UPDATE: I finally got my SUs dialed in. As you might recall, my fuel/air mixture was too to lean at highway speeds, but Idle and WOT were fine. Here’s what I found. 1) One of the float valves was occasionally sticking in the closed position. 2) One of the fuel lines had a flap of gouged-out rubber partiality blocking fuel flow. 3) As I was constantly adjusting the mixture trying to get it right, I ended up with one carb adjusted way too lean and the other way too rich. At idle and WOT, I’m guessing the exhaust balanced out and sent a good average air/fuel number to my O2 meter, making it difficult to figure out what was actually going on at mid range.

I fixed with the float valve and got it working properly, replaced the damaged fuel line, and balanced the fuel/air mixture on the carbs. After a little fiddling with the mixture and syncing the air flow between the carbs, the O2 meter is now where I want it: 13.5 to 14.0 at idle, 12.0 to 13.0 at WOT and 14.5 to 17.0 at cruising. That’s a little leaner at cruising speeds than it was. The spark plugs bare that out. When I replaced the head gasket a couple of years ago, I was surprised to see how much carbon was built up on the head and pistons. I’m thinking a slightly leaner midrange mixture might keep the carbon in check, not to mention improve my milage a bit. Thanks for the help.  

When was the last time you calibrated the lambda sensor ?

Yes, to install new throttle spindles & bushes you will need to line ream them or it's a pointless exercise.

Good suggestions, all. When I noticed my O2 gauge was showing an overly lean condition, I replaced the O2 sensor and it read the same; too lean at highway speeds. Damper oil and springs are all in good order and I'm using the same fuel I've used for years. After richening the mixture in an attempt to compensate for the overly lean condition, my plugs look a bit sooty when I return home from a drive—a result of idling into my neighborhood and driveway, I'm guessing. Yesterday I pulled off the freeway and immediately checked the plugs. The plugs tell the tale; way too white, way too lean.

I was about to use carb cleaner and compressed air on the jets to blow out a possible obstruction as someone suggested. But before I did, I sprayed WD40 and brake cleaner in and around all likely places and got a slight stumble when I sprayed around the throttle shafts. I'm thinking vacuum leak now. To confirm, I bought a can of starter fluid today and am hoping to notice a dramatic change in RPM when I spray the shafts. 

Craig: Are you saying that I'll need to ream the carbs to accept new shafts? I don't have any experience with precision reaming anything requiring an air-tight seal. I was hoping I could just install a rebuild kit (see pic) and call it good. No?

I may have missed it, but did anyone mention that throttle shaft wear can give you those same issues? The throttle shaft and the body of the carb will wear on eachother over time and allow excess air to leak past the throttle plate leaning out the air/fuel mixture. Generally, you can still tune the a/f at idle if the wear is small, but it will most likely show at near steady state throttle positions other than idle as a lean mixture. Easiest way to check is let the engine idle and spray carb cleaner where the throttle shaft comes out of the carb on each side. If the idle stumbles or increases, you most likely have an air leak there. Remove and refurbish or buy new or find a good one somewhere in your garage and replace. I say, "refurbish" because the carb must be reamed and bushed for a new throttle shaft to repair the worn out carb.

Hope this helps. Others will most likely have a different opinion...

Long shot...

Is the linkage between the two carbs tight and properly adjusted? Some slop may allow one of them to open at a different rate, though they'd be under (more-or-less) equal engine vacuum at a given rpm. Maybe one of them is leaning out and you might not notice a difference at the plugs because of the cross-over pipe of the manifold and/or since the lean condition is at a certain range the other ranges "cover" the indication at the plugs (a bit like plugs getting sooty if the idle mixture is rich and you let it idle too long).

If your carbs are HS4's with the spring loaded needles then they will wear over time. The needle can wear a bit flat on one side usually for the first 3/4" or so. The jet will wear a bit oval. Sometimes it's hard to see. As other's have said this probably isn't your problem since you are getting too much air or too little fuel at part throttle (high vacuum under load). It's possible that you have a vacuum leak that only causes a problem at part throttle since the intake vacuum is at it's highest. 
The first thing I always do with SUs it pull the suction chamber and piston and make sure everything is clean and moving properly.

Kelley

Are all four spark plugs the same colour? Could it be one carb affecting the A/F mixture?

One esoteric possibility to consider: have you changed where you buy your fuel? Or has the supplier/state changed their fuel mix regulations?

