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.

Using a jumper or starter trigger on the starter is a lot easier than using the key, unless you have buddy to help. Also, make sure to block open the pistons in the carbs to get maximum airflow and more accurate readings.

It also helps to warm up the engine before you begin, and hold the throttle open while you crank it over to free up airflow.

Steve

I finally got a chance to check the compression. I checked it warm. But, I wasn't able to rig the piston or throttle open.

results:

6- 140

5- 150

4- 145

3- 150

2- 160

1- 155

These numbers seem fairly decent. So, I guess the carbs are indeed the problem?

Now, I am going to change the wires, plugs, fuel filter. The distributor is new. I'll replace the air filter when I put on the flapper air cleaner with the round top carbs after I get the rebuild kit from Ztherapy.

When I was removing the plugs to test the compression, I broke the #1 plug (and began cussing the fuel pump). So, I went ahead and put a new set of plugs and wires on. It will rev all the way up to 4,500 in neutral now. I think it is still 'popping' a bit. But, the fouled plugs were definitely causing a lot of problems. I haven't run it under load yet. So, we'll see how much of the problem was the plugs.

I hope you used NGK plugs. 10 inches is not much vacuum at idle. Should be 18 to 20 inches unless you are running a wild cam. If you want to see what others think of flat topped carbs . start a new thread , asking just that. I think you will find the numbers will be at least 20 to 1 for the round top SUs. These cars have been around for a few years and everything has been done to them. Benefit from others experience . I know at least 30 Z drivers personally and not one is using flat top Hitachi carbs. Black plugs if not oily, indicate a rich fuel mixture, they could be wet but it could be gas , which is likely.

I hope you used NGK plugs. 10 inches is not much vacuum at idle. Should be 18 to 20 inches unless you are running a wild cam. If you want to see what others think of flat topped carbs . start a new thread , asking just that. I think you will find the numbers will be at least 20 to 1 for the round top SUs. These cars have been around for a few years and everything has been done to them. Benefit from others experience . I know at least 30 Z drivers personally and not one is using flat top Hitachi carbs. Black plugs if not oily, indicate a rich fuel mixture, they could be wet but it could be gas , which is likely.

I used bosch plugs. Should I go with NGK in the future or ASAP?

Yeah, I have come to the conclusion that the flat-tops have to go. But, the carbs were not the only problem, and that was kind of my point at the beginning. Everyone (myself absolutely included) wants to mess with carb adjusting/swapping (SUs, weber, holleys, flat-tops, etc.) when they (I) should set the timing properly and replace the plugs and wires (which I did with a huge improvement). For example, the above post which suggested that I check the compression was right on, and I would be/am a fool for not doing that first.

I think the flat-tops are dumping too much gas. Rather than rebuild the flat-tops, I think this is a great time to swap for round-top SUs. I can get the ZTherapy kit and benefit from the knowledge of everyone on the subject of SU carburetors. But if I went with flat-tops, I would be going it alone which might be fine for someone who knows a lot more than me, but not for me.

Now I can start and idle the car which will make the tuning of the new carbs easier/possible. I'll go back and check the vacuum now that the engine is running a lot better. As far as I know, it's a stock cam.

Thanks a lot for everyones help!

Have you checked the condition of the ign points? are they gapped correctly? Are they quality parts?

My 240 went through points every 3 or 4 thousand miles. I kept a spare pair in the car.

I haven't owned a 240Z in 16 years, so I'm relying on memory.