S13 Rev problem?

[freeandam]

I have a bone stock S13 with the KA24E (5spd). Anyways when the engine is cold it revs all the way to 7k with no issues. After the engine reaches normal operating temperature the car doesn't want to go over 3k in any gear (including neutral.) If I floor it it falls on its face. The only way to pass 3k is to apply pressure slowly to the accelerator but it only works in neutral and not in any other gear. Its almost like the engine isn't breathing properly. I changed the mass air flow sensor a few months back and I recently had the muffler and catalytic converter cut off and a straight pipe welded on. So I know its neither the mass air flow or the cat (being as I dont even have a cat.) So what else could it be? And why does it rev fine she. the engine is cold but not after it warms up?

[CSUPUEBLOTIM]

Did you replace it with a NISSAN maf or one from a parts store. Any codes in the ECU?

[freeandam]

I replaced it with an OEM maf sensor that I ordered through advance auto. And I do t have an obd I reader to see if its throwing codes