My big block runs fine at idle and light throttle, but under load (around 3000–5000 RPM) it starts hesitating and stumbling. Idle is smooth, no smoke, and it always starts back up. I’ve checked timing, replaced plugs/wires/cap/rotor, fuel pump and lines seem good, and air filter is clean. Carb is roughly tuned but not perfect. Could this be lean jetting, a vacuum leak that shows up under load, or weak spark? Any advice on next steps or things to check would be appreciated.

 

What model & what year? Accelerator pump (if Quadrajet carb.) might be faulting.

 

Which "Big block" ? 400B, 400E, 400G, 425, 455 ?
Which Carburetion ? 2 barrel, 4GC, Quadrajet, 3 X 2 barrels ?
What compression ratio ? High compression or low compression ?
Ignition ? Points type, GM HEI, aftermarket HEI, other ?
What gas and how old is it ?

 

I had a similar issue with my 400G. It would run perfect until about 4000 rpm and then start to break up. It didn't fall on it's face like running out of fuel, but just start breaking up. I went through everything, ignition, fuel pump, until I found if I removed the fuel filter in the QJet it solved the issue and would fully rev out. Putting in a new filter just made it break up again at 4000 rpm. Good luck!

 

Yes, GAG1602, our crystal ball is broken.
Always provide the mileage, make, model, year, drive train combination, and what has been serviced recently. You will get more accurate, timely answers without tail-chasing.

tbispo pull the rubber end out of the filter and reinstall it.
I too, had this built in rev limiter... for years before I figured that out. I think it's the only thing that saved my G block from my youthful enthusiasm, having driven SBC's before this long stroker. The G is not a high rev'er. It will... once.

 

Built in REV Limiter...I love it!