73 240z running rich

[wfl_colorado]

I just picked up a 73 240z w/ flattop carbs and it is running rich. Don't know much about carbs, but I pulled off the air cleaner and the front carb appears to have an excessive amount of gas being pumped into it. Gas is being pumped in regularly at what seems to be a pretty high ammount. The rear carb does not do this. Is this a stuck float or something like that?

[kenz240z]

I'm not familiar with the flat top carbs that came with the '73. Somewhere in its past the flat top carbs on my '73 were replaced with the round top carbs. Is the front carb leaking fuel? It could be that the float isn't floating any more - maybe has a hole in it, or is out of adjustment - and the fuel inlet needle isn't closing against the seat. That would cause too much fuel to enter the float bowl and leak.

If you are mechanically inclined you might pull the carb off the car and open it up to check the float. Not sure what gaskets & rebuild kits are available for the flat top carbs, you might check on e-bay. Sounds like the carb needs a little attention.

[wfl_colorado]

Thanks Kenny. I think you are probably right, but I haven't ever worked on a carb before. I guess this will be a first. As near as I can tell, there is no way to inspect the float while it is on the carb.

I also bought a set of round tops, but it doesn't appear totally straight forward on how to swap them out. That was going to be my next question. I have searched quite a bit and it sounds like it isn't that big a deal, but again, I have never worked on carbs. A bit intimidated at this point.

Bill

[Arne]

Round-tops should bolt right on to your existing manifold. You will need the center linkage piece for the round-tops, the flat-top linkage piece is different. Some round-tops have the water passages in the bottom, but many do not. And I'm not sure if the water passages in the flat-top manifolds match the late round-top carbs or not. So depending on what you have, you may need to block off the water to and from the manifold. (If you drive your Z during the Colorado winter, the water might be desirable. It'll be worth looking into whether your round-tops have the water passages or not.)

The fuel lines for the flat-tops should work fine with the round-tops. But the flat-top air cleaner won't fit, you'll need to get a round-top air cleaner, aftermarket dual air cleaners, or the air cleaner adapters from ZTherapy.

Really should be a fairly easy conversion. Should only take an afternoon as long as you have all the parts gathered in advance. Much simpler than my next project, removing the injection from my car and re-installing carbs.

[wfl_colorado]

Looking at eng pics w/ roundtops they sure look a lot cleaner than w/ flattops. It appears a lot can get stripped off. Are there any basic instructions for this conversion? If so, where? I have read a lot but haven't seen a lot regarding the flattops.

[bpilati]

My wife had to change the manifolds on her '73 when she went to round tops.

[wfl_colorado]

Bryan,

Do you remember why? Is this the water passage issue or what should I look for?

Tx,

Bill

[Arne]

My wife had to change the manifolds on her '73 when she went to round tops.

Hmm, I know that round-tops will bolt onto the flat-top manifolds, as there are people who claim that the flat-top manifolds are better for horsepower than the round-top manifolds. So the only reason I can see for having to change the manifolds would be the above mentioned water passages. And that can be worked around if you don't need the warm-up assistance.

[kenz240z]

Bill, if you haven't done any carburetor work before then you might consider having a shop replace the flat tops with the round tops on your Z. There's a great shop near me, T&D Auto. They are located in Hygiene, just a few miles west of Longmont. Phone number is 303/772-0404. Talk to Dale or Evan. These guys know Z cars inside & out, & may have any miscellaneous parts needed for the conversion. Hygiene would be a bit of a drive for you, but if you call T&D, they may know of a shop closer to you.

If you plan to do the work yourself, the first step is to do your homework. Read anything & everything you can get your hands on that relates to the Hitachi round top SU carburetors. These carbs are pretty simple in design & operation. But, just like anything mechanical, it can be messed up and cause your car not to run if you aren't careful & precise in setting it up.

[=Enigma=]

I just got done watching the Ztherapy SU Video that came with my SU Tune-up Kit and it has a chapter on swapping out flat tops for round tops. Steve talks about linkage differences, vacuum lines, air box mods/replacement, etc. Sounded like the water passage issue was more about which year round tops you have since the earlier carbs don't have water passages. Rather, the water simply circulates through the manifold only. You can get the video by itself if you want but it comes with a couple of their product at no add'l charge. Definitely worth the money if you have SUs and want to work on them yourself. All the "goods" are in here.

[Arne]

I'm preparing to put carbs back on my 240Z (maybe this weekend) and have looked into the water passages thing first hand. My round-tops have no water passages. My manifold is from a '72 and does have water passages. I don't know about the flat-top passages, but on the '72 manifold if the non-water carbs are bolted up, the water will not flow. The manifold passages need the carb passages to complete the circuit.

So no water will be connected on mine. That won't be a problem for my climate. But the round-top install on a '73 would be far simpler if the carbs going on did have the water passages.

[bpilati]

Hmm, I know that round-tops will bolt onto the flat-top manifolds, as there are people who claim that the flat-top manifolds are better for horsepower than the round-top manifolds. So the only reason I can see for having to change the manifolds would be the above mentioned water passages. And that can be worked around if you don't need the warm-up assistance.

There's another new one for me. No one has ever told my '73 manifolds are better performers. I'm not sure about water passage, but it might have something to do with the angle the carbs are mounted at.