Mixture knob and fouled plugs....

Chrisj13

Chrisj13

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Chrisj13
The lower plugs on our Cessna 172 (lycoming) keep fouling, despite an aggressive lean on the ground and even in the air. It seems like the mixture knob does little to reduce the mixture, then all of a sudden with just a half twist more the engine dies. The best analogy I can think of is that it acts more like an "on/off" switch than a "dimmer" switch. Carb engine...

Does anybody know if it's possible to readjust the mixture cable? Do they stretch or otherwise lose their "positioning" so that the band of adjustment is so narrow the engine will just die instead of slowly reduce rpm? Obviously the engine is getting too much fuel to foul the plugs (always the lower plugs, and they are fouled after just one 30 minute flight....despite being just cleaned.)

Thanks for any help/advice!
 
Are you sure they are lead fouled? Could it be oil fouling?
 
Sounds as is the cable is flexing due to a loose or missing bracket. If the sheath isn't secure, the cable (wire rod) will bend/flex rather than moving through the sheath to move the mixture lever.


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What model 172? There are carbureted and fuel injected lycomings in them.
 
Chrisj13 said:
Carb.
Click to expand...

There is an idle mixture setting on the carb, might be worth having a mechanic look at it. It should be pretty obvious what's going on when cleaning the plugs, evidence such as the shorted one is packed full of oily gunk vs relatively dry deposits. You could try running a fuel additive called TCP which will reduce deposited caused by 100LL but it shouldn't affect oil flooding issues.
 
Sounds like it's running too rich. You have 2 solid clues. First, plug fouling can be caused by a very rich mixture. Second, when pulling the mixture at idle, RPMs should increase, then decrease and possibly run rough, then die. If RPMs just increase then it dies, without running rough or decrease in RPM, the idle mixture is set too rich. Turn it 1/2 a turn at a time and pull the mixture knob at idle slowly, looking for around 50 - 75 RPM rise, then decrease and rough idle, then it dies.
 
Here's a carb lesson. The round brass thingy shown above the carb is the mixture valve. It inserts into the round tube you can see in the bottom and that tube feeds fuel to your main jet (the 45* brass pipe) and meters fuel by twisting the slot opening's alignment with the main jet from full open to full closed or any position in between. It's very unlikely that your mixture will go from full rich to cutoff instantly. To view the travel of the carb's mixture arm is as simple as removing the cowl and watching it while somebody operates the panel control. If you can pull it to cutoff? It probably isn't the cable causing your problem.

At taxi power settings you can't overheat the engine with mixture. At low power it'll simply quit if you over lean it. Don't be afraid to pull the mixture right to the point of quitting during ground ops.

For guys who care, the up pointing orifice in the carb throat is the accelerator pump nozzle. A plunger fits into the cavity seen at lower left and when pushed down it shoots fuel out of that little orifice. If you ever have your carb off and want to see it work? Twist the throttle arm and a pressurized stream of fuel will shoot out.

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