Hello everyone. This is prompted by an email exchange with ZTherapy regarding replacement carburetors.

I asked: "I have the set originally installed on the car (01/1971) complete with linkage. These carbs are 3 screw and have fuel float bowl drain plugs. I also have another set of 4 screw carbs. I believe I would want to stay with the 3 screw units that are original with the car but would be willing to make a change if there were some significant advantage to the 4 screw units."

ZTherapy: "Depending on which intake manifold you have on your car determines which carbs to go with. A 01/1971 car would have come with 4 screws(?). What number is on your manifold? An E46 is 4 screw, E88 is 3 screw."

My 1/1971 240z has an E31 head (original as far as I understand) with 3-screw round top SU carburetors. This car was purchased new by my father but my brother has his hands on it for several years. As far as anyone (father and myself) can attest the long block is unchanged. It is entirely possible that it had carburetors swapped (I do have a pair of 4-screw round top SU's in the extra parts boxes amassed by my brother) but nothing was indicated in the documentation left behind. My brother did document the installation of a 6-1 header and thread insert for a damaged exhaust stud in the head. There was a plan to do an L28 block with a P90 head (still have the head) but it did not manifest before the car was abandoned.

I will send ZTherapy both sets of carbs but am wondering which to ask for in return, the 3-screw or the 4-screw. I am inclined to ask for the 3-screw. A secondary idea is to try and sell a pair (-4-screw) of the carbs on the open market in an attempt to gain a little more cash to offset the carburetor purchase (ZTherapy would pay $150 for the core without center linkage). I do like the idea of them having more stock to work with as I feel they provide a valuable service. There is a 6 month backlog currently and I have a 5-speed to rebuild in the interim so there is no particular urgency.

Any thoughts?

Cheers.

-R

Edited July 18Jul 18 by DrFeelGood

I'm no expert on the chronology of what Datsun did when, but I have high confidence that your car originally had four-screw round tops installed from the factory. I don't believe the three-screw round tops came out until the 72 "model year", and the Datsun parts manual supports that belief... Four-screws until 8/71 and three-screws from 9/71 until they went to the flat tops in 73.

So, that said, I think the four-screw carbs would be the original version for your car.

Ztherapy's question about the intake manifold is also something to look at... What number is on your manifold? An E46 is 4 screw, E88 is 3 screw."

There should be a part number cast into your intake manifold runner. If that number is E46, then it would have originally been paired with 4-screw round tops. And if that number is an E88, then it would have originally been paired with 3-screw round tops. Of course, at this time, who knows what kind of mix-n-match of parts is on it at this point.

Bottom line? For originality, I think you should be running four-screw round tops. But the pair you have pictured have drains on the bowls (which I think is not original). Do the bowls on the installed 3-screw carbs have bowl drains? They should, but if they don't then someone probably swapped the bowls from one pair to the other?

I second the vote for the 4-screw carbs for all the reasons cited by the Captain, that's how my car is equipped. If you go this route and find you do not have the E46 manifold setup, I have a complete pair with the balance tube (stripped) and the connecting coolant tube. PM me if there is an interest.

Thank you Cap'n and jfa, that is useful information. I will look and see what is on the car. There was a 72 in the mix that became a parts donor after a fit of adolescent driving. That date tracks.

My car is pretty close to yours SN wise, 17486. It has the E31 head and 4 screw carbs.

I stopped by the car today and confirmed it does have E46 intake manifolds. That would mean the 4-screw units that are bagged were probably original.

Another question would there be any reason to change the choke cables with a carb change of this sort? The car has a 1972 dash and center console installed. Would one need to change the choke cables for that console? Thanks again and much appreciation for all the help as I navigate a steep learning curve of nuance.

The later model console and the shift rod visible in the pic says the car no longer has its original F4W71A 4-speed transmission. This might explain the choke cable change. Some details: the transmission change moved the shift rod forward, that does not play well with the OE console. The PO elected to change the console to work better with the shift rod and it is likely the OE choke cables mount did not fit well with the later console.

Edited July 17Jul 17 by jfa.series1

@jfa.series1 That makes sense. It does have a FS5W71B close ratio from a 1980 280ZX (according to records.) The previous owner (brother) swapped it from the FS5W71B wide-ratio that the previous-previous owner (father) had installed. In one of the pics you can see the 280ZX shift boot and plate that I replaced with a 240Z unit. i was not aware that the 1971 and 1972 had any real significant change in center console shifter positioning. I do recall the boots are different size so that must account for it. Thanks for that tidbit. I will do some research on choke cables as the ones currently installed are rubbing some of the paint off the inside of the hood.

Edited July 18Jul 18 by DrFeelGood
Added pictures