Modern Vespa

Hey everyone! I haven't visited MV in a while but this is the first place I've turned to in my time of need. Sorry if this topic has already been posted...

Basically my GTS 125 is having issues with idling smoothly after long, fast rides.

Upon first startup the engine fires up like a dream and runs real smooth around town. However, once I've blasted down the open roads WOT for a while the idling will become quite rough when I'm stopped at traffic lights, etc.

Every couple of seconds there'll be a noticeable 'bump' in the idle where it feels like the revs drop then suddenly pick up again.

I do the occasional 120 mile ride from Birmingham to London (UK) on my carb'd 125 GTS which involves around 3 hours of full throttle riding down the country roads. Once I reach a town where the traffic is slower and I have to stop at lights, I've found there is an initial delay in engine and road speed after I twist the throttle and it even cuts out when the revs drop too low. Many times I've had to restart the engine while rolling around a roundabout for instance. Not safe!

This has only started happening quite recently - most noticeably on my last road trip to London. My GTS had a major service a few months ago, I've adjusted the idle speed screw and it's not set too low or too high and everything seems to be in place. I'm wondering if it's a fuel pump issue?.. or perhaps something wrong with the spark plug/ignition lead? I'm no expert but I know some of you are. Any help will be appreciated!

My bike is a GTS 125cc LEADER and has just clocked 30,000km.

You might want to look for a vacuum leak. Maybe the the hoses to the fuel pump or petcock.

Try loosening the fuel cap to see if tank is venting.

The electric choke/enrichener valve maybe malfunctioning.

I'm also wondering if tight valves could cause this?

Not too likely it's the fuel pump (that's a fuel injection issue that doesn't affect carb'd models).

Couple of things-

1. Bad gas?
2. vacuum leak?
3. weak battery? (have your battery tested in a shop)
4. fuel line degrading? (not uncommon on the GT200 etc)

Agree with Harvey and Gary too particularly about the valves. Could be an inlet valve going tight when very hot and this will often show up as a slight idle wobble in the early stages, but only if hot. Performance will be affected a little too and then it will get bad as time goes by. Let us know what you find.