E10 is oxygenated fuel - therefore in the case of SU carbs a richer needle is often called for to utilize the extra oxygen and prevent a lean condition. If one tuned it just right for E10 then switched to alcohol-free fuel, then the reverse might take place (too-rich fuel mixture.)

It's a long shot since it seems most places in North America have long since switched to E10 fuel mixtures, but I'm not familiar with every state and county in the USA let alone North America, so I think it could be a possibility. Easy to rule out if not, anyway.

I'm guessing you checked and topped up the carb damper oil as first course of action? If not the carb piston will rise more rapidly causing mixture to lean out.

As mentioned by others a worn needle or jet would give a rich fuel mix which is the opposite to symptoms shown by your engine.

If you're pretty convinced the manifold doesn't have any vac leaks check the throttle shafts, though I wouldn't think they could be worn enough to matter after that sort of milage. The needle wearing itself smaller or jet wearing itself larger would both cause a richer running condition. Make sure the carb pistons aren't hanging up anywhere and travel up and down smoothly. Try blasting some carb cleaner through a straw directly into the jet, that'll blow anything that might be stuck in it back into the float chamber. Float levels and fuel pressure could also be playing a part. 

Have not heard of needles wearing out ...and mine have done 100K miles+ The jet does wear and need replacing ...every 30 or 40 years (rolling eyes)...

However, throttle shafts do wear and would allow more air into the engine. This usually upsets idle settings and I wouldn't think you could enough wear to affect cruise/acceleration settings...

Cheers, Ian

Sounds like the same thing I am starting to troubleshoot. A/F is great when on the gas, when just cruising it slowly leans out to 16-17:1 with a slight stumble. HIF38, changing to a stiffer yellow spring did not seen to help. Will be following for tips.

I was involved in a lengthy discussion here on this topic 15+ years ago. Hope I have the terms right.

the carb piston damper (with the oil) slows the rise of the carb piston and the airway opening (the variable venturi) on pressing the accelerator pedal that is connected to the throttle butterfly downstream (engine side) of the carb variable venturi area. This allows momentary enrichment, needed for acceleration. (A) Not enough or too thin oil would allow the piston to fly up too fast, momentarily allow too much air, kill the vacuum, bog the engine by running lean. (B) The tapered needle is attached to the piston and also rises. The thinner diameter of the risen end of the needle would allow more gas through the jet orifice, but not if the car is bogging down from too fast (A) . He said WOT 12:1 and never mentioned bogging so that does not sound like a carb OIL issue.

The carb piston SPRING helps control the height of the carb piston at cruise /steady rpms and also smooths out fluctuations. Conceivably, a weakened spring would allow the piston to be higher and thus more airflow. However, the tapered needle is attached to the piston and as it (they) rise together the thinner portion of the needle would allow more gas through the orifice to match the increased airflow. These two effects operate in lockstep WHEN perfectly matched which they were at one time.

I do not know if carb springs weaken or even break over time, but with today's supply chain, anything is possible. If you do not find another answer, I would experiment. They make standard and higher strength springs, or at least formerly did. They are easy to change. I know little of your big engine but I did read that the higher strength spring may be indicated in that setting. If so, it would have been needed all along, so why now?

Clear as mud.

On an SU the action of piston damper richens the mixture when you put the pedal to the metal. I forget what actually happens, maybe somebody who remembers can provide the explanation that escape me. That's why your mixture reading goes rich at WOT. 

a worn needle would be smaller diameter and thus allow MORE fuel to flow through the jet orifice .

maybe the carb oil needs topping off

maybe the carb piston spring has weakened (although that would make it run richer)

maybe the orifice has deposits (but that would lean out AF ratio everywhere - idle, cruise and WOT)

I’ve got 40k miles on my Seven Enterprises built 1380 A+ motor. Since new, my A/F ratio was always in the 14:1 range on the highway and my plugs were a nice chocolate brown. Lately I’ve noticed that my A/F mixture is running overly lean at highway speeds. My wide-band O2 gauge now shows 16:1 to 18:1 on the highway and my plugs are overly white. When I go WOT the A/F ratio drops to a reasonable 12:1, but leans out again when I back off.

I’ve been richening the SUs in an attempt to bring down the A/F mixture, but then I run too rich at idle and lower RPMs. Nothing else has changed. The air cleaner has been serviced, my timing is spot on and my plugs are new. I have no vacuum leaks (that I know of).

It has occurred to me that perhaps the needle profiles have changed over time and need replacing. Is that possible? Any thoughts?

Twin SU® HS4 1 1⁄2” carburetors
Fast road cam
10:1 compression ratio
200+ psi in all cylinders