I'm just kinda pulling an idea out my **** and I'm not too sure how it would work but I figured would ask and see.

on a carbed car that has a vaccum line below the carb itself if you could run a vacuum line into the dash and put a variable air bleed on it to be used when doing highway driving could something like this possibly help out with fuel economy?

what I'm thinking it would do is when doing highway driving you usually aren't under heavy load and so might be able to open the air bleed up a little bit to creat somewhat of a vacuum leak. this should reduce the suction on the main venturi so less fuel should flow.
granted for city driving it would be a nono but under steady hwy drives might it help?

isn't this something like what the hwy mode on the EFI thirdgens do? well in a way I guess they prolly don't do it by creating a vacuum leak. :-p

 

EFI doesn't do it via a controlled vacuum leak, but the end result is essentially the same.

If you set up this "mixture control" system, be very careful about head temperaures. Running extremely lean for extended periods can cause severe damage. A functioning EGR system can offset some of that.

This would be done best with some feedback, as in something from a wideband O² sensor and meter/display so you could monitor mixture, but if you know the risks, it's your call. The EFI systems with Highway Mode invoked still monitor the O² and maintain a target EGO, but it is toward a ratio closer to 17:1 than the typical 14.5:1 in normal cruise mode.

NOTE: Many carburetors have just such a mixture control system. That silver lever in the photo is specifically for that purpose on this Holley:

 

On further thought, if you try this alteration, you might want to use the control valve to admit crankcase air to the intake (like the PCV would do). This air is already warmed (which will aid in fuel atomization), is already filtered, and contains blowby gasses that are combustible but will also have some limiting effect on combustion temperatures, somewhat like the EGR system. As for the valve you might use to control this, a '50s vintage vacuum control as in the type that was used for windshield wipers might be effective. They were designed to maintain fairly fine control over the flow of air. As a safety, you may want to install some sort of blocking valve that closes on low coolant temperatures and when load conditions are high enough to demand a rich mixture.

If you're serious, you can probably devise a system with a TVS and small solenoid valve piloting a larger valve, since a typical TVS wouldn't flow enough air to affect mixture at cruise by very much. That larger valve could be the type used in Air Injection systems. Those used on many GM vehicles are vacuum piloted and would flow more than enough air (vacuum) to affect mixture.

 

the only main concern on is if I rememer right the car already runs close to 16-17:1 under cruise conditions from what I understand.
also which hose would be better to run it under the ported or manifold vacuum? I would almost think ported would be the better option being it should allow a lot more controll over what happens with smaller amounts of air.

with running this lean under cruise though with 8:1 compression could it really cause that much of a problem? understood under high load yeah I could easily screw things up quick i am just thinking under low load it might not have that big of an effect as far as problems but I might be mistaken and from what you say it sounds like I might be.


also what do you mean by your last post with a TVS? what is that part? might I also ask for a little better explanation as to what you are talking about. it does sound like a little better method then what Iwas going to try. that was to run a manual valve inside the cabin and open the vlave up a little bit during cruise speeds but with how there are some hills around here I don't know how well that would work. the other problem I have is I wouldn't even know what to tell the guys at the autoparts store the valve thing I am looking for

 

A TVS (temperature vacuum switch) would make the system just a little more automatic. IT could be installed to prevent lean mixture until the engine is warmed.

If you are planning to just try it to test viability, a simple ball valve or gascock at the end of a vacuum hose in the passenger compartment would work. Once you establish whether it is effective, you could refine the system with the more automatic controls.