DATSUN 720 L20 ENGINE TIMMING

timming marks are located on crank pulley, but is easier to adjust the timming using a screwdriver on piston #1

has the timing chain been replaced

It sounds like the timing chain was replaced one tooth off. The only other option if the timing chain is correct would be to adjust the ignition timing. Since the timing chain was replaced I would think it to be the first problem.

18-20 city 22-25+ highway. Instead of the L20b, you would be way ahead to install an SR22DE or DET (Turbo) motor. Much better engineering and horsepower. IMHO

Probably same. Datsuns are famous for keeping the same carburetors (and other parts, as well) with only minor year to year modifications. My '79 Datsun pickup 620, right now has a carb from a '73 Datsun 620 parts truck, runs okay-for a hitachi.
Check ebay for new, remanufactured, or used- pretty sure they would have a lot of carbs for your car. The best place to buy new or remanned carburetors, especially for our older datsuns, is rockauto.com. Very nicely planned (and huge!)catalog, too, with easy access.
Don't know about your 280Z, but with the 620's 2.0Liter engine(L20B), weber carburetors are all the rage now for them. They are excellent carburetors and very reliable, even claim to increase mileage over stock, but for $250, I guess I'll have to motor on with the old hitachi a bit longer.

There are two things you can do.1. Set the timming back a little or 2. Start using a gigher grade of gas.
I use to have one of these cars and had to do the same thing.

1-3-4-2

Remove the rocker cover.
Turn the engine until it's at TDC on No.1
Both No.1 rockers should be loose.
Remove distributor cap and look where rotor is pointing, this is No.1 plug lead.

http://forums.nicoclub.com/zerothread?id=428078

Here is a link to a Datsan Fourm hope this helps

if it is a l20b engine it is about a m7 or m8 or m10 with a 12mm head about 28mm long or longer with a lock washer and the big washer that holds the exhaust and intake be careful the head is aluminum so it wont handle a heavy torquing