The International Cessna^® 170 Association

While doing this years annual, the mags checked out OK and nothing was done with them. Recently I am experiencing a bit more (approx 25 to 50 RPM) drop on the left mag. Altitude at home base is almost 5000' MSL so I always taxi with about 1" of mixture pulled to prevent fouling. A spark meter was placed on leads from both mags and the right mag. definitley shows a hotter spark but the left looks healthy. Recently on engine run-up I am getting a 100+RPM drop on left side and after opening throttle to 2000+RPM and leaning, both mag. then check out at <25RPM loss with smooth running engine.
Question: What is causing the fouling, weak mag or plugs?

Thanks for any suggestions.

[quote="n3437d"]While doing this years annual, the mags checked out OK and nothing was done with them. Recently I am experiencing a bit more (approx 25 to 50 RPM) drop on the left mag. Altitude at home base is almost 5000' MSL so I always taxi with about 1" of mixture pulled to prevent fouling. A spark meter was placed on leads from both mags and the right mag. definitley shows a hotter spark but the left looks healthy. Recently on engine run-up I am getting a 100+RPM drop on left side and after opening throttle to 2000+RPM and leaning, both mag. then check out at <25RPM loss with smooth running engine.
Question: What is causing the fouling, weak mag or plugs?

Thanks for any suggestions.[/quot

The primer is onthe left side, make sure it is not leaking.

Recently I rassled with an ignition problem myself, which I thought was mag related but it turned out to be cured by a new set of plugs. How many hours on your plugs? Mine had 450,they looked pretty good but apparently looks can be deceiving.
Regarding leaning on the ground--I recently asked a question about this on another topic. The Continental book seems to indicate that at idle up to about 1,000 rpm the idle circuit in the Marvel carb is running the whole show. Above 1,000 rpm the main jet circuit gradually taked over. So does pulling the mixture control out on the ground,when I assume the rpm is generally not too much above 1,000 rpm if that,really accomplish anything other than to make you feel warm & fuzzy?

Eric

zero.one.victor wrote:Recently I rassled with an ignition problem myself, which I thought was mag related but it turned out to be cured by a new set of plugs. How many hours on your plugs? Mine had 450,they looked pretty good but apparently looks can be deceiving.
Regarding leaning on the ground--I recently asked a question about this on another topic. The Continental book seems to indicate that at idle up to about 1,000 rpm the idle circuit in the Marvel carb is running the whole show. Above 1,000 rpm the main jet circuit gradually taked over. So does pulling the mixture control out on the ground,when I assume the rpm is generally not too much above 1,000 rpm if that,really accomplish anything other than to make you feel warm & fuzzy?

Eric

When you pull it all the way out, does it lean enough to stop the engine?

Yeah,Tom,the idle cut-off feature works,of course. But I don't notice any change in idle rpm until the rpm goes up just as the engine starts to die. So I question whether leaning the mixture at idle really helps with plug fouling or anything else. Like I said before,I leaned on the ground myself until just recently,but after reading in the C-145 book about the idle circuit I don't bother. Didn't seem to adverselyaffect the way it runs on the ground.

Eric

I don't know about everybody else but I've not been in very many airplanes that will start moving and taxi at idle. If you have to push the knob in you might as well adjust the mixture. Very few people taxi at idle, most people I see taxi at about 1000-1500rpm and hold the brakes. In that case it would be better to lean. You'ed be supprised how fast the plugs will fill with lead when people taxi around at high rpm and hold the brakes when they don't lean. I know they need to pull back on the knob but they never do.JMO

zero.one.victor wrote:Yeah,Tom,the idle cut-off feature works,Eric

Then use it. get that little rise, and leave it there. It'll help when that new engine of yours gets a lot more hours on it.

On the inside of the Marvell carb the mixture well is prior to the main metering jet. If you think that leaning at idle is ineffective why does your engine quit when you pull the red knob all the way out?
You must lean for takeoff at the higher density altitudes anyway.
With regard to the mag problem the point gap sets the "dwell" timing and when the little plastic cam (yes it is plastic )wears the spark gets weaker.
( you did not specify what kind of mag however)
Get a new mag it is cheaper than re-building them. New mag sets come with wires. The problem with plug testers is they supply pressure, and spark but not heat so they will not diagnose all spark plug problems. I have re-built both magnetos , and Carburetors and what I learned was that unless you have a low time unit with a specific failure it is cheaper to buy an O/H unit.

Hardly ever use primer but it is checked regularly. Does not appear to be leaking.

Having been a "flat lander" pilot for many years I hardly ever used misture except to kill the engine.
Having flown in Colorado, Idaho and now Nevada where home field is 4700+ and a long taxi to favored rnwy(about 3/4 mile), the plugs do foul quickly when traveling at normal idle speed and rich misture. They do not foul when mixtue is pulled back.

Eric, good point on the plugs. I can find no record of new plugs being installed. During the last three annuals the plugs have looked OK, no unusual wear, very minor lead build-up that is easily cleaned, nice gray dry color. I am not a technical person other than understanding theory. The spark box that the mechanic placed on the leads indicated a bright strong blue spark form right mag and a strong but not as blue spark from the left side. Also after almost 100hrs between annuals the timing was "exactly" where it was at prior annual which would indicate no to minimal wear on cam. You did give me the idea to put in new plugs as an experiment. Then a new left mag. if that does not work.

My plugs looked pretty good too,I thought--nice tan/grey,no deposits to speak of,minimal erosion of the center electrode. They tested just fine in a pressure-box type tester. They just didn't run worth a damn in the airplane! Too bad I couldn't just fly that darn tester around.....

Eric