SYM Fiddle III - Stumbles at Steady Throttle

[pgcorky]

I recently bought a "project" scooter - SYM Fiddle III 200i. It's fuel injected, so I won't need any carb advice  !  More of this story - seller bought this scoot new in 2017, but mostly let it sit in his garage; only 65 miles on it when I picked it up. He was upfront about the throttle issue, but I thought the price was low enough that I could take a chance. 

The scooter starts right up, idles fine, strong acceleration - all good. But the moment I let up on the throttle I can tell she's bogging down. When I apply throttle at speed, she hesitates, surges, hesitates, surges - but once I get on the throttle hard - she's going again.

I did suspect simply stuff like bad fuel. I drained the tank, but in 91 octane gas and a bit of Seafoam. No improvement. The maintenance records show that the previous owner did spend a bit of money on a new fuel injector, new fuel pump, new air filter (it's spotless). I sprung for a brand new ECU! Nothing has changed!

I also connected a code reader to see if there are any stored codes or current problems. Nothing there either. So I'm stumped and looking for ideas! Thanks all.

[FrankenMech]

Throttle position sensor?

[pgcorky]

One thing that's a bummer - I can't seem to get my hands on a SYM Service Manual - so it's not been easy to figure things out and to understand best procedures. Having said that, I found what looks like a SYM document that describes re-setting the throttle position sensor - so I'm going to give that a try!

[FrankenMech]

The throttle position sensor can develop worn spots or even dirty/corroded spots that will give false readings. Test the output with a voltmeter, analog types work best for seeing bad spots, digital meters are almost worthless.

Factory service manuals are like gold and guarded jealously by the manufacturer. That is one reason the newer EFI scoots are hard to service.