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FlyingMonkey

Cold Weather Roughness?

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Has anybody else noticed any engine roughness when operating at significant;y colder temps than you are used to?

On Saturday I flew from my homebase of KWDR (Winder, GA) to 3M5 (Moontown, AL), a distance of about 146nm each way.  The air temps on the ground were about 32F, which is colder than I usually fly in the winter by about 10-15F.  Oil temps were on the cool side, mostly around 165-180F (after our first stop at about 100nm I put more tape on and got oil temp to 220+).  I was flying with slower aircraft, and most of the trip was made at 4500-4800rpm (I know, not ideal).

Almost immediately on getting settled into cruise, I noticed that there seemed to be additional roughness and vibration from the engine, which I had also noticed the last time I flew in colder than normal temps.  Not enough to be alarming, but enough to notice.  I decided to play around with it a little bit.  First I added full carb heat, and that seemed to help a tiny bit, but might have been my imagination.  Then I added half choke, and aha...the engine got a little smoother.  I went to full choke and the engine smoothed out noticeably, and rpm came up a bit.  I also noticed that the engine ran smoother (without choke or carb heat) above 5000rpm.  I did not leave the choke on because I didn't know how good it is to run like than when in cruise, and also didn't know how much extra fuel was being dumped in by the enrichment circuit.  So I left the carb heat on and lived with the roughness.  

Later in the day when the temps came up to about 45F, the engine ran much more smoothly, like I'm used to.  At no time did the engine appear to be in distress or perform poorly, it was just a bit rougher when OATs were cold.

I'm assuming that one of two things is going on:

1)  In the cold air, the mixture is getting lean, and the 4500-4800rpm range is where it's particularly lean, since lower (4200-) and higher (5000+) rpm ranges were both smoother.

2) The colder air was exacerbating a slight out-of-sync issue between the carbs, and the 4500-4800rpm range happened to be the portion of the range where the sync is worst, since you can't sync for all RPM.  I usually do my sync at 3500rpm BTW.

 

Any thoughts or suggestions?  I'll re-sync sometime in the next couple of days, but if it's a mixture issue I'm not sure there's anything I can do about it, other than setup and sync my carbs when it's really cold out and hope for the best.           

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Yep sounds to lean on cold dry days, one size up on main jet for wide open

 and possibly raise needle position may help. If your EGT are higher during

this time maybe a little to lean mine does same thing little higher EGT

on dry cold days, runs ok. I sync. at 5000  and at idle. My temps run really

close at that RPM.

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If I encounter this again, does anybody know if it's bad for the engine to give it some choke and leave it on in cruise?  It's a problem on so few days here in Georgia I don't want to "engineer" a real solution that might make things worse the other 95% of my flying time.

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On cold days it is harder for fuel to vaporize so this doesn't surprise me.

I have flown in 20 below C, and had to leave carb heat on to stop the roughness.

I imagine those using the warming jackets on their carbs don't see this issue.

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Makes sense Corey.  If the carb heat isn’t quite enough, do you see any problem with using the enrichment circuit of the choke system to smooth things out?  It’s kind of the same way I use the manual choke in cold weather on my wife’s 1967 Ford pickup truck...

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Andy, I get this roughness too.  During cold flights I get roughness.  If the temps get warm, this roughness goes away.  I'll monitor this to see what others say about using a small amount of choke to enrich the mixture.  Thinking about the choke circuit....is this circuit designed to only be able to add more fuel when the throttle is closed?  Wondering if the choke enriches the mixture when the throttle is opened? 

Corey, was it you who posted the video showing the flow of fuel thru the carb in a recent post?  Did this show how the choke works?

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I thought that the main circuit overrides the enricher circuit?  Having said that leaving the choke on does seem to effect cruise performance???

My Rotax loves cold, the colder it gets the better she runs and the higher the top speed.  My mechanic pulled carb heat as he was crossing the mts at 12,500 and I pushed it back in and asked why.

For me cold temperatures = best performance.

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29 minutes ago, Ed Cesnalis said:

I thought that the main circuit overrides the enricher circuit?  Having said that leaving the choke on does seem to effect cruise performance???

My Rotax loves cold, the colder it gets the better she runs and the higher the top speed.  My mechanic pulled carb heat as he was crossing the mts at 12,500 and I pushed it back in and asked why.

For me cold temperatures = best performance.

It may have to do with your normal DA and the fact your carbs are setup in the high DA environment.

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The enrichment circuit is not completely overridden by the throttle being open. The enrichment circuit is effectively an open circuit with a plate that closes it when you have the choke lever off. While it would become considerably less effective with the throttle open, there's nothing stopping the engine from drawing some air through it, and thus a little fuel from the enrichment hole.

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Andy, it appears that you have a good suggestion to use the choke if the engine is running rough during cold operation.  I'll have to give this a try unless anyone thinks this has any downside?  I do know that the engine will run very rough if the choke is not fully shut when running the engine at low speed so I imagine the choke must be closed when power is pulled for landing.  It might be a good idea to check the spark plugs after a flight where the choke was used?

