Originally Posted by
firstflight
Just received the email below from out local FAAST Team representative concerning ground operations.
"During the discussion a couple of operating habits came to light..... Leaning on taxiing out for takeoff : a carbureted engine has a fixed idle jet. That means a fixed amount of fuel goes to the engine. If you are below 1400 rpm the mixture control does nothing. You should be below 1400 RPM taxiing. Fuel not only cools but lubricates. Unless there is specific information in the POH/AFM that tells you to lean on the ground...it is not a good technique!
So if the mixture control doesn't do anything below 1400 how does this guy stop his motor at the end of the flight? A log in the prop? The fixed idle jet (orfice) is goverened by two things: flow thru the venturi section of the carb and fuel avialable to be pulled from the idle jet. The mixture control limits the amount of fuel available to be pulled thru the orffice. Hence you can (and should) lean on the ground.
He is correct in one reguard; a rich mixture DOES cool the motor but it is at the top end of the RPM range not the idle/taxi portion of the flight!
The last issue is plug fouling. This is fairly rare yet I hear about it all the time. Clearing a plug should be done at no higher than run-up RPM and should only occur for less than 30 seconds. Repeated plug fouling or lack of clearing per the manufacturers recommendations means you have a mechanical problem. Get an AMT to look at it. It might be the wrong plug, too cold of a plug, poor timing, etc.
...fairly rare yet I hear about it all the time. Which is it?? Someone should tell this Boob that forced use of 100LL in a low compression motor meant to use much less lead will foul the plugs in short order. Leaning on the ground (which bozo doesn't think is possible) TCP and lean runups post-flight help to keep the plugs clean. Unless you are using oil AND fouling the plugs an A&P ain't gonna do anything but lighten your wallet.
YOU ONLY HAVE ONE ENGINE AND YOUR LIFE DEPENDS ON YOUR KNOWLEDGE OF HOW TO CARE FOR IT. "
Then this guy is a dead man walking...
At our club we insist that our members lean during taxi - I am a little surprised that they are also suggesting that the idle jet is set to 1400 rpm. Should this not be set to 600 -650 rpm ? Since this is from a source of some authority just wanted to check if I'm missing something ?