1981 Turbo ZX - 21 gallon fuel tank. I have the car running pretty good (no missing) but at 3500rpm it hesitates like lacking fuel. I was really bad yesterday with less than 1/4 tank. I filled her up this morning and even though it still bogs down, it is 200% better than with 1/8 tank of gas. Question is: Is the fuel tank supposed to be vented? When I take the cap off after driving it goes psshhhhh. If it is vented, is it through the cap or somewhere else?

You need to look through the emission control system section of the factory service manual. The carbon cannister/fuel tank vapor vent/vacuum release valve/check valve are areas to check for the problems you describe. It would be easier to read than to explain.

Turbo and non turbo would be the same where emission controls are concerned. Here's a link to the FSM if you don't already have one:

http://www.xenons130.com/files/1981%20280zx%20FSM.zip

In case you need the turbo suppliment:

http://www.xenons130.com/files/1981%20280zx%20Turbo%20Supplment.zip

Thanks, Stephen. I have read through the FSM and will perform these checks this weekend. I'll let you know what I find. Thanks again for your help.

Since the easiest check was to suck on the gas cap to check the checkvalve, that was the 1st thing I did. It seemed to work just fine. Then my son wanted to drive it so I said it's not running right but go ahead. He came back and said he did not see anything wrong with it. So I drove it and low and behold, it ran just fine. I have to assume that the checkvalve in the gas cap was stuck and I unstuck it. Life is good.

My 280zx does the exact same thing, what i do is when i first start it up let it sit for a few seconds and rev it usually theres some back pressure once it gets out, she runs just fine

And you're okay with that? If it were me I'd figure it out why it does that and fix it.....