Just rebuilt the c223 in my '84 p'up after the head gasket blew. Now it won't run unless the pedal's on the floor, and then only about 1000 rpm. If I hold it wide open it will start and run, but as soon as I release the pedal even a little bit, it immediately dies. Also the manual pump on top of the fuel filter just pumps bubbles all the time, no matter how much fuel I pump through it. Any ideas? I think its in that hand pump, maybe a bad seal. The fuel filter's brand new, with a new o-ring on the sensor at the bottom, I cant find any fuel leaks there. I know it will run ok, if I fill the filter with fuel, it runs fine on that until it needs to bring fuel from the tank, then it can't pull enough to keep going. I've tried blowing out the fuel lines, but that didn't make any difference.

Sounds to me like your sucking air into the fuel lines somewhere. Air leaks can be hard to find, basically just start with the most likely culprit and keep checking till you find it. Sounds like your on the right track. I would think it would be between the filter and tank if it runs good when you fill up the filter.

Also, an air leak won't always leak fuel from the same spot. Air needs a smaller hole to get through than diesel does.

Just some items to consider
-did you crush a hard fuel line when you stabbed the motor back in?
-Is the tank vent line/fuel cap OK?