The International Cessna 170 Association

Hi Everyone!

I'm a new 170 owner (or should I say a new owner of a classic 170?) and would appreciate some help figuring out my C-145 during the run-up. I realize this might be picky for a 66 year old airplane... but about every second or third time I go to fly both mags drop almost 150 RPM during the run-up, and the engine runs noticeably rougher. Running the power up and leaning the mixture doesn't make it any better or worse. However, other times I taxi out to fly, the run-up is smooth as silk with the mags dropping about 25-50 RPM during the check. Never any problems in flight.

<150 RPM drop is within limits, but I'm curious why some run-ups are so much "nicer" than others. Regardless of how the initial run-up goes, I check again after landing, and it's smooth with a 25-50 RPM drop after every flight.

Here are some of the usual suspects that I think I'm avoiding...

I lean aggressively during taxi and ground operations - I also lean IAW the operating instructions in flight.
No operations above 1000 RPM until the oil is warm
I let the engine idle for at least 2 minutes before shutdown
Mags were overhauled about 200 hours ago
Timing was verified correct for each mag and all plugs were pulled and examined at overhaul - nothing abnormal noted.

I've got a good A&P who's happy to dig into it for $100/hr... but I'm pretty sure I'm not the first one to have this issue. Any thoughts?

I was having run-up abnormalities last spring and had a mechanic take a look at it. The culprit was the spark plugs. Despite the fact that they were not very old, all 12 failed the resistance test. New plugs made it run like it should. I wouldn't assume the plugs are good just because they were ok at overhaul. I found out they don't always last as long as they should.

Could there be three problems? I would think that 25 RPM drop is unusually low. I'd be checking the grounds and the "P" leads. How accurate is the tach? The six cylinder is smooth, should'nt be rough on a single mag, silky smooth compared to my Lycoming.

Yeah - the grounds are good, I check them before shutdown every few flights... the drop might not be much more than 25, but it's definitely a lot less than the 150 I get every few flights You're right though, when it's smooth, it's smooooth.

Plugs are a cheap inspection... I think that'll be my first move.

Clover,
Have you tried, when you have this problem, using the carb heat to see if that makes a difference? Carb ice can be a sneaky problem.

I've been told the C-145 is susceptible to carb ice on the ground during long taxi's. Thanks for the input - something to think about...

Gee, I love Carby ice .[well the excuse of it any way] when I land in some place [not your average runway or field but on a beach or empty road ] and the authorities turn up for a please explain etc etc ." It's carby ice and the engine was running very rough " I then go into the technical details of carby ice ." It should be melted soon. Some times it takes hours to melt .

CloverCrusher wrote:...
Plugs are a cheap inspection... I think that'll be my first move.

Visual inspection of spark plugs are not very thorough. Spark plugs can fail under pressure (such as at the moment of firing at TDC) and, while Champion long argued that their carbon-pile plugs were a good design... the Unison Spark plug company presented a convincing argument that plug resistance/longevity is superior with their product.
The argument convinced me to try the Unison/Tempest plugs (and that was quite a thing to get me to abandon my love-affair with Champion Spark Plugs)... and after trying them.... I'm a proponent. (They also have a more rust-resistant barrel than Champions.)

If you are running Champions, I suggest you test them with a pressure-tester.

Another possibility is your harness. Have you had them tested for leakage? It's a simple thing to do with an E5 high-voltage tester which many A&Ps have in their toolbox.
It might be overkill, but I use mine at every 100hr/annual inspection or anytime I've got the harness disconnected. While I've been lucky, I've found two bad leads on brand new harnesses we were installing after magneto 500 hr inspections.

Sorry for the delay - I appreciate all the inputs. I did find that a combination of aggressive leaning and carb heat on the ground has yielded consistently more acceptable run-ups. Some days are still picture perfect and others a little more rough... but all are much better than before.

Going to see my A&P for unrelated upgrades this month... I'll have him dig into the harness and plugs and I'll report back.