EFI revs hanging

[scotto75]

Hi everyone,

My neighbour has a 2020 125cc Mutt and the revs are hanging for 2 seconds after the throttles returned. It starts and idles perfectly. No backfiring. There’s a slight hesitation when accelerating, but nothing untoward. It’s just that whether you return the throttle or put in the clutch, the revs stay at that rev range for 2 - 3 secs before dropping down to idle.

Don’t know much about Mutt’s except it’s a GN125 engine. I’m a carb and points type of a guy and this is all electronic, fuel injection and ECU type nonsense.

My thoughts were that it is running lean so I removed the spark plug. The ceramic was clean and the ground electrode was white, so I’m pretty confident that’s the issue.

I’ve sprayed easy-start around the manifold where the injector and throttle body are mounted, but no signs of air leaks. There’s no warning lights or the speedo and, as mentioned above, it idles and runs really nice.

Could the injector need a clean? It’s been sat around for a few months so suggested sticking a bottle of fuel treatment in the tank maybe?

Any other ideas would be most useful! Thanks!
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Honda ‘73 CB350, ‘80 CG125, ‘88 CG125, '73 Triumph T140V

[Nobby the Bastard]

Probably a sticky cable or outside chance it's an air leak on the intake between the throttlebody and the barrel.
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trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050

[scotto75]

[Nobby the Bastard]

Typically,the lambda sensor has the greatest effect on mixture but if it fails it registers in the efi as lots of oxygen so it makes the mixture too rich rather than lean.
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trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050

[Robby]

If it has an kind of separate idle air circuit then it would be worth looking at, a stuck open valve there could cause the problem.

It probably also has a PAIR system, which feeds air into the exhaust to reduce emissions. These typically require a vacuum feed, so if that vacuum feed line is broken you will have an air leak.

[WD Forte]

check the resistance of the tps from off to full throttle a few times
If the track is damaged or dirty or the contacts are poor you could get
erroneous signals to the ECU.
the tps is basically a voltage divider so I'd also check the voltages between the tps and ecu change smoothly in response to throttle movement.
It's a Chinese bike and we all know how variable the quality can be on them so 3-4 years may end of life for some parts

As it's EFI it probably has a diagnostic connector somewhere so reading fault codes could give some clues
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bikers smell of wee