Got a 1987 Suburban ¾ ton, swapped with a 70s model carbed 454. Been running and driving just fine until the other day, going down the road about 55 it started stumbling. Let off the gas, eased back onto it and it was fine. A few minutes later it did it again, turned off onto a side road and couldn't get it over 25. Pulled into a gas station, topped off the tank, checked everything out and it seemed fine. Started up fine, idled and revved at idle with no issue, put it in gear and it rolled fine, stumbled as soon as I hit the gas in gear but wouldn't actually die. Cap, rotor, fuel filter and pump were both just replaced a couple weeks ago. Plugs and wires are barely a year old. Running a mechanical pump with electric backup, tried with the backup on and off with no change. Tried adjusting the carb and it didn't change anything either. What do y'all think could be the cause? Need to keep this thing on the road til my 78 K10 is finished

 

How old is the ignition coil? When was the distributor timing last checked and adjusted?

 

I already tried messing with the timing but it didnt improve anything. Going to try replacing the coil to see if that may be it

 

Is this a point distributor? What carb? If it's a point distributor, it won't be the coil, as it won't start if it's bad. Adjusting the carb, air meter valves?