Here is something interesting, I am going to assume its maybe vacuum leak related, but here goes.

Basically either the car will idle so low it wants to die (especially when coming to a stop), or if just a minor twich of the choke and it goes from ~500RPM idle to nearly 1200-1500 RPM idle.

If I give it a small tap on the gas at the light when its idling low, it will hang around the 1500 RPM mark once again, and if/when it drops its usually pretty slow. Power is pretty good on the street, but I am still suspicious of vacuum leak or maybe incorrect jetting. These are dual Weber downdrafts (32/36), and am assuming the jets are unchanged from the donor setup but am not positive.

Now, here is further mystery for me, a non carb person and new to Z land (Haynes manual on the way), the air filters have a barbed plastic connector at the bottom, as if a line goes to them for some purpose, wether EGR or what I am not sure, but they are unhooked at the moment so basically letting in a small amount of unfiltered air.

I can snap a pic if this helps, but would you assume this is for EGR and should be plugged up? Or is this for something else that I'm not finding in the harness?

73 240Z, P30 engine (verified on block), idle screws are backed out about 2 1/4 turns

Another 1/4 turn rich helped greatly, but still idle RPM is not always consistent from one stop to the next, occasionally it wants to die, other times a nice "1k RPM" idle (according to the tach).

I'll check Napa for a color-tune.

I was experiencing the same stuff with the carbs on my '73. I went to Z Therapy and we established that the nozzles and needles needed replacing. I put in the SM needles from Ztherapy and it was the best $60 I've ever spent on that car...

The color tune is going to do you no good in this sitch...however it is a good tool to have around. With an idle that comes and goes you have a vary in the amount of fuel that is introduced into the chamber. 2 1/2 turns should run the car. I would bet that you have worn nozzles.

You can research this at Ztherapy.com, Steve is very helpful and has been working on these carbs for years, I highly recommed that you check them out.

Feel free to call me, the symptoms that you have are close to the ones that I had...I know it can be very frustrating...:stupid:

E-mail me in the event you want my number.....

Good luck,

~Brian

Someone else suggested a possible worn linkage, allowing leakage.

I wouldn't doubt these could use a freshening up though, so I will probably tear them down hopefully this weekend and find out whats going on, as far as even what jets etc it has (I'd assume whatever it came off of donor wise, a 260Z perhaps?).

I am having similar problems on my car (74 260, 72 carbs), when she is cold she won't run hardly at all un less the choke is on. Am i just not letting the car warm up long enough? or is there something about the cold weather that makes one need to make anadjustment on the carbs? I am clueless when it comes to carbs, thanks,

Zlish

Hey Zlish,

If a SU carb is set up correctly, it will NOT start when cold without using the choke. Some days when its a warm summer day you might get lucky. For those of you that can fire up the ol' Z without choking the beast (now this is SU's mind you...) you are running rich.

And, pumping these carbs in the morning is a waste of time because they have NO accelerator pump. I had to break the habit being a Chevy guy also.....

So, Zlish, I would have to say that choking is normal, mine in the '73 take a while to be "happy", usually not until it's at at least 120 degrees.

Hope it helps....

~Brian

When the carb is cold, fuel doesn't want to atomize, so the choke makes the carb run richer to help make up for that.

Mine most definately wont start without the choke, unless I stick my foot in it to get it doing 1500-2000 RPM, and even then without the choke it would probably take me several restarts to get it running constantly.

Thanks guys, kinda figured i was just not letting her warm up long enough. I wish i could go back to high school and take that auto class i never had time for because of sports. Just wasn't into cars until i turned about 38 (last year), now i can't get enough and the learning curve is slow when you are not mechanically inclined! I find myself doing alot of trial and ERROR, emphasis on error, but it sure is fun!

See ya,

Zlish

You're just lucky it's not fuel injected!!!:classic:

If there were an easy and reasonably inexpensive way to log and re-program the ECUs I'd go FI in a heartbeat...

Oh well

easy and inexpensive

Doesn't exist with ECU.

Unless you pick the systems up at a garage sale!!!

Bri

Yep, I will stick carb for now, but did consider a Megasquirt box ~$100, but then need the injectors from a later Z and then a fuel pump and then....