I'm running a 383 stroker with a TH400 and it seems to decrease in power (and almost sputter a bit) when climbing above 2500-3000 rpms. If I take off hard on the throttle, it pulls strong through 2500 r's but, then seems like its missing a bit or sputtering through the rest of the gear. When it shifts and rpm's drop, it pulls strong again thn starts sputtering above 2500 again.

How do I figure out if this is spark, air or fuel that it seems to be starving for?

Could it be timing?

 

More like your vacuum advance is not opening up as needed. I just went through the exact thing. I cleaned tested and reinstalled and didt replace though the parts are very inexpensive I always try cleaning fixing 1st. Could even be a bad vac line between carb and dist. Not allowing it to advance.

 

Hmm. Just realized I somehow swapped the 2 vac hoses on the passenger side of the carb. The distributor was hooked up to full manifold port at the carb base instead of the upper port that dist vac advance should go to. Haven't test drove it yet but, would this cause the issue in OP?

 

No, after around 2,000 rpm's the vacuum advance has no further effect, it's all mechanical advance from there up. So, check your mechanical weights and springs under the rotor, and make sure nothing with the mechanism is hanging up.

 

Also pull a plug and check for a white tint, Fuel pump/fuel filter can limit the fuel supply.