Rough idle when the engine is cold?

[kevinh]

Hi ya'll,

Over the last few months I've noticed that my O360A1A has been idling a bit rough when it is cold. If I keep the RPM up around 770 then it is okay and by the time I reach the runup area it seems fine (I can lower the RPMs down to 640 ish and it seems smooth).

Any ideas on what would cause this? I'm going to have the cowl off on Saturday and I'd like to look into this at the same time. I've checked the gap on the plugs. I have a LSE ignition and one mag, the rough idle when cold seems to be the same on either the mag or the LSE.

Kevin

PS: I'm sure I've read about this before, but I'm failing in crafting a good search at this time.

[fodrv7]

On the ground at idle it is worth considering pulling the mixture lean until you see a rev rise and the engine smooths out. It will also avoid carboning/oiling up the plugs.
The reason you need to do this is that the engine is set up to be a little to rich at MSL for cooling purposes on Take-off. In fact the rev rise on leaning is a good test that your engine is set up to be a little rich on the ground.
When the engine is cold, the excess fuel (rich mixture) does not vapourise and causes the rough running. When the engine is wwarm the problem goes away.
Pete.

[gmcjetpilot]

It's a Lycoming. Could it be that the temps are lower in recently? If its colder than it sounds normal. You did not say how cold it was a few months ago and what it is now.

It's hard to say. Rough is relevant. It seems from your description all is well. When you get to the run-up area (ie warmed up) it's fine, right.


IDLE SPEED, WARM UP, MIN and SPEC
In my opinion your idle is too low for warm up, its recommended at least initially, 1000 rpm. You should not exceed 1,200 rpm cold, but you need 1,000-1,100 for proper splash lubrication (ref: Sacramento Sky ranch Lyc/TCM engineering manual)

When you idle below 900 rpm you risk lead fouling (or fowling for Thanksgiving, gobble gobble ). Low RPM's anytime, regardless of outside temp can lead foul plugs due to lack of heat in the combustion chamber. Higher rpm (+900 rpm) activates the lead scavenging additives in the fuel. At low idle the plugs are too cold. (carbon and oil is not really the issue). Normal min idle is about 650 rpm warmed-up, but you should not idle at min RPM for an extended period.

Higher RPM has the advantages of better lubrication and lead scavenging.

Air-cooled engines need heat to work. The fuel and oil has to be warm. The fuel is cold, as is the sump, so fuel is not atomizing properly. You could be super rich and leaning would help, but just raising the RPM is the solution to rough low RPM idle, plus you get better lubrication and lead scavenging.

Car fuels are seasonal and have summer/winter mixes. In winter automotive fuel goes to a winter mix with high vapor pressure. Higher vapor pressure helps starting, but its a negative when hot. It's subject to vapor lock to a greater extent. Av-gas does not have seasonal mixes as far as I know.

Airplane engines do struggle a little with cold weather, until warmed up. Pre-heat is goodness. Try to preheat the whole plane, engine and fuel tanks (carefully).

YOU NEED HIGHER IDLE FOR WARM UP
If you get smooth idle down to 770 or 640 RPM you are doing good, but you really should idle a little higher when you first start up.

Sorry can't help; I don't see a problem, could be wrong. Try lycoming list on yahoo groups. I have to admit I'm not too picky. If the engine is cold and a little rough I increase the RPM. If all the parameters are green and run-up is good, than it does not matter. After run-up I always check min idle and pull the throttle to the stop to see if it idles but than go back to my normal 800-900 RPM idle. I rarely leave it at min idle. During taxi I'll go to min idle when coming to a stopping, but as soon as I'm stopped, back to +900 rpm.

[mahlon_r]

As the ambient temp drops the idloe will get naturally leaner. Try richening it up a couple of clicks.
Good Luck,
Mahlon
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."

[gmcjetpilot]

Originally Posted by mahlon_r

As the ambient temp drops the idloe will get naturally leaner. Try richening it up a couple of clicks.
Good Luck,
Mahlon
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."

I would think that as well, denser air and all, but the thought is the fuel is so cold it is pooling and not vaporizing. Regardless I see that leaning is probably would not help; I assumed he was full rich. Regardless it does not sound like a problem, it sounds like a cold Lycoming.

Leaning on the ground in general:

I normally lean aggressively at any temp with a Carb-ed 360. Lyc says not needed, but I feel better doing it. I guess the main argument against leaning during taxi (not regarding this rough running question ) is forgetting to en-rich-en for take-off. I don't think its an issue. I lean so much at idle that if I forgot it would not run at full power, besides check list and good procedures should provide protection. I learned this technique with a Piper Tomahawk O235 over 20 years ago. It was prone to lead fouling, leaning helped.