'70 318 BBD falls flat under load

I was running a '74 BBD on my '70 318. It has been running great even though the BBD had not been rebuilt before I installed it. I grabbed it off the shelf from my son's car.

I wanted to go back to the correct carb as far as the bowl vent was concerned and to get away from having so many blocked off, unused vacuum lines on the '74 carb. It had also started to leak and seep around the bowl and everywhere else it could.

I just put a commercially rebuilt BBD on. She fires right up and idles beautifully. When pushing the throttle the engine is instantly responsive and runs great.

I took it for a test drive and got around the driveway fine but at the end of my driveway is an uphill. As soon as I gave it more throttle to go up the hill, the engine immediately died. It started right up again and I drove back to the garage. The engines responds well to the throttle as long as it's not under load.

I can put the brakes on and put it in drive and put it under load and it will stumble and try to die when applying the throttle. As soon as I release the throttle before it dies it returns to a perfect idle.

I can see that the accelerator pump is supplying fuel when the throttle is pushed and at throttle under load it will continue to stumble long after the accelerator pump has delivered it's load.

This is the first time I've had an under-load drivability problem with a carb. Any ideas where to check? Thanks.