Smoky Lycoming o290D2

[T18]

I have a problem with my lycoming O290D2 in that everything checks out on the ground, however when flying it apparently trails black smoke and produces partial power.

The plugs appear to be fine and the mixture has been reset. The compressions are as they have been for the past three years

Any pointers on what to check next. the aircraft is a two seater on permit.

Thank you...T18

[cockney steve]

So..... is the fuel consumption heavier ? is it using more oil?

it's no good giving half the symptoms and expecting an accurate diagnosis, let-alone from miles away on the internet :}

BLACK smoke is usually exess fuel IN RELATION TO THE AMOUNT OF AIR INDUCTED.....that's important!
the carburettor/injection pump may be fine, but a collapsing air hose/restricted air filter/broken or loose sealing -strip/baffle reducing the airflow, would probably richen the mixture.

Blue smoke - oil. you'd expect a noticeable increase in consumption, black, greasy deposit in exhaust stubheavily coked plugs......
gather your evidence methodically , analyse it logically and the answer should be self-evident.

[T18]

Cockney Steve, thanks for the response, the airflow is sound..however there is a greasy black deposit on the underside of the aircraft, the oil consumption has gone up a bit. I haven't seen the smoke but have been advised that it's black..it may actually be blue.

Does this mean that the piston rings may be damaged or stuck?

I will check the plugs next visit to the airfield.

T18

[stevef]

Worn or sticking rings/valves will be reflected in compression test readings. How many hours has the engine run since overhaul/top overhaul? I'd ask for another differential compression check - it's less than an hour's job in total.
Worn valve guides can give you blue smoke, mainly noticeable on the over-run but I doubt it would account for the power loss.
If the smoke is black, I'd look at the air filter to begin with.What happens when you lean the mixture?

[NutLoose]

Check the breather isn't blocked as well... Could be numerous things, what's the colour of the oil like when it's changed, is it very black?

Guides... Rings... Glazed bores... Could be oil related

Fuel wise

Is the carb in good shape and the bolts ok holding on the float bowl, I've had one in the past drawing fuel past the gasket on an 0-235... Primer not drawing fuel through giving you an over rich mix... Or even the induction tubes loose in the sump.


Key Reprints - Lycoming (http://www.lycoming.com/support/tips-advice/key-reprints/)

[T18]

Hi Nutloose,
your comment on glazed bores strikes a cord. The top end was done not so many hours ago and I wonder if I may have been too kind when running-in the engine.

The oil seems to get dirty very quickly, I should add that the carb was rebuilt recently.

I will investigate further. Many thanks to all.
T18

[cockney steve]

Well, you're a lot nearer a diagnosis now:)
Very good suggestions there.....Worn valve-guides,- usually inlets, they're below atmospheric pressure at the induction-tract, so oil tends to be "sucked" down the guide,most gets carried into the induced charge(the fuel will tend to wash some of it off) and some will build oily carbon on the back of the head and stem....in extremis it can restrict the induction-flow so dramatically the engine will not rev or produce useful power! Exhaust , being pressurised, tends to push the oil back up the guide ,into the rocker-cover,whence it came.
Stuck rings-usually reduced compression and heavy oil consumption.Again, the state of the plugs will give a good secondary pointer to the likelihood of this. Rings can be successfully freed, cleaned and reinstalled,once the piston has the grooves cleaned and any drain-holes in the skirt/grooves cleaned.

@ NutLoose Good call:ok: Modern car engines, with positive crankcase scavengong, make one overlook the old "vent over the side" crankcase-breather.....of course, if either system suffers a blockage sufficient to pressurise the crankcase, oil will tend to be pushed out of crank main seals, rocker-covers, pushrod tubes etc as well as tending to get pushed up the bores as well.

OP, yes! if you've re-ringed and not glaze-busted, it's possible to get rings/bores which won't bed in to each other....compression losses and high oil consumption and, of course, with the excess of lubricant, it makes the "wearing-in " even more unlikely.

[T18]

Good morning Nut loose,

I believe it's pointing to glazed barrels, I didn't realise you could free up the oil rings so that's handy.........what are you doing saturday?:rolleyes:

I will keep you posted on the outcome.

Many thanks,
T18

[NutLoose]

Wrong end of the country :), when they did the top end what did they do?

[easaman]

Marvel Mystery Oil (16oz in the gas tank) helps to free the oil/piston rings!
Another more rough method is MEK in the air intake!
Cheers easaman