I try the kicker and still nothing.
I've gotten it started twice after jumping it and trying 10 minutes worth of various methods of cranking and feathering the throttle. After I got it started and ran it around for 10 minutes, it seemed ok, but let it sit overnight and it won't start again.
1. I'm eliminating battery as a problem because we installed a new one with full charge. It happily cranks the engine until voltage goes low and that's all.
2. I have reset the computer by providing 12v to the jumper wire on the harness. This reset the computer but did not make the bike start like it should.
3. I have removed and inspected the carb. It is spotless. I cleaned it nonetheless.
4. I have checked and cleaned the auto-bistarter (automatic choke). That did not work. So I replaced it with a brand new one. This did not solve any problems.
5. I have checked the fuel pump and it appears to work normally. (normal clicking, I have observed that it is pumping fuel from the tank to the carb)
6. I replaced the air filter because it was dirty, but this did nothing.
7. I have verified the ignition system in terms of grounding the spark plug and observing a healthy spark when cranking, so I believe the ignition coil, spark plug, lead wire, etc. to be fine.
Finally after getting the bike running one time, I let it warm up and then just let it idle for a few minutes. I killed the engine and checked spark plug color and the spark plug is soot black. Knowing that this is a rich condition on that carb circuit, I removed the airbox and air filter completely and ran the bike again but the condition was the same - sooty black plug. Elevation is 5700ft and I think it's got stock jetting.
So my theory is that the bike will not start because it is flooding on the idle/mid carb circuits. Since the jet needle is not adjustable, the only thing that would cause such a rich condition at idle would be choke problems or air intake obstructions. Since the problem did not clear up when I removed the airbox, it has to be a fuel problem.
The floats are not sticking because I do not have fuel running out of the carb. Therefore, it has to be an automatic choke problem (auto-bistarter). This is why I replaced it, AND YET THE PROBLEM REMAINS! (yes I reset the computer after replacing the choke)
So now I am left with one option as I understand it: replace the computer.
I would rather sell this flakey peice of junk than spend $300 on a computer, especially if I replace a computer and don't know 100 percent that it will fix the problem. But nobody wants an already flakey by nature scooter if it won't start.
So, out of desperation, I post here in hopes that another mechanic or scooter rider can point me in the right direction. I'm sure there must be other things I don't know about, as all of my testing has been done without access to a manual.
Anybody have any advice? What is the troubleshooting flowchart for when the engine will not start?
Something controls the choke. There has to be a thermostat somewhere that the computer reads. When the thermostat indicates that the engine is cold, the computer must send voltage to the auto bistarter that would cause it to withdraw the needle from the carb body, which lets more fuel mix with the intake charge. When the thermostat reaches a certain point the computer shuts voltage to the bistarter an it springs shut and the needle closes the choke passage. That's my thinking anyway.
There is a coolant temperature sensor. That's the only one I am aware of. Would that be the culprit?
These scooters are way too flakey. My Honda Civic has nearly 300k miles. I have honda bikes that are over 30 years old and still run fine. What gives? Certainly someone has some light to shed on this problem?