The International Cessna 170 Association

I have a drop on the left mag. It is consistent and does not flucuate. I sounds as if a plug might not be firing, but they all checked out when cleaned and tested. The engine has not been run much in a year, so it could need to be run. I guess it could be the mag. Thoughts on testing or anything else?

You didn't say how much drop on the left or how much you are seeing on the right. How did you test the plugs? Just because you checked the plugs doesn't mean that they are firing, there are other ignition components that can be the problem. Have you performed a cold cylinder check to determine which cylinder is not firing? Have you tried "known good" plugs in place of the ones not firing or performed a high tension lead test on the harness to that cylinder?

Do you have an A&P mechanic to consult with?

Go to your nearest Harbor Freight Tool store and buy one of their infra red thermometers with a laser light in it. It will quickly find a cold, or cooler, exhaust riser to help determine your problem. One of the least expensive but handiest tools for anyone dumb enough to work on airplanes.

So here is a proceedure.

First things first. Using a multimeter insure that the P leads have not been switched and when you are selecting the left mag it is actually not grounded and your running on the right mag.

Second start the engine on both mags then switch to the bad mag and run for 15 seconds. Get out and physically feel the exhaust at the cylinder to find the cooler cylinder. If you can not tell which cylinder is cooler repeat until you get the exhaust hot enough to feel the cold cylinder. Of course take precaution that any cylinder you touch doesn't burn you.

Third trace the plug wires from the bad mag to that cylinder and plug. The left mag should be running the lower plugs and the right should be the upper plugs but check to be sure because they've been known to be changed.

Once the plug is identified replace it with another and try it out. If you don't have another plug swap the suspected bad plug with another cylinder and repeat this process to see if the problem follows the plug.

If you don't find a bad plug then it could be the plug wire and if not that then you in the mag with a bad cap or distributor most likely the cause.

I think we narrowed it down to #4 ignition wire. When on the left mag "bottom cylinders", the engine ran like it was missing a spark. I removed each bottom plug one at a time and ran it the engine for a few seconds. Each time the miss was real bad, as if it were missing two sparks. but when I got to #4, it seemed the same as when all were connected. I them swapped the plugs in that cylinder to make sure it wasn't that plus and still the left mag. Also, some times it is fine and others is sounds like a miss. With all that, we are guessing a bad plug wire is all. Thoughts?

Sounds like you narrowed it down to a wire or defect in the cap of the mag for that wire. Likely the wire.

Aryana, I just realized I actually have that same cable tester and didn't know it. I bought out a flight school and maintiance shop 2 years ago and it was new in the box, just found it tonight. I will test it tomorrow.

On a related note, when I called to price ignition wiring harnesses with Spruce, there were two types. The type that each cable screws in the back of the mag and the other is all made together. Any suggestions? The second one priced out at about $327 I think it was, for both sides (12 wires). Does that sound right?

I have the Bendix mags and I think the back cap can be either the cap that allows each wire to screw in or it can be the solid piece. I am not expert yet. Anyone else?

My cap have the cover that lets each wire be taken one and off, not the solid state cover, but if I replace them, I am told to get with the solid state, that is 12 less items that can unscrew due to vibration etc. Plus, the solid state is cheaper. Thoughts?

I used the eastern tech ignition wire tester tonight and just as I thought, that one wire had no fire. Now I got to decide to replace the one or everything.

Blake, if you can determine the age of that harness it may help you make the decision.
If it's over 10 years old, or has over 1000 hours of operating time on it... go for an entirely new harness.
If it's significantly less than that...and the rest appear to be in good shape, and not been tied up in knots...
then replace the single defective lead. IMO.

I am sure they are over 10 years old. Also when I was getting quotes, they asked what length. I have not measured them. Is there a stand length for these harnesses? I told her about 2 feet. Thoughts?

BlakeinAlabama wrote:.... Thoughts?

I think you're in for an expensive learning-curve if you don't start being more specific before ordering parts. (Don't ask how I learned this.)

Ignition harnesses are different for different models of engines, and individual leads can be specific to cylinder and to whether it goes to the top or the bottom plug of that cylinder, and whether it comes from the right or the left magneto.
There are standard harnesses which are made up for specific engine families, with the cheaper harnesses being more "generic" (meaning all the leads in a complete harness will be the same length) and the better-designed ones following specific installation routing and lengths.
Consider for a moment how your wish your leads to be routed. Did you buy an airplane that had it's third or fourth engine replacement with the installers wadding up the harness into whatever route worked for them and tying them together with ty-wraps?.... or did it follow neat, original or logical routing using cushioned "Adel" clamps similar to this one...
The problem with ordering one "about two feet" long is that it might be adequate for the left mag to reach the #2 cylinder's lower plug(tho' I doubt it) ....but it certainly will not reach the #5 cylinder, especially with the need to route it around accessories, baffles, and along a neat pathway coordinating alongside the rest of the individual leads in that harness.
If you order one 5' long and it routes a short distance, then you'll have to coil up the excess and/or loop it back onto itself, and/or let the excess dangle in the engine compartment, all of which will subject it to excessive wear and abuse.... So measuring an individual replacement lead is a good idea, or specifying which engine, cylinder, plug, magneto, and aircraft installation and let the supplier/mfr'r risk the determination, or obtain a "universal" replacement and trim it to length.

Most harness you can remove the inner core wire and replace it. just a matter of knowing how. the inner wire can be bought in bulk, and you can also buy individual leads completely assembled and install them in you mag cap.