definately a fuel supply problem, pump, filter, blockage behind gauze under needle and seat........ start with the simple first.

Question , when you reach 55 how is the engine running . Missing , backfiring, or just flat and wont turn any higher revs ? Did your Mechanic check the fuel pressure the pump is putting out ? You need 4 to 5 lbs. pressure. If the vacuum hose to the advance is this rotted check the rest of the vac. hoses also and if in doubt change them , there cheep. If you change any of the rubber fuel line be SURE they are for fuel . Vacuum line will fail if used for fuel . With the engine running is the fuel filter full , half full , what . One thing you can do is to drive the car and take it up to speed and when it flattens out shut of the key and kick it out of gear and have a look at the filter and see if it is full or what. If you have a blockage at the tank the filter will be less than full . This could also be from a weak pump but if the pressure at idle is 4 or 5 lbs I don't think it is the pump . I agree with the others I think this is fuel related. Gary:rambo:

If you change any of the rubber fuel line be SURE they are for fuel . Vacuum line will fail if used for fuel .

I've experienced that firsthand. I bought a carburetor rebuild kit from Victoria British, and the fuel hoses they include are not rated for fuel! I was driving the car, getting ready for a date that night, and went to go get some gas. My Z runs rich, but this smelled like raw fuel. I stopped at the gas station, and gas was pouring out of the nipple on my carburetor floats. Luckily I saved the hoses before I rebuilt my carburetor, and used those instead. I pulled the hoses off, and they were cracked about 12 times due to fuel rotting them.

ok.. i replaced the fuel lines.. and im back at zero. the exhaust cracked an popped, normally it doesn't do that. It still gets to 55 range only and im stumped again. ill keep checking.. the fuel pump is the mechanical one.. i just replaced it not so long ago. its new and so it the gasket. I recheck to make sure it was not loose.

I let you guys know what i can figure out.

What is you setup now? Stock 71 w/ stock SU's?

yes stock 71 su's