The International Cessna^® 170 Association

Twice now over the last four or five flights I've had the engine run rough on takeoff for about a minute or so before it suddenly clears out and runs normally. There is a significant power loss.

Both times were from a cold startup, just warming up during taxi to about 250 degrees on the CHT. Mag drops at 1800 were fine as normal. The idle seems to be a little rougher as the only hint. I've started pulling the mixture a little for taxi like I do at 5,000ft home base in AZ even though I'm at sea level now.Today I did a full power runup which was smooth at 2350 and it was also smooth on takeoff. This will be my procedure until I figure out what is wrong.

I'm due for new plugs so I'm going to do that right away. (any suggestions as to brand and type?)

Does a valve start to do this as an early symptom of sticking???? That's my real question.

If you've washed your engine down lately you may have introduced moisture to your magneto, but that should dry out after a few flights. If you have an oil seal leaking at the front of you mag, it may be causing the problem, which may clear up during a flight as the distributor gear "slings" the oil off (or worsen as the leak continues). Get a mirror and check the lower vent-plug of your mag for evidence of oil in the mag.
Lastly, a failing condenser in the mag can also give all kinds of crazy performance until it finally dies. Of course, a talented magneto man can trouble shoot all these problems.

Dave Clark wrote:Twice now over the last four or five flights I've had the engine run rough on takeoff for about a minute or so before it suddenly clears out and runs normally. There is a significant power loss.

Both times were from a cold startup, just warming up during taxi to about 250 degrees on the CHT. Mag drops at 1800 were fine as normal. The idle seems to be a little rougher as the only hint. I've started pulling the mixture a little for taxi like I do at 5,000ft home base in AZ even though I'm at sea level now.Today I did a full power runup which was smooth at 2350 and it was also smooth on takeoff. This will be my procedure until I figure out what is wrong.

I'm due for new plugs so I'm going to do that right away. (any suggestions as to brand and type?)

Does a valve start to do this as an early symptom of sticking???? That's my real question.

A sticking valve most often shows up on the ground, at startup and during taxi. But, it would not clear up on a mag check and then show up again on takeoff. A stuck valve is a VERY rough idle, and sometimes no idle at all, it is running so rough it just won't run at low RPM. Of course, a valve can stick at any time, including in flight. But if it does, remember that there are 5 more cylinders running fine, stay cool, head for the nearest airport, and make a normal landing pattern, aproach, and landing. Then, of course you are carrying the few tools necessary to fix the problem well enough to get back home and you are back in the air in a half hour or so.

George and Joe
It has been sitting outside and in the rain for the first time in a long time so that might be it. I'll check the mag plugs for oil but when I run it up from the slightly rough idle the mags check ok at 1800, or seem to. The rough idle is as if it's rich, not a severe roughness at all. The roughness on takeoff is like a fouled plug, not severe, but a noticeable power loss and lasts about 30 seconds or a minute then smooths out instantly and full power is there.

Thanks for the inputs.

Dave, I would also check for water in the fuel, know it's obvious and you have already done that. Hope the new plugs clear it up. Champion REM40E's work well in my C145. Does not sound like you have to worry about a valve. Sounds like either fuel or electrical. Hard to understand why the full power runup improves the situation.

I would not discount the sticking valve problem although I'd look elsewhere first. Your symptoms sound like some I had recently. After checking everything else and changing the plugs I checked the valves to find one very tight and another tighter than the rest. Cleaned all valves and guides and the engine is now running much smoother.

You might drain your carburetor bowl to check for water/contamination.

Do you have a 1 piece or 2 piece venturi in the carb?

Are you running on auto fuel? If so, it tends to fowl the plugs at idle. It is worse with the one piece venturi. I lean it until it gets rough and then go back rich until smooth while it warms up on the ground. You can't hurt it by leaning at low power settings while taxiing etc. I keep it so lean that I get a BIG mag drop when I check the mags and that reminds me to go rich for the take off. I also try to remember to lean it again after landing as I taxi in.

