my '79 300 cid L6 idles well but when i rev or try to drive it boggs down. what can i do to find out whats wrong with it?
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Assuming it's a carburetor, I would think it's not getting the right mixture when you step on it. Either adjust or rebuild. If it is a throttle body, I would assume the same, adjust or rebuild. Timing could play into it also. Make sure it is breathing right. I would start there.

 

The carburetor should have something like an accelerator pump that adds gas as you push down on the pedal. There is a lag between pressing the pedal and increased fuel through the jets. A little raw gas smooths that out. Take off the air cleaner and depress the accelerator linkage one time while looking down the throat. You should see a stream of gas spurt down the throat.