Rough running

·         Guillermo

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Model: 2200A
Serial No: 22A 3251
TT: 180 hours
Aircraft: Land Africa with slats (similar to ICP Savannah)

Hello,

Sometimes, in specific conditions, I notice the engine runnig in a rough or irregular way.
It happens specially afer 1 minute or so climbing at WOT, just in the moment I reliev slightly the throttle, but I mantain the climbing attitide.
Is much more likely to happen in winter.
Usually it disappears if I level the plane, even at the same throttle setting, althougt not always.
It disappears 100% if I resuce or augment throttle and I level the plane.

I have noticed the problem sometimes in the last 50-60 hours, and never noticed it before.

I have suspected three factors:

- Carburettor ice: I discard it for the moment, as it happens also with carb heat (not hot air induction, but a couple of resistences attached to the carb, in any case i take the air from inside the cowling)
- Detonation. I have run for several hours with 98 auto gas (usually I run it with fresh 95 auto gas) and the behavior is the same.
- Mixture problem. I havent changed the carb setting ( factory setting: M245/ N290/ NEEDLE 4A138AOD-5 marked in the carb) nor the intake. I usually find carbon deposits in #3 cylinder spark plugs (the zone where spark is produced is clean, isolator and electrode, but with black deposits all arround). After cleaning it is enougth to fly 2 or 3 hours to come back to the deposits.

I think the engine is otherwise strong and healthy. Good in leak-down test, the 4 CHT between 135ºC and 155ºC (usually #1 the coolest and #2 the hottest), rocker camber clean with honey colour in some zones...
I fly almost every week, and put arround 60h/year.
I use a wooden prop, I get 2950-3000 rpm in climb and 3150-3200 level at WOT. I cruise usually at 2700-2800 rpm.

Could you please give me some advice on this problem?

Thanks,

Guillermo

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·         Doug Smith

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Hi Guillermo,
It does sound like it might be related to mixture... what attitude are you flying, and what temperature? Cold temperatures can make the engine run leaner than it would in warm air.
Thinking about it, the Bing carby sets mixture by the amount of air flowing through it, and it figures out air flow based on the pressure on the sense ports at the carby intake. If the air is disturbed over those ports - so if the intake air is turbulent or if there is too much swirl in that air (which can happen if there is a pod type filter) - then the carb will get the mixture a little wrong. So i guess the first thing i would look at is the intake, to make sure that the intake air is as smooth as possible.
The other thing that might be worth a look is the ignition system - make sure the gaps on the coils and plugs are right, the cables and distributors are ok etc. I think mixture is more likely but it's a good idea to also check elsewhere.
Regards,
Doug.

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·         Guillermo

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Thanks Doug.

The airfield is 2300' altitude. I usually fly under 5000'. Last week (last time I noticed the problem) it was 2ºC at take off. I dont have OAT so I don't know wich was the temp at 5000'.

I will take a look at ignition, but I find it in shape in maintenance inspections.
I will also look at intake. Is quite simple: K&N filter, pod type, of generous dimensions and a 2", 90º bent blue silicon hose. Maybe the hose is too deep inserted in the filter. Or maybe this 90º generate turbulence.
I would like to make some test in order to confirm the turbulence subject, maybe run without filter and hose for one or two flights (the runeway is asphalt and the enviroment is not dirty ) or maybe modify the intake. The problem with that is the limited space. I have read about a "cobra head" intake tube, do you supply it?

Regards,

Guillermo

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·         Mark

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Hi,
I’ve had rough running issues similar with different engines and tracked the fault to the spark plug leads having a high resistance. Use a multimeter to check the resistance of each lead, normally below 12,000 ohms, but above that the coils have problems firing the plugs, the erratic nature of this fault ( temp, power settings, altitude etc) runs well at idle but at WOT rough running occurs. It’s a simple check and doesn’t cost anything.
Mark

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·         Doug Smith

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Hi Guillermo,
I seem to remember that some types of pod filters are designed to create swirl in the intake air - so a test flight or two without the filter is not a bad idea, provided there's not too much dust or dirt around. You might also be able to flip your hose so it points at the other side of the engine - if the symptoms change it will tell us that we're looking in the right spot.
We do made a Cobra Head duct for installations where there's not much room - your best bet is to contact your local agent about getting one.
Regards,
Doug.

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·         Doug Smith

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Never hurts to check for cheap / simple gremlins before going deeper!
D.

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