Assuming you are doing this while in park or neutral, the rev limiter is kicking in. If this is happening while driving down the road, you may have a restricted exhaust due to a damaged catalytic converter.
15 LBS. at idle, and at least 40 LBS. at 3000 RPMS.
Advertised HorsePower is :Factory 4-barrel , 225 HP @ 4600 RPMS. Torque is: 355 FT. LBS @ 3000 RPMS.
maybe your rpms gauge is broken.get it looked at by a professional
it was a bad ignition rotor
major vacuum leak?
Advertised horsepower is: 175 HP @ 3600 RPMs.
Chevy stopped using cable driven tachs to read rpms back in the very early sixties.
You have a vacum leak. A big one. Check all your hoses. Idle should be under 1k rpms
Advertised horsepower is--225HP @ 4600 RPMs. 4 barrel carb. Advertised Torque is--355(ft. lbs) @ 3000 RPMs. 4 barrel carb. Advertised horsepower is 215HP @ 4400 RPMs. 2 Barrel carb. Advertised Torque is 320 Ft. LBS. @ 2400 RPMs. 2 barrel carb. Both engines are 1970 truck engines. NEUTZ.
If your testing this with the vehicle in park it won't go over 3000 rpms because the rev limiter kicks in so you don't do damage to the engine. If you are in drive and you step on it when it's safe to do so, it should go well over 3000 rpms.
How about it shifts into 4 overdrive gears are higher so RPMs come down.