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The choke does nothing in the higher rpms. If you don't believe that next time your in cruise rpm then pull the choke. Nothing will change.

32F should have been an issue. many Rotax owners including CT owners fly in much colder weather without any issue.

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Been lurking on this site a bit ( mostly hanging out at sportpilottalk.com) but .. last  week I was flying my Sting at 10 F and couldn’t notice any difference in terms of engine. Week before it was down to 3 F ( with wind chills well in negatives) , my damn canopy cracked a bit ( which I presume was in part due to extreme cold and some bumpiness .) My oil temps were around 165 or so cause I had to go slow and run low rpm ( 4800-5000 or so ) due to bumpy air but the engine was just as smooth as ever.

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 Warmi, sorry to hear about your canopy.  A bummer for sure.  I think some of us are only talking about a "slight" roughness and this doesn't always occur.  It is found at the mid to high 4,000 rpm.  Not there > 5,000 rpm's.  I also think that this roughness may be due to the carbs needing balancing but have not tested this theory - I have not stood in the prop wash in 20 F weather to mess with balancing.  Then too, those of us with CTSW's may feel the roughness more than those flying CTLS, due to the better engine isolation of the CTLS.  Not sure what engine isolation the Sting may have but think it might be similar to the CTLS?  As soon as the ambient temps warm up, I don't notice the roughness. 

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When operating at low throttle the mixture is lean anyway then add very cold air and the mixture is very lean.   

Carbs will altitude compensate but not density compensate like injection will.

The answer is to just increase throttle till it smooths out or if you don't feel like that just go higher for thinner air which will also fix it. The altitude compensation of Bing carbs is not that good anyway so climb to find the best spot. 

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Andy, I fly at temps as low as the high teens (Fahrenheit).  I don't get roughness.  Have you checked the float bowls for water or debris?  Just a generic thought...

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Hi Fred.  Agree about checking the carbs - either for contamination or for balance.  I know cold ambient accentuates roughness caused by contamination but haven't looked into the effect from poor carb balance.   If I happen to feel some roughness when I start out on a cold morning I have noticed that this will go away later in the day after the ambient temps have warmed up.  The roughness seems to not always be there so perhaps there might occasionally be some condensation that needs to work it's way thru the fuel system?

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I also experience this with my CTSW in Michigan. In my case it is most noticeable with lower temps (as low as 0F) and if oil temps never got above 200 in the climb (not enough radiator tape), in mid revs (say, 4600rpm during descent). I assume it is combination of slight carb imbalance and not enough moisture boiled off in the oil. Not rough enough to be alarmed, but noticeable. Carb heat seems to help slightly.

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3 hours ago, FredG said:

Andy, I fly at temps as low as the high teens (Fahrenheit).  I don't get roughness.  Have you checked the float bowls for water or debris?  Just a generic thought...

Yes, they are clear and my fuel samples from the gascolator are completely free of water or debris.

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Flew today with temps in the  low 'teens.  I had fairly noticeable roughness and applied choke.  Application of the choke reduced the roughness in the high 4,000 / low 5,000 rpm range.  Application of carb heat had no effect.   

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If you are going to fly in the teens and lower than I would drop the needle clip one notch down to the #4 position. This will slightly richer the mixture and should reduce the mag drop and smooth the engine. 

You’re just running a tad lean especially in the 4000 rpm range. He 5000 range is richer.

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Thanks Roger.  We not only have cold temps, we got 18" of snow Thurs/Fri on top of 8" we had.  I'd like to check carb balance and check the needle setting but this is not a good time to do it. :ph34r:

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Just had a chilly flight here in Michigan (13 degrees F) and experienced the same phenomenon of roughness in the high 4000's  with higher than normal EGTs (high 1490-1510) 

interestingly, I climbed to 7500-8000 and my engine smoothed out and my EGTs dropped to low 1400s where they are normally in the summer. 

Density altitude at the field was -4000, so it's easy to believe it was running a little lean down low.  

Im thinking Roger is right about making it richer in these sort of temps. Thing is, I fly in this so rarely it hardly seems worth it. Maybe in this cold I'll just fly higher.

Goodness knows I got up there fast today with just me and 10 gallons fuel onboard. There's something pretty cool about departing a 6000 foot runway and blasting through pattern altitude well before the end of the runway passes below

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Interesting thread. We operate 4 CTLS in Wisconsin and fly every flyable day. Never experienced this (knock on wood). The only thing we have noticed is sometimes the idle rpm changes a little.

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The engine normally runs lean in the 4K rpms and if you fly there in cold and make it even leaner then it will be rough. No reason to fly in the 4K rpm range unless you are coming in for a landing. It smooths out in the 5K rpms because it runs richer there. The engine is designed to run in the 5k rpms.

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