The International Cessna^® 170 Association

Hello all,
The other day I went flying, but before that checked the mags on ground at 1700 rpm. There was only a drop of 25 in one a little more on the other, but not even 50. During flight I felt some strange vibration on the pedals and decided to check each mag in flight. Well, the left one was misfiring every 5-10 seconds. I played with the mixture but it made no difference. I had the mags overhauled back in November, but suspect that I have a harness problem. The spark plugs are fairly new also.

Over this part of the world no one teaches you to check the mags during flight, so I don’t know how do you operate, but I am thinking this may be a good idea since the standard test on ground did not indicate anything wrong. On the other hand, things tend to break down when you mess with them, so I would only check the mags in flight within gliding distance on an airport.

Any similar experiences? Anyway this is my suggestion to the fellow 170 drivers. Good flight,
Donovan

I occasionally check mags in-flight. (If you have an EGT gauge, don't be surprised to see a marked INCREASE in temps due to late-firing effect. .... more combustion takes place in the exhaust than with both systems firing.)

And I have noticed a mag that will get rough in-flight such as you mentioned when an oil-seal is leaking oil into the distributor. This can happen with freshly-rebuilt mags due to injured seal upon installation. Pull the dist. off and see if there's oil.

Additionally, this is a symptom of a bad condenser.

Could also be a plug breaking down even a new one. Swap then top to bottom (which should swap them beteen mags if yours is wired right) and see if the "bad mag" changes.

Bruce,
It didn't take long for one of Us "Vultures (Moi)" to pounce on this!!!!

Swap then top to bottom (which should swap them beteen mags if yours is wired right)

Johneb

johneeb wrote:Bruce,
It didn't take long for one of Us "Vultures (Moi)" to pounce on this!!!!

Swap then top to bottom (which should swap them beteen mags if yours is wired right)

Johneb

Yes John we have to be pretty quick sometimes to through out the remaining crumbs of advice that GAHORN somehow over looks.
Maybe he's baiting us into participation.

Thanks for the suggestion on the distributor, will check. What we did was to install a harness of a different 170 and the problem apparently went away. So it is the harness right? Sorry if it seems a stupid question.
Thanks,
Donovan

That's what I thought once, then I opened up the magneto.........

AGB wrote:Thanks for the suggestion on the distributor, will check. What we did was to install a harness of a different 170 and the problem apparently went away. So it is the harness right? Sorry if it seems a stupid question.
Thanks,
Donovan

Donovan , you might be surprised to know the number of (experienced) folks who have accidentally gotten the firing order ...out of order.... and did a
lot of trouble-shooting, parts replacing, etc etc ... before they realized that when they'd cleaned their plugs... they'd merely gotten two spark plug leads swapped by accident. Be CERTAIN to check the bad mag's firing order in comparison with what is stamped on your engine and with what the "good" mag has.
George

gahorn wrote: Donovan , you might be surprised to know the number of (experienced) folks who have accidentally gotten the firing order ...out of order.... and did a
lot of trouble-shooting, parts replacing, etc etc ... before they realized that when they'd cleaned their plugs... they'd merely gotten two spark plug leads swapped by accident. Be CERTAIN to check the bad mag's firing order in comparison with what is stamped on your engine and with what the "good" mag has.
George

On my brother's 170, I have routed/clamped the leads so they can't be accidentally swapped.

AGB wrote:Thanks for the suggestion on the distributor, will check. What we did was to install a harness of a different 170 and the problem apparently went away. So it is the harness right? Sorry if it seems a stupid question.
Thanks,
Donovan

A quick and dirty way to test a harness for leakage is to remove the cowling and run the engine at night in the dark while someone stands outside by the engine, I have seen them light up like a Christmas tree, and the shorts somtimes move around as the throttle is advanced.
Rob

Thanks for the replies, it turned out to be the harness. But I have not fixed it yet because I have not been able to find a harness here in Brazil. I found one with a straight fitting and mine has a 90 degrees elbow. I would like to ask you, if is possible to install the straight fitting in the 170 cowling? Would I have to do something to the baffling? Does anyone have a picture with this installation?

I am afraid to order the harness from Spruce and it would not fit and I would not be able to return the item all the way from Brazil.

Thanks again for your help.

Donovan

The harness you describe (with a 90-degree elbow) is probably an "old style" harness. Modern harnesses have short, straight connections which fit beneath the cowling just fine. The harness which Spruce sells should work just fine for you.
Be certain to communicate with Spruce the actual engine and magneto-models you have, to assure a good harness. (While the shipping might be a lost expense, Spruce is very good at accepting returned merchandise provided the error is theirs. I recommend you communicate IN WRITING or EMAIL the exact engine model and magneto models and spark-plugs you are using.)

As always, thanks for your help!