I am trying to balance the carbs on my '71. I warmed up the engine.  Then I set the jets so they were flush to the bottom of the carbs and turned them both so they were flat with the bottom of the carbs. I backed off the idle on each carb so the adjustment knob was not touching the linkage.  I also loosened the linkage so it wouldn't interfere with each carb and they could respond independently.  The car has about 62k miles on it and it runs and drives nice.

When I have the car running and look in the  throat of the carbs, the front piston is much higher than the rear carb. Maybe by 1/2 inch. When I install a syncrometer to the front carb, the air draw is off the chart.  There is no way I can turn the idle back because it is as far as it will go now.  I can get the rear carb to adjust to anything I want.

The piston is not stuck.  When I shut the engine off, both drop down to where they should be.

Any ideas from anyone would be appreciated.

You need to check your float levels and make sure the float valves are working properly. Everything is dependent on fuel level

 Did you interchange any of the parts (pistons, suction chambers, etc.) from one carb to the other?

Thanks for the help everyone.  I will open the float bowls and take a look.

No I assumed not mixing the parts between the carbs was important. I made sure I did not mix & match anything between the carbs but as I write this, I cant say someone else didn't a few years ago.  Maybe I will try switching the pistons and see what happens (keeping track of which parts belong to which carb).

I just noticed the insulators are mounted upside down.  There is a pair of holes at the bottom of them that are now at the top.  Cooling perhaps?  Maybe that is allowing airflow where they shouldnt?

Google "quick and dirty su tuning classiczcars.com"

Those holes are on bottom on my '72. That very well could be a problem.

The trick I learned from quick and dirty is get the front and rear running equally shitty separately as Phillip so brilliantly describes. He's a wordsmith, explaining methods my dog could understand if he could read and I'm working on that. 

 

Thanks Siteunseen.  That post is very helpful.  In my case, my idle adjustment screws are completely backed off so the linkage does not touch them.  The front carb still draws much more air than the rear eve though the idle is backed off.  Valves are adjusted and plugs are good.

The air screws are #10 on the diagram.  Also shown in the last 2 pictures of the first post in this thread.  Those are the ones I turn to balance my air flow.

 

I don't think you should be able to get the car to even run if the linkage is all the way pulled off. I think if the adjustment screws are all backed out so far that they aren't making contact, then the butterflies should be shut so much that the engine can't even get enough air to idle.

I'm wondering if something is put together wrong, or if someone messed with the "never adjust these" stop screws on the carbs.

Can you take a couple decent res pics of the linkage?

Captain Obvious brings up a very good point.  I was just recently helping a fellow board member tune/balance his carbs ('72 style carbs) and one of the issues he was having was that even with the idle screws completely backed out, his car was still idling hi (1000-1200 rpm).  We discovered that his throttle linkage was "on" even when the throttle wasn't being used.  In other words, his throttle linkage wasn't returning back "off" fully.  We popped off the throttle linkage (the little plastic cap) and suddenly the throttle plates closed completely, the rpms dropped way down (to where the engine would barely idle) and we were able to begin using the idle adjust screws to begin balancing and setting up the carbs.  I think he was able to remedy the problem by adjusting the throttle where it goes through the firewall and connects to the gas pedal.  It might be something to check out as it does sound like your throttle plates are partially open even at idle.

Some older SUs have a fast idle issue when the butterfly shaft bearings are worn and allow air to enter.