Can anyone tell me what is going on with my carb setup? - Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z
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post #1 of 8 (permalink) Old 11-20-2019, 12:53 AM Thread Starter
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Can anyone tell me what is going on with my carb setup?

I am the second owner of a '73 Z. It has been running good but there's been some fuel cutout at speed. I started to do a full tune up, and when I pulled my plugs I noticed that one carb bank (the front 3 cylinders) was running rich while the other was running lean. So I went to adjust my throttle opener adjusting screws and found that it was zip tied to the throttle shaft. Why? This is the front carb fyi. So I went to researching su carbs and possible mods, only to become more and more confused by my current carburetor configuration. The more I look at it the more different from anyting I found it becomes. For example the EGR control valve doesn't look like a block off plate or the stock EGR control valve. I do know that he did an engine bay clean up, and got rid of the 73 square body carbs. Can anyone tell me what mod/mods the previous owner did that would explain this?
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post #2 of 8 (permalink) Old 11-21-2019, 04:34 PM
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I'm not a carb guy, butt my general rule for situations like this is to return everything to stock configuration and work from there. If you're dealing with someone else's mods you have no idea where you are.

Since those aren't the original carbs it's a bit different, but those carbs have a stock setup. I'd go to that and then figure out of changes are needed.
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post #3 of 8 (permalink) Old 11-22-2019, 06:02 PM
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I would say it's time to learn how to adjust the the early 4 screw carbs you have there. They are not difficult to adjust and there are plenty of guides online. ztherapy is a good place to begain if you need an expert.

12/70 build 240Z
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post #4 of 8 (permalink) Old 11-23-2019, 06:16 AM
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For fuel efficiency on multi carb setups, butterfly's open sequentially. For power, all butterfly's should be synched to open together.
Ergo, over time the plugs show rich on front cylinders. Change plugs, run 100 miles, and see how they look. Decide from there if balance needs to change. Beyond that, ID your carbs, buy balancers, learn how to adjust air/fuel mixture and synch and idles.
I am confused as to why your Z would have British carbs on it.

As for the power issue, check your fuel pump and lines, and rebuild the carbs. It sounds like it could be a fuel loss issue. SU carbs do require regular maintenance due to deposit build up. Granted, most people wouldn't think of ten years as a maintenance cycle.
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post #5 of 8 (permalink) Old 11-23-2019, 12:06 PM
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Nissan/Datsun used a Skinners Union carb setup on Zcars from 69 to 74 before switching to EFI in 75 (Bocsh L Jetronic). These can't be sequential as each carb feeds 3 cylinders. The need to be balanced. It sounds like you know about old pontiac ford and chrysler multi carbs which are bad a$$ themselves.

1975 280Z F54/N47 MSA 6-1 Header 2.5 Exhaust
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post #6 of 8 (permalink) Old 11-23-2019, 02:39 PM
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post #7 of 8 (permalink) Old 11-27-2019, 09:21 PM
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Looks like you have a '73/'74 least the cross pipe is...with earlier '70/'71 4 screw carbs....
part of your adjustment problem looks like its also the later style linkage...
one option would be to replace the linkage with an earlier version....
Its been awhile since I played with one of those mix matched ones....
another option would be to adjust the other screw to the left in the picture...the little one behind the carb dome screw.... the mixed linkage makes things a bit screwy, as the later style is for the flat top carbs which operate with one carb essentially "slaved" to the other for mixture control, etc....

Last edited by Eze240; 11-27-2019 at 09:38 PM.
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post #8 of 8 (permalink) Old Today, 01:23 PM
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The linkage that is tie-wrapped is part of the deceleration device mechanics. HOWEVER your decel device has been removed. (See the hole at the back of the linkage). That’s ok but I think what I would do is cut the tie wrap off and remove that front spring covered screw or at least back it off so it doesn’t contact the front carb linkage at all. The screw to the RIGHT of that screw (closer to the rear carb) and also spring loaded is the screw that controls the BALANCE between the front and rear. Generally speaking, using a airflow meter you set the front carb speed, and the rear carb speed separately and with the rear spring loaded balance screw kind of loose. Then you run the speed up to around 2500 rpm and balance the two together using the rear spring loaded balance screw.
In other words remove the tie wrap connection that has been made between the two carbs so that that piece of linkage is free floating and not influencing either carb. Hope that makes sense.

John Hull
1972 240Z Original Owner
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