Carb problems..

[jonathan14]

Does anyone know what size jett i should start with.. Ot is a vip 49cc with a current 72mm piston aftermarket everything.airfilter, heads been ported cool air,exaust and runns like crap at full throttle

[Zino]

Since you didnt list what carb
Here is general info

If this is a 2 stroke
Do you have a range of Jets that go up to 20 % bigger to 10 % smaller .
Start out as large as possible and work your way down.
Until it starts to run right .
When you get 2 jets that run close to the same take the bigger jet .

With porting and new exhaust that usually requires up to a 10 % up jet the the air filter requires even a bigger up jet.
If this is a 4 stroke you still need a range of jets to work down .

[GrumpyUnk]

What does 'cool air' mean? If you took off the shroud, put it back before you toast your piston & rings.
If you mean cool air fed from 'outside', never mind.
The engine runs mostly on the main jet at WOT. You might want to check that the needle has been moved up in the slide. Most have a movable clip or stackable shims to be able to lift the needle a smidge higher. Higher needle leaves more room for fuel flow around the outer diameter of the needle, thus enriching the mixture.
If the main jet is too large, or the slide cannot move up, you'll have too much fuel, and it will run poorly. If the main jet is too small, it will likely misfire. If the diaphragm on top of the slide is leaky or punctured, the slide won't raise and allow more fuel flow. If the diaphragm does not fit properly, the slide won't open and you'll choke off the engine at high rpms. It will get richer and richer if the slide doesn't move for any reason. The slide also lifts the needle, allowing the taper to affect the fuel flor, so it has to move or you won't develop full power. If you fiddle with the slide/needle/diaphragm, be sure to point the slide properly so it fits the bore and bottom of the venturi and the diaphragm seals.
Substitute diaphragms from another brand may not fit properly even when the slide does. If that's the case, it will not operate as desired or designed, and you may have a non-op slide. It will run ok at lower speed, and as you start to get up in the rpms, it will start to choke off, so that closing the throttle a bit makes it run better and develop more power. How 'bout that - close the throttle for more power...
tom