The International Cessna^® 170 Association

Hello all,

New '48 owner here! Recently purchased my first certified bird 4179K. have really enjoyed flying the 170, but started noticing some issues on the last few flights with the ignition system. Now, I am fairly green to the mechanic side of things so you will have to forgive me if something doesn't sound right. Working next to my A&Ps here's the scenario that I have and what we have determined.

Problem:

Rough/bad mag drop when checking right mag during engine run-up. The mag test was slightly noticeable at first, progressively worsened over next 2-3 flights before making a decision to not fly anymore. There was a slight vibration noticeable in yoke and rudder pedals on last flight. Conducted a "burn-off" procedure thinking maybe it could be a fouled plug. No significant changes. Infrared temp readings on cylinders led us to #5 cylinder showing 70 – 100 degrees colder than the rest.
Took cowl off, noticed a slightly loose top plug lead; tightened lead and ran engine. Problem still persists. Still thinking right mag problem with the #5 cylinder, pulled plug and tested 1020 ohms. Just to be sure, swapped plug with bottom and tested. Problem still persists with right mag.

Went into it a little further testing the mags at various rpms, which gave some interesting feedback. All issues noted here where when testing the right mag. The left mag did not give any issues… yet (keep reading).

~700-800 rpm: minimal drop but slightly rough.
~1000-1200 rpm: Very noticeably rough. Leaving it running on the right mag when testing at this rpm the degree of roughness would vary, sometimes violently dropping 300rpm at times, then might clear up a bit, then back to being rough. It was not consistent.

Noticed exhaust leak on #5 cylinder with a noticeable gap and missing nut. Replaced nut and tightened. Ran engine, problem still persists with right mag. Used infrared thermometer to measure temps while running and all temps were fairly consistent.

Checked timing on mags, 28 and 26 BTDC so it's not the timing.

Installed new plugs. Just for the heck of it, we did a compression test while replacing plugs. All compressions good except for #4 which was ~50/80; leaking around the intake valve. Still doesn't explain the ignition issue. Test ran with new plugs, problem still persists.

We then tested the p-leads on the mags with the ignition switch and found out that they are wired backwards to the ignition switch so it's actually my LEFT mag that is causing the bad drop. So now the problem is on the LEFT mag.

To confirm, we then conducted tests isolating the switch out of the equation, problem still persists, so not a bad switch.

Then we ran on each mag individually and determined it to be running rough when only on the left mag when advancing throttle past ~800 rpm. Very rough at times like before.
The A&P’s at the field suggest that it is probably a mag issue and suggest getting it overhauled. My pilot friends who reported somewhat similar issues in the past on their airplanes suggest to replace the plug leads.

So we are pretty confident that it is either the mags or the leads.

Not having a high tension tester, we are unable to test the leads at this time. Should have access to one at the end of the week.

Any advice, suggestions, etc. at this point? Are we on the right track? Just wanted to hear from some of the membership on what we have done thus far. I have Bendix SF6LN-12 Mags installed with ~400 hrs I believe. Thanks!

Drew
Starkville, MS

Bad capacitor/condenser.

Just an update here. Went ahead and got my mags overhauled. Took a while to find someone to work on these mags but finally got in touch with the guys at Fresno Air Parts. Took a while to get them back but man did they look good! Got the mags reinstalled and everything ran great for about 15 hours and now having more ignition issues. A bad/rough mag drop on the right mag (top plugs). We re-tested leads and we were not happy with the results, so a new set of harnesses just arrived today. Hopefully this resolves this issue! I’ve missed a lot of the flying season this year.

Good move on the mag overhaul. It’s a common thing to delay/postpone mag overhauls for many owners. The majority of folks run ‘em until trouble shows up (thereby denigrating the advantage of dual ignition systems) and only addressing points/timing at inspection time.... Others “repair” mags rather than true overhauls.
A real overhaul cleans and completely disassembles the mag and replaces coils, points, condenser and re-magnetizes, (Yes, the magnets slowly lose effectiveness and should be refreshed) following the mfr’s manual completely. (And don’t forget those impulse couplings... some of the older couplings have issues with the flyweight/trigger being soft and rivets which can let-loose. 500 hr inspections apply.)

I had the same issue, turned out to be a bad spark plug that only had ten hours on it.

I had another issue where lower plugs would foul with lead nuggets after a long taxi, lean that engine when taxiing and don't forget to push the mixture in before take off.

I found that using MO gas or TCP helps and I lean it out and let it run for a few minitues before shut down

To completely overhaul mag in my country it’s nearly just as expensive as to buy new. By the time the AD,s and the required new parts and labor/testing are attended to the cost some times exceed the cost of new. Plus some mags are not supported by their manufacturers.

Just so I’m not leaving anyone hanging, I figured I’d give everyone what I hope to be the last update on this issue for a long time! I received my new wiring harnesses, got them installed, yet still had a bad mag drop. We called the guys at Fresno Air Parts and decided to send the mag back for re-inspection. Turns out that the ”points had closed up.” I received my mag back from them Monday and we got it installed last night. We verified timing was correct on both mags (26 degrees for top plugs, 28 degrees for bottom), and the test run went wonderfully! Can’t wait to do some more flying this year.

Thanks to all for the info and help!

Drew
Starkville, MS