Rough running engine

[Zsomething]

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.

[e_racer1999]

from what i hear, the flat top mani is BETTER flowing than the earlier mani. it should just be bolt on! of course you are going to need a new air cleaner, as the RT sus have round mouths versus square.

EDIT- also, where did you hear flattops were better for performance? there are some old datsun comp SUs that happen to have flat tops, but those are a COMPLETELY different beast.

another upgrade you might want to look at is upgrading your dizzy to a ZX electronic one.

EDIT AGAIN - oh, and WELCOME!

[Zsomething]

I've also heard the newer intake is better. It looks like coolant runs to the manifold and to the carbs. Should I just JB Weld over these ports?

I have a flapper air cleaner that I can put on with the round tops. But, the 2 screws look like they are too short to hold the top on. Are there brackets missing?

[Art@NissanSport]

Don't JB weld, use plugs

[Zsomething]

EDIT- also, where did you hear flattops were better for performance? there are some old datsun comp SUs that happen to have flat tops, but those are a COMPLETELY different beast.

Some people seem to think that they could eek out just a little bit more with the flat-tops. (can't find it now. But, I think I googled 'round top vs. flat top' and found some flame-war on the topic that was pretty informative). The conclusion was basically that if you really know how to do it, you can get flat tops to really run. But, most people don't have the skill, time, patience, equipment, or luck to make it happen.

I guess to each his own. For me the choice is pretty simple: better after-market availability and wider use of round tops = fewer headaches and more information

[e_racer1999]

I've also heard the newer intake is better. It looks like coolant runs to the manifold and to the carbs. Should I just JB Weld over these ports?

as was mentioned, plug these puppies up

I have a flapper air cleaner that I can put on with the round tops. But, the 2 screws look like they are too short to hold the top on. Are there brackets missing?

on the new (old) air cleaner, you SHOULD have a couple strange looking bolts that have a ~2" head with a hole in it. those screw on the inside 2 holes of the carbies and accept the wing bolts that hold the "top" of the air cleaner on. i'll try to find you a pic.

[Arne]

It looks like coolant runs to the manifold and to the carbs. Should I just JB Weld over these ports?

If you are eliminating the coolant flow to the manifolds and have by-passed the hoses, there is no need to do anything to the coolant ports on the manifold. Just leave them be. Without coolant running to them, they do nothing.

[e_racer1999]

^haha, arne is right! for some reason i was thinking about the plugs on the ends of the balance tube :/

[Zsomething]

on the new (old) air cleaner, you SHOULD have a couple strange looking bolts that have a ~2" head with a hole in it. those screw on the inside 2 holes of the carbies and accept the wing bolts that hold the "top" of the air cleaner on. i'll try to find you a pic.

I was just logging in to say that, after looking at the carbs more closely, the air cleaner mounts into the long hex nuts on the side of the carbs. The setup came apart. So, I never put 2 and 2 together.

Thanks!

[sblake01]

Some people seem to think that they could eek out just a little bit more with the flat-tops. (can't find it now. But, I think I googled 'round top vs. flat top' and found some flame-war on the topic that was pretty informative). The conclusion was basically that if you really know how to do it, you can get flat tops to really run. But, most people don't have the skill, time, patience, equipment, or luck to make it happen.

That was an old discussion, quite a while back and I'll bet I was involved in it. I worked with a guy back in the day that knew how to make flat tops work and picked up some of his tricks but I haven't touched a pair in over 20 years. We were dealing with the smog carbs (73 240Z/74 260Z) not the Datsun comp carbs. You could find them cheap and in pretty good condition since everyone was swapping them out for rountops. My buddy still has a Z shop to this day but doesn't deal much with them anymore.

[Ramses]

Here is something some of you may not have thought of. My vacuum was perfect. The interior of my cab smelled like unburnt gas and driving it made tears come to my eyes. I finally checked the compression in my cylinders and I found out that I had great compression in 1, 2, 5,6 and no compression on 3, 4. It seems that when #3 got compressed the fuel mixture would leak into #4 and vise versa. I had to run the car well passed the normal advanced side to get it to fire on those cylinders. If I put in where the FSM states then it would run like crap and cough and sputter all the time.

Moral...Check the condition of the plugs. They can tell alot. Run a compression test before spending any more money. If you have the same issue, then consider performing a valve job on the head. Worked on my '73 and she runs much better without the fumes.:knockedou

Good luck

[Zsomething]

Moral...Check the condition of the plugs. They can tell alot. Run a compression test before spending any more money. If you have the same issue, then consider performing a valve job on the head.

The plugs are black with carbon. I think one of them was bridged by a carbon piece, because just pulling them and putting them back in made things smoother. I'm pretty sure the plugs are shot. I've got new plugs and wires ready to be put it.

I'll check the compression when I get home. To check compression, is the procedure:

pull the plugs, attach gauge, attach jumper to starter motor for 5 seconds, check gauge.