Modern Vespa

My GTS 250 super wobbles a little at 40 mph is that normal

Well, a peek in our Wiki would show a list of models. Clicking on that link and drilling down through GTS-250 and Presto!

https://modernvespa.com/forum/wiki-wobble

If it has Pirelli tyres then no. If it has Savas or something else, then a wobble while decelerating though 30mph with one hand on the bars is normal. See this article from the Wiki:

FAQ: Is it normal for a GT/GTS to "wobble"?

And, just to make sure, are you keeping the tires (especially the rear) inflated to spec?

Depending on what model Pirelli tires it could be normal. My old GTS 250 had Pirelli SL tires and with that tire it definitely has the wobble. My current 300 Super with the Pirelli GTS tires does not have the wobble.

OK only have lx 150 - but reading this before and after it seemed
the 150 had least trouble, but now I had two fears, 1st the 250 wobble,
and second my fear the front wheel will just fly off ( counting cars
I've had some form of no 2 for 45 years).
So on the 1st I drilled holes on bar ends and put 17.5 oz they
look great. That day they arrived I saw [redacted] had them for 1/2 $.
ON the second, I took out the 8.8 bolts and added new 12.9
for @ 50% strength increase, about $5.50 feel real good , may
even help with the wobble I won't get! I do now own an $80
torque wrench and am lookin for something to do.
I do love my Vespa - really!
I did buy that holster and am weighting for a pershing rifle scabbard
for my $20 cane. Is weighting with an a or an e ?
Thank the Web I can bitch to people @ the world.
So far on the US map I have been top to bottom of DE
Its on the right has 3 counties I been to them all.

You are waiting for the scabbard.

Adjust air Pressue no More wobbles