The Cessna 120-140 Association Forum

For all you engine guys out there, have a Stromberg carb question. I decided to connect up my mixture control (like most, was wired full rich). With full movement from full rich to full lean ... absolutely nothing happens, no effect at idle or at full power. I expected a sudden full lean at full power for the engine to stumble. Engine acts like it is still wired full rich. Took the mixture cover off to look inside, both mixture plates are present and the upper moves with the arm. Any ideas what's going on? What do I look for next?
Thanks in advance, Jim

The mixture will not kill the engine at full lean. If I remember correctly, you have to be above a certain altitude for the mixture to have any effect.

I re-read Neil's Stromberg article on how the mixture control works. If the chamber is sealed correctly at idle there should be no effect, which is precisely what happened, no change. When I pulled to full lean at full power I did so at run-up, about 1200' density alt. I think that's my problem, I'll try it again when I'm above 5000' and see what happens. Sometimes it's best to talk things out to help get it straight in my head. Getting old is a pain but beats the alternative! Thanks, Jim

It is normal that it has no effect at idle, however, at full throttle I would expect it to kill the engine. In fact, the effect is reduced at altitude.

Fuel is delivered due to a higher pressure in the float bowl than in the carb throat where the jet is. The mixture control bleeds pressure from the float bowl into the throat and reduces the pressure differential.

I'm guessing that some of the passages involved are clogged.

I agree with John. When I installed mine it had a definite effect at full throttle but it took about 5seconds to occur. Time can go slow when the plane is straining against a full throttle!

Be careful once you are flying with it. I find if I lean it too much, any increase in throttle setting can cause a stumbled.

Mine is connected, and it works, it won’t kill the engine like normal aircraft and has been stated only is really effective at higher power settings.
However I don’t believe it should be used, the possible pennies saved from fuel is way offset by the possible damage of running an engine too lean.
Most engines and I assume a C-85 is no different, you can do as you please with the mixture as long as they are below about 65% or so power, above 75% though and you can destroy one with the mixture.
I don’t know how to determine % power on a C-85, but I do know that if left full rich it doesn’t seem to foul spark plugs and doesn’t get too hot from being too lean.
I’ve tried a few times up at 10,000’ or so to lean it for max RPM, but have never gotten it to increase RPM by leaning, making me believe that it’s not too rich, it seems as if somehow or another it’s an altitude compensating carburetor, I know it’s not, but it seems to function like one.

Oh, mine has a definitive effect at sea level too at normal cruise RPM, say 2200 or above, I’m not sure what it does at run up RPM

It is very good at maintaining the mixture at altitude. I recall the Continental operators manual specifically states not to lean below 5000 feet and then, not unless you can achieve an increase in RPM. Power output decreases with altitude anyhow, so the higher you go the less likely it is you can do any damage by leaning.

How to kill the engine with the mixture control: at idle, pull the mixture control all the way out then smartly firewall the throttle.

Most or many aircraft recommend against leaning below 5,000. I just bought a J model Mooney, and guess what it recommends against leaning below 5,000.
Although that’s not really correct, it’s percent power and not altitude that you can lean, so get down to low power and lean away. I got the Mooney down to 6 GPH at 120 kts, leaning the snot out of it, 20” manifold pressure and 2200 RPM

Having said that of course power goes away with altitude in a NA aircraft, at 5,000 ft, you have lost 5” of manifold pressure, but to get to 75% where Lycoming anyway says you can safely run peak EGT means 7,500 ft if you keep RPM up. I can’t see how Continental would much different

But my 140 doesn’t have fuel flow or EGT, and it doesn’t need if, it being simple is a lot of its attraction, no mixture to worry about, no prop to worry about, no cowl flaps, boost pump and of course no wheels up landings.

So I leave the mixture alone as I don’t believe there is anything to be gained from messing with it.