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At 05:40 PM 12/6/01 -0800, Percy Wood wrote: >The NE corner of IL doesn't sound too high, Joe. I'll go with what I >learned in >primary (taken in the Cessna 150 I owned at the time) "Do not lean below >5,000 feet MSL." I've seen over 10,000 in my 415-C crossing the Rockies. Oh dear, the CFI who gave that advice needs his pee-pee whacked. A 150 has a Marvel carb. It does needs leaning below 5000', especially if run on blue gas. Maybe he gave the advice in the olden days when running rich didn't hurt so much. Now, a Stromberg carb in good fettle has a sort of self-leaning quality below about 5000 feet. So even though YOU are not leaning, IT is. The mixture control starts in where the self-leaning property leaves off. (Stromberg has long been known for carbs which have some interesting properties such as this. So has SU. They tend to be simple and to work better.) However, the Marvel carb has no such property. Especially if you're running on 100LL, proper leaning is critical. You might get away without it running red gas, but you'll have loaded up plugs and perhaps sticky valves in short order if you do not lean aggressively on blue gas. (Again, below 5000 feet, the properly working Stromberg SHOULD take care of this for you.) In N99387, it's impossible to get the EGT up to the 1650 or so degrees that heralds decent economy without a good bit of leaning. I start to lean at around 2500 feet. By the time I get to 9500MSL, the mixture control is WAY out before the EGT gets to the desired point (and that is well rich of the point where RPM starts to drop off or the motor goes rough.) Conversely, N99387 run full rich at even 4000 feet is CLEARLY losing RPM to a too-rich mixture. With a Marvel carb, you also may want to lean on the ground. If I idle or taxi at full rich for any appreciable time, I usually pay with a fouled plug that must be cleared via going to 2200RPM and pulling the mixture lean prior to passing the mag check. It's quite predictable :-) The rule for leaning on the ground is 'lean to where any increased power results in a stumble.' There is NO reason in the world to run rich on the ground. The motor doesn't need the cooling of a rich mixture. It just craps out the plugs. So, my results from having learned to lean aggressively on the O-200 are that I burn between 5 and 5.8GPH, depending on my diligence, etc. That's up to about 8500MSL. When we pulled the plugs recently, we found them to be the ideal soft grey color with no signs of carbon or lead fouling. I burn less fuel and go faster in N99387 with an O-200 than I do in N2906H with a Stromberg and C85. I have mixed feelings about the 'lean to taxi' thing. It's too easy to forget to go rich before takeoff. UNLESS you really do it up to the point where the engine is just going to cough and sputter on advancing to full power. So that is probably the right thing to do, to guarantee a reminder. In the cases where I have foulded a plug due to rich taxi mixture, a full-power run-up cleared it instantly. That suggests that any crap that accumulates on the plugs during taxi-backs while doing pattern work blows right off in the takeoff roll anyway. Oh, yes... ...when going to full power to climb, you generally go full rich. Except that the engine operating handbook sometimes says that for take-off at high altitudes you should lean for best RPM at full power! Watch that EGT! Greg ==^================================================================ This email was sent to: [EMAIL PROTECTED] EASY UNSUBSCRIBE click here: http://topica.com/u/?aVxiLm.aVzvvT Or send an email to: [EMAIL PROTECTED] T O P I C A -- Register now to manage your mail! http://www.topica.com/partner/tag02/register ==^================================================================
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