Leaning Procedure

[170C]

I'm sure this subject has been discussed at length previously, but I just completed my annual insp & one thing that bothered me was how much lead was in one of my lower plugs. IA said he'd seen worse, but I don't know how much more could be cramed into a plug. Bottom line is I apparently haven't been leaning as aggressively (especially on the ground) as I should and NO I don't use nor plan to use TCP. My IA said his method of leaning, learned from yrs in the Air Force, was to get to crusing altitude, turn one mag off (not sure why one mag off is better than having both "ON"), lean until the engine began to lose rpm & get rough, then enrichen the mixture until the engine smoothed out. Then be sure to switch back to both mags. His contension is that if this method is used one will never damage an engine by leaning. I plan to give this a try upon my next trip, but just wondered if others use this method and if others, expecially those who may have been any branch of the military where recip's were used, had ever heard of or used this method.

[buchanan]

I flew Skyraiders with 3350's and we leaned until rough and then pushed up intil smooth with BOTH mags on. We always went full rich when we struck but strike altitude was usually no higher than 6,000'

Buck Buchanan

[GAHorn]

Frank, turning one mag off has the effect of retarding timing. The flame propagation is slower and altered such that some of the fuel/air mixture leaves the cylinder unburned becuase it's not totally consumed before the exhaust valve opens. That's why the EGT increases when one mag is turned off.
I don't recommend your mechanics technique at all. And it's contrary to both Cessna and TCM's instructions.
Further, while leaning aggressively will reduce the amount of fuel burned per rpm, it does not reduce the amount of lead in that fuel. After you run the same amount of gallons thru your engine, you've run the same amount of lead thru it as before. Nor does it change the effect of that lead.
Lead fouling of spark plugs is caused by the deposits of glass-like beads of lead in the cavity of the plugs. (It also builds up in other places as well including valve stems, guides, muffler/pipe interiors, and oil sumps as a grey sludge.) The only truly effective way to reduce those deposits when using avgas is to simultaneously use TCP. TCP converts the lead to lead-phosphate, which is a fine, white powder which blows on out the exhaust.
TCP is sold at Texas Air Parts up across from Meacham. It works out to add about 5-cents per gallon to the cost of fuel, but saves unknown amounts of maintenance problems. It's the only snake-oil additive I know that truly works. although a competitive product called "Decalin" does the same thing, it's neither FAA nor engine-mfr-approved. (And while the mfr does not make this claim, tcp is known to provide high-pressure lubrication to sliding parts, like valve guides, etc. It's the exact same stuff that's added to EP (extreme pressure) gear oil and automotive oils to decrease wear.)
I've got some exhaust pipe risers that were run without TCP I'll show you sometime that came out of an O-300. The reduction in inside-diameter is marked.

[bsdunek]

IMHO, the best tool for leaning is an EGT. Mine is on the rear right cylinder which is supposed to be the hottest one. A six channel would probably be better.
I lean just past peak temperature so long as I am not operating over 75% power, which is really never except for take-off and climb out. As I understand it, you can't hurt the engine under these conditions.
I do like the use of TCP. It's not expensive, not difficult to use, and seems to work great. We have self-serve fueling at our airport so I'm up on the wing anyway - just squirt it in!
I also use Marvel Mystery Oil in the crankcase. I know it's just a solvent/oil mix but I've never had a valve stick. I also use it in my emergency generator at home which used to have problems with a sticking exhaust valve due to lack of use. That's over now.
Again, IMHO,

[bradbrady]

