Here is a guide to what you can expect if you plan to convert from fuel injection to early (round-top) SU carbs. My particular experience with this is a bit unusual - my early '71 240Z had been converted (rather poorly) to fuel injection when a prior owner had an '83 L28 installed. Rather than de-bug the injection installation, I decided to go back to carbs. But this guide will also be applicable to installing round-tops on any injected 280Z (or even ZX).
Basic parts list - one pair round-top SUs, center linkage, pair of manifolds (including the balance tube with linkage), heat shield, return springs, fuel pump or regulator, fuel filter, fuel rail, air cleaner(s), choke cable assembly, manifold gasket, 8mm x 1.25 bolts or studs, assorted metric fittings and plugs, fuel, vacuum and coolant hoses, etc.
- The fuel injection fuel pump delivers over 30 psi, the SUs prefer about 4 psi, max 5 psi. So you will need either a low pressure pump or a fuel pressure regulator. A pressure regulator is a viable option, if your existing pump is good. But you don't want a cheap regulator that might go bad and start delivering 30+ psi to your SUs. The best scenario in that case would be a rich or flooded engine. The worst is a catastrophic engine fire! Better to have a cheap low pressure pump that can't over pressurize. Many people are using a pump from carburated Mazda RX7s, but those are getting hard to find used. I found a new aftermarket pump at a local parts house that is 4.5 psi for $30 US. (This was at the end of 2005.)
- Your injection probably has coolant lines running to the manifold and/or throttle body. You may or may not want to connect them to your SU manifolds, depending on your climate, and whether your carbs match your manifolds. But you'll have to deal with them either way. Either re-plumb them to the manifolds, re-route them or cap them off.
- If your car has an oxygen sensor or EGR in the exhaust manifold, you'll have to deal with that as well. The oxygen sensor can be left in place, or you can get a plug for its hole at most muffler shops. The EGR (if any) will have to be plugged somehow. Or you can eliminate all this by installing a header.
- You'll need different bolts (or better, studs and nuts) for the intake manifolds. The injection manifold bolts into different holes on the head. Luckily, the holes you need are still there, under the current manifold. (Actually, I have heard of at least one P79 head that apparently did not have the holes for the manifold studs. My P79 had the holes. YMMV.) The studs or bolts need to be 8mm x 1.25. If you use studs, 40-45mm long is good.
- You will need to re-route the breather hose from the valve cover, as well as the PCV hose. You will probably need new hoses for those. PCV should go into the balance tube on the manifolds, the breather goes into the air cleaner. You may not be able to use a factory hose for the PCV connection as the block vent tubes (under the thermostat) vary quite bit from 240Z to 280Z and ZX.
- You will have to fabricate a choke cable setup. I've been told that the choke cable/handle assembly from a late 240Z or 260Z can be mounted in the console of many 280Zs. For a ZX, you'll have to get creative.
- The vacuum advance needs to go the fitting on the front carb. If you are converting an '81-83 ZX (or it's engine), you should find and install the E12-80 ignition module, as the one on the late ZXs retards the ignition 8 degrees if the injection brain isn't connected.
- In some cars with injection there is a slight mismatch with the throttle linkage. To make certain that you are able to open the throttle fully, you may want to get the linkage pieces from the firewall of a carb'd car too.
That's it. Using this as a guide you should be able to do the conversion in a day. Enjoy!