Modern Vespa

Hello, I have just returned to Cambodia from UK with a fistful of parts to finish my build.
Problem I have is it will idle but will not rev at all, it bogs and cuts out. I'm not feeling any suction on carb with thumb over. The compression from new piston is good. I have tried 2 carbs, made sure gas is flowing through them and spark seems strong. I'm guessing it maybe a seal, here is a short vid of it idling and then trying to give it gas. Can anyone give an idea before I order in the seals and gaskets?
https://youtu.be/sUpXba86aQw

Oh, hello everyone, my first post, forgot to be polite 😊

HI

A few suggestions linked below, also did you have piston out and could you have put it in backwards? Exhaust pipe plugged up? Not moving air suggests a clog somewhere, lean can run you just won't like results.

https://scooterlab.uk/engine-pressure-testing-leak-cheap-tech/

https://modernvespa.com/forum/post2499165#2499165

Welcome.
As we have no idea about your scoot yet is there any chance you could share things like what motor, carb, jetting & other things that will surely assist us with some type of potential diagnosis?
Photos are good as well, as we have terrible imagination

Hi, yes sorry, it's a 67 VBB but I'm in Cambodia so imagine it's a Vietnamese one. Not sure if it's an original engine but I doubt it. I had it in bits in my kitchen for 3 years, before that it ran great was just rusty, took me a long time to get it fixed up
It has a cylinder head and new piston.
A 20/20 spaco carb (I've tried 2) with jets which came with it. It will fire up but will not rev at all, just splutters and dies (like in the vid) I've cleaned the tank and checked fuel is flowing, fuel is in the carb, I think it isn't being sucked into the engine as not really any suction with thumb over inlet. It's spitting a lot of fuel back out from there. I know I should have changed seals and gaskets but didn't really have the tools. I will attempt it and order from Saigon scooter centre but just wondered if I'm missing something. Not sure if it's wired correctly as some of the colours of wire from stator didn't match but I figured I had a good spark so dismissed that.

+1, Clogged muffler perhaps?

I've also tried with the exhaust removed

piston in backwards. Shouldn't see a hole in the side of the piston from the exhaust end when exhaust removed. Turn the flywheel and check.

The arrow pointing down to the exhaust port

Arrow pointing to exhaust is correct. If not, or if you can see the boost port opening in the piston from the exhaust port, you have it backwards. Take top end off and flip the piston.

We're the pins that hold the rings on the top side of the piston when you installed it?

Yes they were aligned correct. Thanks for your imput

Post a short video if you can.
Perhaps it will help

I posted a YouTube link on original post at the top of this thread, did you see it or would you like another?

Problem maybe electrical. Check the grounds for the ignition but maybe a bad ignition coil.... strictly guessing from afar. New coil may fix it. Put a timing light on it and watch if at idle its sorta steady and giving revs, timing lights jumps everywhere.... then it's prob ignition.

Ok will give that a fry and let you know, many thanks