first attempt tuning SU's, could use tips(see video)

[RJK]

Hi folks-so to give some background, I've read up and watched videos of people tuning 2x SU carbs on a 240z, and had my first attempt at it today. Have a bit of confusion, and would love some tips. Background:

 

-'72 240z, stock SU carbs(to my knowledge?). Were rebuilt about 2-3 years ago and balanced at the time. Fuel pressure regulator in line pre-carbs. Cannot verify actual PSI, have not tested with gauge yet.

-Over the last two years, the car has run progressively worse. Last summer a changed out the spark plugs, and it temporarily helped. Recently I pulled all the plugs, and 1-3 looked clean, but 4-6 all were black at the tips. I am assuming the "bumper side" carb is running richer. Last month or so, the car would have an extremely lumpy idle. If I got on the throttle at all while starting, it would flood out and I'd have to wait to get it to start. Only starts with ZERO throttle, zero choke, and then I have to start it 5 times and massage it and give it flowers.

 

As I have read it, the mixture is controlled by a "nut" on the bottom of each carb. See photos; I can't feel a "nut", but there is a "dial" that I can turn by hand. I'm touching it in the _6813 photo. I got the car to idle today, and put an airflow meter on it. firewall side carb read 10, bumper side read 16. Turning this dial on the bumper side carb had no effect on the idle speed, or the airflow. by adjusting what I understand to be the idle screw( touching this in _6812), i got the airflow meter down to 10, so they were pretty close. the car held an idle, and seemed to run smoother, but I still don't think I'm adjusting the mixture, and if I am, it is probably completely out of whack, as I probably ended up turning that "dial" a solid half turn (counterclockwise from top).

 

while the throttle is responding closer to normal, as you can see in the video, I was still able to get it to stall out from a quick burst of throttle(which I have been constantly on guard from when driving the car). Also, what I assume is a water vapor or fuel overflow tube dribbled a bit of liquid from the firewall side carb(again, see video).

 

Would love some pointers here. Thanks a bunch folks!

[Mark Maras]

It appears you've got the carbs balanced. The mixture screws at the bottom of the carbs are the secondary mixture adjustment. The primary mixture adjustments are the float levels. I'd check those next. They don't have to be exact, just close.

[Patcon]

Yes, what Mark said.

Float levels are super important on SU carbs! And yes the dial on the bottom adjusts the mixture by lowering or raising the the nozzle (the black stem connected to the fuel line). Once you have them balanced at idle. You need to raise the idle to 1800 or so and adjust them at fast idle.

Also there shouldn't be fuel coming out of the float bowl overflows. If there is, that's a problem. Too much fuel in the bowl will make it run rich

Maybe @siteunseen will find you a good thread on carb adjustment. He's the local librarian! 😉

[Jeff G 78]

If the carbs are '72 3-screws, they will have different float heights between the two carbs, though the fuel height will be the same.  There are a few ways of measuring.  I posted some pics years ago on here where I cut clear water bottles and marked the proper heights on the plastic.  I then set the top half of the float bowl onto the plastic so I could tell exactly when the floats closed the inlets.  I found that to work better than trying to measure the float height with the top half inverted.

I'll see if I can find my post later tonight and edit this post with the link.  The fuel level tool is ideal, but they are unobtanium, so your choices are to measure the float, or measure the fuel level with a clear plastic bottle.

[siteunseen]

I learned the "quick and dirty" adjustment way. It was right up my shady tree mechanical way of thinking. Once I read it a few times then did a couple, all that carb talk made sense.

And to @Jeff G 78 height difference in the '72 carbs I robbed float lid off a spare set I had and made both lids rear carb float level. The rears have shorter ears to compensate for acceleration. Going up a steep mountain one time I the front carb ran low and I was only running on the back carb, cylinders 4, 5 and 6. I had a 3 cylinder 240Z for a minute. 

 

Edited October 19, 2023 by siteunseen