About plugs. For the past 15 years I have been sending my plugs to Aircraft Sparkplug Service, 16152 Valerio St.,Van Nuys, CA 01406. They recondition the plugs, regap them, pressure test them, repaint them and send them back with a new gasket for $3.50 each. If a plug is bad they will replace it with another for about double that. In 1500 hours I have never had a plug problem in my 170. Many agricultural application outfits have their plugs serviced by this company. It is cheaper than buying new plugs and as best I can tell just as safe.

Thanks to all for the replies. I am using auto fuel with Marvel as I have been doing the last 200 hrs when not travelling. The last load was Shell and I think I'll go back to Chevron. Don't know about the Venturi, all logs and tools are in AZ and I'm in WA for the summer. Carb bowl was just drained in May at the annual. No water in tanks, gascolater or anything nor has the plane ever taken on any sitting outside in the rain over the prior two summers. The engine doesn't "clear up" at full power I just meant that I'll be doing that as a check before takeoff untill it's no longer acting up because the two times it happened it was hard to tell until well into the takeoff. New plugs are on order and I'll be operating it lean on the ground from now on just like I do at 5000 ft back in AZ. I'll keep you posted.

Hi Dave if it is that humid, you may want to try and pull the carb heat after start-up until it runs smooth. When it gets real humid here (Montreal area Elevation 200'), and it does (surrounded by water), my engine runs rough at start up. Last little while, it has been 35 to 40º C with loads of humidity and down to 14-20º C range at night. RThis morning, for instance, at 7 am, it was already 25º C; started up, ran rough; pulled carb heat till it ran smooth (30-45 seconds). Did run up and took off, kept runninng smooth till destination, 30 minute flight. Hope this helps. JD

I read your original post again and I remembered another similar problem I had. It was an intake leak. one of the intake rubbers was simply pushed against the metal-not over it. After about 200 hrs. it began to run rough similar to your problem. I simply stumbled on to it after trying everything else. With the rubber correctly located on the metal and clamped the problem went away.

One other item you might wish to check is for a sticking or poorly performing lifter. Carbon buildup may cause a lifter to stick intermittely, air entrainment in the oil if the oil level is low or if the oil pump has an air leak on the inlet side, will also cause a lifter to perfrom poorly.

Thanks to all who have contributed ideas. I replaced all the plugs with new about three weeks ago except for the lower right which will have to wait until I can pull the cowl. I've put at least fifteen flights on it since and only had a minor roughness once when taxiiing out. Looked down and saw I'd forgotten to lean. Leaned it and it cleared up right away. So I'm attributing the problem to plug fouling and will continue my leaning for ground ops and full power runup when lined up for takeoff untill I get back to Arizona with my hangar full of tools.

One thing I'll try to do here is to play with the idle mixture a bit as it is a little rough at sea level without leaning. Thanks again to all.

Dave Clark wrote:Thanks to all who have contributed ideas. I replaced all the plugs with new about three weeks ago except for the lower right which will have to wait until I can pull the cowl. I've put at least fifteen flights on it since and only had a minor roughness once when taxiiing out. Looked down and saw I'd forgotten to lean. Leaned it and it cleared up right away. So I'm attributing the problem to plug fouling and will continue my leaning for ground ops and full power runup when lined up for takeoff untill I get back to Arizona with my hangar full of tools.

One thing I'll try to do here is to play with the idle mixture a bit as it is a little rough at sea level without leaning. Thanks again to all.

Let me jump in here and make a recommendation re: your idle mixture setting. Adjust it so as (on a sea level, standard day) that when idling at 800 rpm, you can s-l-o-w-l-y lean it out, and see a 50 rpm rise before it quits. That's they way it should be. Yes, at Pagosa Springs, CO it'll be idling too rich. That's also the way it's supposed to be. At higher elevations you'll need to lean for takeoff. But at lower elevations, on a high barometric pressure day, you won't be running too lean that way, and that's how the mfr. wants it. Good luck.