MG Car Repair: misfire and backfiring, vacuum gauge, vacuum test

Question
My 74 midget sputters and backfires starting at 2800rpm then contiues through 3600rpm then smoothes out. The car has a hard time getting up to 45mph and wont go any faster. This problem started gradual and got worse. Ive removed and cleaned the carborator and reinstalled it but it still runs the same. Could it be the ignition, and what do i do?  Thank you for your help!

Answer
Hi Cale,
You need to do a manifold vacuum test. Put a vacuum gauge on the intake and run a long hose out from under the hood and put the gauge under a wiper arm so you can read the gauge. Then drive the car again up to it's speed that it levels off at and read the gauge and slowly push the throttle all the way down and read the gauge and tell me what those readings are.
Howard