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Mixture controls

One of my students asked me if anyone still makes a control to allow a driver to remotely adjust the mixture on a carburetted engine, kind of like small aircraft have used for decades. He wants to improve his fuel economy, and figures if he could lean out the mixture (once he’s at cruise speed) he’d get a lot better mileage. I think I saw an aftermarket system like this on the 'net, but now I can’t find it. Anyone out there have any information on this kind of thing?

This falls into the hobby/experimental field,good for understanding how ratios other than the 14.7 stoichiometric affects performance and economy and how different fuels have different stoichiometric ratios.

Do you know the technique used on aircraft carbs.? I don’t

As with all “pojects” I suggest your student work out on paper every aspect of the project (cost,reliability.monitoring,potential benifits)

As a “hobby” a wonderful idea,but I think the idea will fall short in overall cost saving.

What kind of car and carb? Of course, fuel injection already does this. Cars haven’t had this since the '20s. Most modern carbs do a pretty good job of this already. Hard to see how you’d modify a carb to allow remote mixture adjustment. Maybe try it out on a lawnmower engine. But too lean, and POP goes the piston!

This would only be feasible on a car without a 3-way catalytic converter, which needs stoichiometric. As Texases aluded to, older cars had two stage carbs. The primary worked at part throttle and was set slightly lean. Open the throttle wider and the richer secondary kicked in. Wider still and the power valve opened up for more enrichment.

If you’re talking about a plug-in to the the OBD system that lets you fiddle with the fuel injection set points, then you are really playing with fire (sorry!). Here’s a question for your student- if all it took was leaning the mixture on cruise, why wouldn’t the automakers have done it already?

Is someone struggling to find a rocket science solution to such a simple idea? Add a T fitting to the PCV plumbing and install a metering device.

On my Cessna 172 there is also an exhaust manifold temperature guage. If you lean it too much you see the temps rise, then stop leaning just under max. On a car engine I can’t say what that would be and , if too hot, could burn pistons.

Just since nobody else has specifically said it, leaning out the mixture at cruising speeds will result in the engine running less efficiently which will end up if anything reducing gas mileage. It could be an interesting science project, but I agree that a lawnmower or go-kart is a better test bed than a vehicle anyone’s too attached to.

Todays cars have a plethora of sensors and a computer programmed to take those sensor signals and operate the engine as efficiently and as lean as possible while still performing smoothly.

The program processes the crankshaft’s position, the speed of the crank, the manifold’s absolute pressure, the amount of air going into the engine, the temperature of the engine, the driver’s demands, and even checks the output in the exhaust to see if it’s all working properly. It them controls the injector’s spray at each individual cylinder with exactly the amount needed and exactly when it’s needed. It also controls the spark and, in many cars now, even controls the valve timing.

You may want to ask the student if he can do all that homself.

There are some motorcycles that are still made with a manual choke. Perhaps the carb(s) could be adjusted (after the engine is warmed-up) so having the choke pulled would give it a normal air/fuel mixture. Then as the rider reached higher speeds, he could push in the choke to lean out the fuel. This would at least give him a chance to see what it does, and if he did it with an old Harley-Davidson, he could then get to work rebuilding the engine.

Show your student a Honda Civic HX. I don’t think they make them anymore, but I know they were available at least between 1998 and 2000. The Civic HX had a special engine that was designed to run lean. It also had a CVT transmission that got better fuel economy than a manual transmission, but that is a separate issue. You should show your student Honda’s 3-Stage i-VTEC + IMA hybrid engine at . If he can wrap his head around the three stage valve control, he might get some better ideas.

Without knowing what kind of car, engine, and carburetor is being discussed there are 2 ways of experimentation the way I see it.
One is by altering the amount of fuel and this could be pretty tricky.
Two is by introducing additional air beneath the carburetor throttle plates. This would be much simpler.

How about an electric air bleed solenoid controlled by a toggle switch on the dash or by a microswitch on the throttle linkage that would allow the switch to energize and then bypass additional air into the intake automatically whenver the throttle is opened a set amount?

Just an idea to play around with if your student wants to play around some. However, you should be aware that TOO lean at highway speeds could lead to problems; overheating, pinging, or even top end engine damage when aluminum pistons start to give up their lives.

If the car in question is an EEC controlled one then he could be opening up a real can of worms because these cars are already running very lean to begin with.

I think you just described GM’s Varajet electronically-controlled carbs. They have a TPS sensor. That, along with an oxygen sensor, MAP sensor, and coolant temp sensor, all feed a fairly simple computer. The computer adjusts a solenoid on the carburetor, trimming the mixture lean or rich.

I know that electronically controlled carbs have a black cloud hanging over them (pun, yes) but when the Varajet works, it works really well.