, My IA said his method of leaning, learned from yrs in the Air Force, was to get to crusing altitude, turn one mag off (not sure why one mag off is better than having both "ON"), lean until the engine began to lose rpm & get rough, then enrichen the mixture until the engine smoothed out.
Pokey,
I'm sure that you know that the mags on a C-145 or O-300 are split, one set of harness goes to top and the other to bottom. so splitting the mags does nothing for what you want to do. The lead is in the lower plugs! TCP is one snake-oil that does work!!!(in my opinion) when Brent and I bought N8434U we used it for a few months as a primer a/c. We then O/H the engine. My thinking was that an a/c engine should have aviation fuel, (with out aditives) within 400 Hrs, we pulled two cyl's to clean lead deposits from the (new) valves and guides. I then decided to use auto fuel, and run for another 400 Hrs with no problem! (note : flying at least 10- 20 Hrs a week) Untill I got a real job and the shop burned (no corrolation I had the job for two years before the fire), but the A/C sat for six months and the auto fuel degraded the carb enough to ruin the wheel pant on the 172. (With over flow fuel from the carb.) So now I am a firm beliver in the action of TCP. But I burn as much auto as I can, but try to keep it to a 2/3 to 1/3 mix. If not 1/4 to 3/4 mix. So that the actual lead is at least close to the recomended amount.
brad

[blueldr]

I spent an entire career flying and operating reciprocating engines and airplanes in the Air Force and I have never heard of leaning an engine with a magneto shut down. That includes time as a flight maintenance mechanic, flight engineer and pilot on all sizes of engines from a 165 HP Kinner and everything in between up to the Big Daddy of them all, the P&W 4360, naturally aspirated, turbo supercharged and turbo compounded. None of them were ever leaned on one mag, by choice, by tech order or s--t house rumor.

[Romeo Tango]

One mag leaning is not a great plan. The C145 is a simple engine without all the impulse coupling of newer engines, but it still will be hammered by running on one mag for leaning. Newer engines will be readily wacked!

We fly out of high-pressure sea-level at Palo Alto and recommending leaning for taxi on all aircraft. That means seeing a inch of mixture control rod. Runup is full rich for us at our altitude. But at a place like Tahoe we regularly take "position & hold" and run up full power and then lean to optimize.

Keep it at 1000 RPM on the ground, lean a bit for taxi and that will solve most of your problems. If they persist, you can do a 1500 RPM runup at shutdown and slowly lean to starvation - that will burn off most lead and carbon deposits.

[GAHorn]

Several things came to mind as I read thru the posts.
The magnetos on our engines are "split" at 26 and 28 degrees BTC, so one is firing slightly more advanced than the other....so which one do you choose to lean on?
Our engines do have impulse couplings. But those are only operatonal at starting. They deactivate at running rpms and they have no effect on leaning.
The hottest and leanest cylinder on our installations according to Cessna and TCM is the No. 2 cylinder, which is the rearmost on the pilot's side. This was confirmed on a survey taken by the 6-probe instrument owners in our group about 5 years ago and included sampes from all models of 170 (with both types of cooling baffles.)

[170C]

Thanks for all the input & comments. I, too, had never heard of turning one mag off for leaning purposes (it wasn't intended to reduce lead build up). I will keep on leaning as in the past, but more aggressively on the taxi, with both mags on and lean until rough then enrichen till smooth.

[N2865C]

A good tip I picked up somewhere (probably on this site) is when leaning on the ground always lean enough so that when you advance the throttle to more than 1300 rpm's or so the engine dies. That way you can't forget to go rich on takeoff.

[GAHorn]

While EGTs are certainly useful when leaning, they are a lot like the FAA micrometer, when leaning a fixed-pitch, carbureted engine.
The 170 Owner's Manual offers a very good method to lean the engine. Set power, lean to max rpm, then slowly lean to first sign of rpm-loss, then enrichen back to max rpm.
This virtually duplicates, and is as reliable a method of leaning as, using an EGT to lean to peak, then enrichening back to just rich-of-peak.
(I do not mean to imply that an EGT does not have additional usefulness as an engine monitor.)

[bradbrady]

A good tip I picked up somewhere (probably on this site) is when leaning on the ground always lean enough so that when you advance the throttle to more than 1300 rpm's or so the engine dies. That way you can't forget to go rich on takeoff.

John,
I never heard that advise, but it seams to be good practice.
brad