The International Cessna 170 Association

Heading for breakfast this morning I was in cruise at 1800', 2400 RPM and leaned, when I developed sudden moderate roughness and a drop of maybe 100 RPM. Carb heat and messing with the mixture didn't have any effect. I made a precautionary landing at a nearby airport. Trying to diagnose the issue on the ground, I found that the intensity of the roughness corresponded to engine RPM. It seemed a little rougher on the left mag, but the actual drops were well within limits for a mag check. I tried the normal technique to burn off fouling with no luck. Unfortunately there was no mechanic at the airport and my mechanic wasn't reachable by phone. After letting the engine cool and having lunch, I ran it at 2000 RPM on the left mag for a couple of minutes and checked for a cold cylinder. Front left (#6, if I have my ordering correct) seemed cooler but definitely not cold. I didn't have any tools and there was nobody to borrow any from, so I tied the plane down and got a ride back to my car.

I'm heading back out tomorrow with some tools. My plan is to check the spark plugs. Beyond that I think I'll have to get the (apparently extremely expensive) local mechanic involved. Any other ideas for stuff I could look at? The plugs are Tempest REM40Es installed 30 hours ago at annual. Mags were overhauled about 60 hours ago.

You could check for intake leaks (make sure all the hardware is tight). If it seems confined to a mag then run the engine a few minutes on that mag and see which exhaust stack is cooler. Those fancy temp guns work great for this, otherwise sometimes you can just touch it with a crayon and if it doesn't immediately melt that's your culprit. (I have little kids so crayons seem to be easier to find than my temp gun). When you find the cold one swap plugs, let it cool, and try it again. If it's a plug it will follow, if the same cylinder is cool than it's the harness. Only other thing I can think of is a stuck valve. I don't seem to ever have them (knock on wood!) so I'll leave that to those more knowledgeable to help you diagnose.

DEM

When were your mags last overhauled? The oil seals dry out over time and when engine oil gets into the mag it plays havoc with ignition and can give these symptoms. (Been there/done that.)

Of course, water in fuel and other issues should be checked such as shorted harness-leads, etc.

The mags were overhauled this spring, about 60 hours ago. The harness was tested when the mags were off for the overhaul but it is getting old, so that is definitely worth looking at. Water in the fuel seems unlikely. I flew an hour yesterday after filling up, then hangared the plane overnight and sumped the tanks this morning during my preflight.

Tis the season for carburetor ice.

A quick and cheap way of checking the harness is cowls off and run the engine in the dark. Worn harnesses light up very spectacular.JUST keep clear of the prop.

The local A&P might/should have a High Voltage Cable Tester (Ign. Harness tester) he can check your leads.http://www.aircraftspruce.com/catalog/t ... kkey=12726
It inputs a high voltage to the spark plug lead and any leakage is displayed during the test. If a lead is bad... it's a simple replacement.

(This is one time I think of the old idea of having a "lending library" of test/tool equipment. I have such a tester I might be willing to lend if I was confident it'd be well-cared for and promptly returned. If a Member needed a tool or piece of test equipment it would be so nice if there were an affordable way to ship it to the Member on a "loaner" basis. The problems would be 1-shipping costs both ways, 2- security/insurance against lost/damage, 3-timeliness of return and/or shipment to the next borrower. ... but this is a topic for another thread...)

Of course, in this case with the Member broken-down "on the road" it further complicates the idea.

A quick compression test wouldn't take much and would confirm integrity of each cylinder before the "remove and replace" ordeal starts.

Well, it's 60 degrees Fahrenheit in the Chicago area today, but winds 20G30 and only going to get worse. Rain, too. Tomorrow is forecast to be 30 degrees and windier. The days are short and I have a day job, so I would be working out on the ramp in the dark. As much as it pains me to pay for something that I might be able to do myself, I'm going to leave this one to the mechanic.

I experienced the same a few years back with my 48. I had an intermittent mag coil, money fixed it. One other time a weak plug, money fixed that too. Unfortunately it can be so many things.....I would start with a full wallet and work slowly toward the engine area......

Sometimes a bit of lead makes for roughness too.

Cheers

brianm wrote:Heading for breakfast this morning I was in cruise at 1800', 2400 RPM and leaned, when I developed sudden moderate roughness and a drop of maybe 100 RPM. Carb heat and messing with the mixture didn't have any effect. I made a precautionary landing at a nearby airport. Trying to diagnose the issue on the ground, I found that the intensity of the roughness corresponded to engine RPM. It seemed a little rougher on the left mag, but the actual drops were well within limits for a mag check. I tried the normal technique to burn off fouling with no luck. Unfortunately there was no mechanic at the airport and my mechanic wasn't reachable by phone. After letting the engine cool and having lunch, I ran it at 2000 RPM on the left mag for a couple of minutes and checked for a cold cylinder. Front left (#6, if I have my ordering correct) seemed cooler but definitely not cold. I didn't have any tools and there was nobody to borrow any from, so I tied the plane down and got a ride back to my car.

I'm heading back out tomorrow with some tools. My plan is to check the spark plugs. Beyond that I think I'll have to get the (apparently extremely expensive) local mechanic involved. Any other ideas for stuff I could look at? The plugs are Tempest REM40Es installed 30 hours ago at annual. Mags were overhauled about 60 hours ago.

Either mag should run smooth individually. If they both run a little rough, then it is something common to both mags. I would check for a fuel problem or possibly a sticking valve. It's not common for both spark plugs to foul in a cylinder at the same time.

When you messed with the carburetor heat. Did you leave it on and lean the engine out? Have you done maintenance on it before this occurred? There is to many recommendations to go after things. Can you repeat the problem?

If you're getting roughness on both mags, the first and easiest thing you can check is whether the primer is locked. If it's not, it will cause engine roughness. If it's just barely latched on the ground, it can unlock in cruise due to vibration and cause fairly sudden moderate roughness.

Please Post the Solution to this problem.

Before you spent money can you replicate the problem? Then Take notes on each attempt to solve the issue.

Have well thought out Scientific Method.

Then share with the forum.

Have a happy solutions.

Quick update: 69V is still on the ground. Compressions were checked and are good, ignition seems fine. They found a small amount of water in the fuel system. I sumped before the flight and the plane was left outside in the rain while waiting for the mechanic, so I think it isn't related. The best static RPM we were able to get was 2000. I have a 7655 prop, so that's about 250 missing RPM. Manifold pressure seemed a bit high as well. We also noticed that there was only a 50 RPM drop when applying carb heat at 1700 RPM. A thorough inspection of the induction system is up next.