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Rough engine



My 1977 L48 3-speed automatic runs smooth at idle, with an occasional misfire and runs smooth at regular speed, but when I floor it and go into passing gear, it misfires,spits and sputters and finally comes out of passing gear and resumes a smooth ride. The plugs and wires are new. It has a new rebuilt Holley carb.....Any suggestions.
I would check the float level in the carb. If it were out of adjustment, meaning low fuel level, then under acceleration it would be starving the engine of fuel. You mentioned sputtering and that sounds very much like running out of gas. Smooth cruising speed would be unaffected by a low level in the fuel bowel. The best part of this problem (float level), is its' a no $$ fix! Rick
When you replaced the spark plug wires did you keep them off the exhaust manifolds?? If one is touching it burnt the element in the wire also check your timing..Good Luck:cool
Check for vacuum leak at base of carb. Take some carb cleaner and while the engine is at idle, spray some cleaner at the base plate. If the speed goes up, you have a leak that will give a lean condition at higher speeds. Also check that the vacuum line to the auto trans is reconnected.:cool :cool
One more little quick check


Might check the timing, also if the dwell is wrong it will show up at WOT. One thing about our GM cars dwell is easy to adjust ;)

Good Luck
Thanks for the input guys.....I'll check all your suggestions...:w
Higher engine speeds require greater fuel supply; have you check / replaced fuel filter? Check fuel pressure at higher rpms to insure correct fuel pump pressure. Check bronze / stone in-line fuel filter at carb inlet.

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