Zsomething

Sorry I posted this in the wrong place earlier:

I just bought a '73 240z and it runs rough. The previous owner says it need a (flat top) carburetor tune-up. It idles ok after it is warmed up. But, somewhere around 2,000-3,500 rpms it starts to sputter and pop.

The previous owner mentioned that he put a new distributer on the car. So, I figured that I should check the timing. I monkeyed with it a bit and settled on 10 degrees advance then a cranked the tightening screw down (and realized that it was loose before, which seems to me like it could have caused some problems at higher than idle rpms).

In the process of checking the timing, I pulled the plugs and checked the air filter. The air filter is very dirty and smells like gas, which seems odd to me. But, I have heard the flat-tops can spit gas into the air cleaner. The plugs are very black with carbon and they smelled like gas (not sure if gas smell is normal for plugs).

The vacuum seems ok. From idle to about 3,000 rpm the vacuum reads about 10 psi solid. I haven't checked the compression. But, the steady vacuum needle makes me think compression is pretty close for all the cylinders.

So, I it seems to me that the flat-top carbs are the problem. My plan is to replace them with a set of round-top SUs that came with the car. So, I am going to get the rebuild kit from Ztherapy.com, rebuild the SUs, and swap out the flat-tops.

How much replacing of hoses, lines, etc. that don't come with the kit will I need to do? Will there be a lot of plugging of coolant tubes that currently go to the carbs? Is using SUs with the flat-top manifold really just a bolt-on replacement? Should I just use the manifold that the SUs are already attached to instead of the flat-top manifold? Are there any parts on the round-top carbs (which are of unknown origin) that I should check before I attempt to do the refit?

And finally, am I blaming the carburetors too soon? I know the flat-top carbs are notoriously bad for maintenance (although I have heard some people claim flat-tops are better for performance applications). But if there is some tuning that I can try for the flat-tops, I would love to try it before I go and replace the whole setup. Should I replace the air filter and plugs before or after the carb swap?

Mainly, I would prefer to know that everything but the carburetors work before I replace them. That makes troubleshooting (when I bolt them on upside-down or something) a lot easier.

Thanks in advance. This is my first post. But, some of you have already helped me a ton (via the archives).