Modern Vespa

Hi all I have been having issues of my bike not going over 55 turned out to be a cdi I have replaced that but now there's another problem. on the first run it ran great got over 60 but ran out of space I then parked up to celebrate went back out 5 mins later pin the throttle it accelerated for about 20-30 seconds and started to engine break I then blipped the throttle couple times and kept it at 1/4 throttle seemed to run fine

Figured out that if I blip the throttle and keep it at 1/4 for a min I can then pin it for about 20-30 seconds before again it starts to bog down

Don't wanna take it to the mechanic yet as I already know their gonna charge a fortune to diagnose and fix it hoping it's something simple as a carb clean spark plug or fuel filter
Any help please

Is this on the stand with the backwheel off the ground? If so, its probably some sort of rev limiter.

Off the stand riding at 30-60mph and no rev limiter due to cdi change

It can be helpful if you share a bit more details of your Vespa.
From what year, what mileage, do you have service history, that kind of information.

If you can run it for 20-30 seconds full throttle before it starts bogging, then most probably it is not a rev limiter. A rev limiter comes in right away when the engine meets that max rev limit.

So it could well be that at full throttle there is not enough fuel flowing to the carb. The fuel level in the float chamber in the carb starts to drop and at a point the fuel-air mixture is too poor and the engine starts to bog.

The most easy explanations are the fuel tank cap venting is blocked or the fuel filter is clogged.

You can test these first: drive the scooter with the cap off or partly unscrewed (make sure not to loose it). If that does not make a difference then replace the fuel filter. It is cheap and not difficult to replace.

Hi all coming back too this thread after a while I've been riding the bike as is all winter so now I can explain the problem a bit better. During winter the bike doesn't get hot enough to replicate the bog to the point that I thought the bike was fine.

stay with me…

Now that the summer months are coming and it's heating up the bike is able to get up to temp and replicate the bog after 25 mins of riding

When the engine is cold the bike has a lot of torque and pulls amazing of a stop but after 25 or so mins of riding, it seems like it floods and starts slowing down at WOT unless you play with the throttle

This problem only appears in summer as I done a full 180mile round trip in the winter and it was a beaut throughout all of it.

Please any suggestions you can make are greatly appreciated and will be looked at

The main thing that changes with outside temperature is air density. What does your air filter look like?

My first Vespa was the same as yours. All the bogging down problems came from when I installed that same aftermarket exhaust that you have. I ended up using heavier rollers and adopting a two stage throttle pull style of riding. Gun it from a stop and then drop the revs to let the rollers settle back and then hit the throttle again.
A better solution would be to obtain a stock exhaust. The GT125 really needs the back pressure and restriction of the stock exhaust.