I recently bought a 125cc Honda scooter. I was driving home yesterday when it suddenly just cut out. Now it only starts if I keep the throttle open. The moment I let the go of the throttle it dies immediately. Even with the throttle open it won't accelerate or rev up. Also if you keep the throttle open it will die after about 30 secs. I really need this thing working.
Please help anyone.
A bit strange... I will start to say that I'm no expert in this so maybe I shouldn't reply at all. However I will anyways since no one else has. So usually it's one of three things: air, fuel, spark.
Spark: Probably it has spark since it runs at full throttle, could maybe be a weak spark that's causing the issue. Relatively easy to control by just taking out the spark plug and placing it against the cylinder (keep fingers away) and try starting. You should see a spark in the gap.
Air: It probably has air to, otherwise it wouldn't run on the extra fuel that the full throttle gives it.
Fuel: Maybe it gets to little fuel. But would it really run on full throttle then? You can test the fuel by running some kind of starter gas in the air intake. If it runs, well then it's most likely something with your fuel system. Like a clogged up fuel filter or so (however I would guess that that would run better on low revs and worse on high...)
I'm thinking that maybe it could be that there's a leek between the carb (of it's carbed) and the cylinder, so it sucks in air into the air/fuel mixture. This could be found by - while the engine is running - spraying starter gas around your intake manifold and cylinder (not in the air intake though, and not to much (it's flammable)). If the revs go up, there's a leak.
But more that this it could surly be about a million different reasons, like offset timing, bad piston rings, etc etc etc...
Hope this might help you at least a little bit in your troubleshooting.
As Solar Mike pointed out in the comments, this sounds a lot like a clogged carburetor. If the fuel jets are clogged, the fuel/air mixture will be too lean, which sounds like the problem you're having.
To fix that, just take out the carburetor, disassemble it, clean every speck of dirt out of the inside, then do a special check on the jets. Those are the (usually brass) parts that the fuel runs through. There are usually two of them (one for idle and one for throttle). Make sure they're clean. Then reassemble the carburetor and put it back in!
The other option that comes to mind is that there's an air intake leak between the carb and the engine. That would also make the engine pull a too lean mixture in.
To check for that problem, you can get a can of starter fluid (it's basically just gas in a spray can). Spray a bit (maybe one or two seconds of spray max) around any connections near the carburetor. Don't get inside the air intake though, as air is supposed to go through there, and that wouldn't tell us anything. If the engine is pulling air in from any place it shouldn't be (aka not the air intake), we've added some gas to that air by spraying the starter fluid. The engine speed will go up, and we'll know we have a leak. Then it just comes down to fixing the leak. Note that you do have to have the engine running to start with here.
