Adjusting SU air fuel mixture, just don't get it

[Healey Z]

I've watched the video, read the posts, searched the net and I just don't think I am doing it right.

With car warm and idling, I stick my finger in the SU and raise the plunger and listen to the motor. Frankly, I can turn the nob either way a time or two and it doesn't seem to change at all.

Is there a better way, or what should I be doing different?

[Gary in NJ]

The domes don't tell all, it's just one tool.

First make sure that your floats are at the correct height. Then set the mixture to a reference position (2-1/2 turns). Then sync the carbs with the unisync. From there you will fine-tune the mixture knobs to achieve the best mixture. This can be done by raising the domes, however the best test is to take the car for a drive a judge the throttle response. A good plug reading should follow. After you have a setting that you like, do a final carb sync.

[Zedyone_kenobi]

With the engine at idle what you are looking for is the highest possible idle you can achieve. Before you can do that though, you must balance the amount of air going through the carbs.

If you have a unisyn or similar tool, get the air balance just right by adjusting the idle flapper angle adjustment screw on each carb until the flow through each carb is the same.

What I do is hook up a digital timing light that has an RPM readout. Then I start with both screws two full turns from closed. I do everything in pairs. Whatever I do to one carb, i do to the second. I turn it one time to the lean direction on each carb and wait a few seconds to see what the rpm reads. If it goes up you have moved in the right direction. This is a coarse method. One turn out or one turn in should produce a noticeable effect on your digital read out. The factory tach is just not a fine enough instrument for this in my experience. Continue to zero in on the mixture ratio until you can not get the car to idle any faster.

Picking up the plunger is explained in the video. I think if you raise it and it does not stumble it is running too rich. IT should stumble a bit when you pick up the plunger.

Others with more experience with this are bound to chime in!

[Diseazd]

.........then balance the front to rear linkage at 3,000 rpm with the uni-syn.

[Healey Z]

Thanks for the responses. I did get a uni-syn and have the carbs well balanced at idle and 2500rpm.

I'll give this a shot tonight

1. hook up my tach (analog though)

2. screw the mixture knobs all the way in, then back off two turns.

3. adjust looking for fastest idle

I can say the car runs pretty darn good right now, but I get the occasional stumble/flutter and it drives me nuts. My car senses tell me it is fuel. Hopefully that will solve the problem and I won't have to open up the floats.

[240260280z]

This vid shows you how it works...and he hits a limit and inbalance:

Here is my fav: http://www.classiczcars.com/forums/showthread.php?41940-Quick-and-dirty-SU-Tuning

[Healey Z]

I had driven the Datsun Healey to work today and was looking forward to tinkering with it when I got home. I kissed the wife, changed my shirt, grabbed some duct tape and headed back to the garage. I popped the bonnet (us English car owners need to call it something different than a plain 'ol hood) closed the SU's all the way up, put a piece of tape on the knob and then spun them down two turns. My analog tach, circa 1980 was no better than my dash and my ear and frankly I couldn't tell if the idle was going up a tiny bit or down a tiny bit with a 1/2 turn.

I decided to turn both of them 2 1/2 turns, close the bonnet and took the car for a spin. The stumble is gone, the flutter is history. Thanks to all for the advice and I'm happy with the way it is running...project complete!

Edited October 20, 2011 by Healey Z

[Bruce Palmer]

I would venture a guess that a lot of our carbs would run fine at 2 1/2 turns down for ever.....

All tweaks should be followed by a road test..... Hard to omprove on the "seat of the pants dyno"...

[Diseazd]

I had driven the Datsun Healey to work today and was looking forward to tinkering with it when I got home. I kissed the wife, changed my shirt, grabbed some duct tape and headed back to the garage. I popped the bonnet (us English car owners need to call it something different than a plain 'ol hood) closed the SU's all the way up, put a piece of tape on the knob and then spun them down two turns. My analog tach, circa 1980 was no better than my dash and my ear and frankly I couldn't tell if the idle was going up a tiny bit or down a tiny bit with a 1/2 turn.

I decided to turn both of them 2 1/2 turns, close the bonnet and took the car for a spin. The stumble is gone, the flutter is history. Thanks to all for the advice and I'm happy with the way it is running...project complete!

Don't forget to check and see if your choke mechanism is hanging up after using your chokes to start the car. This is very common and the car will stumble if one carb or the other fails to pop back in. If it stumbles, reach your hand under to where the adjustment wheel is and push up. If it's stuck down, you'll hear a distinct snap when it pops back in. This could have been your problem in the first place and you fixed it by accident.

[Duffman]

Bruce, if it requires 4 to 5 turns down to run the best, where would you focus your attention? I have adjusted the float level multiple times and still am looking at 4 to 5 turns down for both carbs to run the best. Plugs look light brown. Crazy ...

[Gary in NJ]

Bruce, if it requires 4 to 5 turns down to run the best, where would you focus your attention? I have adjusted the float level multiple times and still am looking at 4 to 5 turns down for both carbs to run the best. Plugs look light brown. Crazy ...

Some questions:

1) Are you sure that you are adjusting the float height correctly?

2) What needle(s) are you using?

3) Are you using the stock/OEM intake?

4) Any (real) modifications to the engine?

5) How does the car run?

[Duffman]

Gary, here are answers:

1. Yes, have reviewed the Ztherapy video multiple times.

2. I believe standard-sized needles, carbs are borrowed from friend

3. Yes, standard stock intake

4. No real mods, except electronic ignition

5. Car runs well when adjusted at 4 to 5 turns down