Carb Ice or Something Else?

[ARFlyer]

Yesterday, I was flying a friends plane on a very nice cross country flight. I had noticed before leaving that the L mag was running rough on run up. However, after leaning it out and running the temps up the issue went away.

Well after getting fuel at our intermediate I performed another engine check with the same results on the L mag. The takeoff was extremely sluggish taking over 1500' just to get to 40 knots. The odd into was that the RPM gauge was showing full power. I aborted and ran the engine up again, which checked out with no roughness or major powerless. The second takeoff and climb out was as expected.

On our climb up to altitude I noticed that the cylinder head temp was pegged out, but the oil temp was only mid range. At cruise the slight engine roughness came back. So I pulled the carb heat thinking it was icing. As expected the engine became rough and over time improved. However, the temps were still not cooling down, so I pulled the power back to mid cruise range. At that point the engine roughness came back with vengeance. So I pull the carb heat once again, which basically killed the engine. So I turned back and landed back at our last airport.

So carb heat or something else? Even my friend is confused and he owns the plane.

 

[poadeleted20]

Your decision to turn back and land was wise. Sounds like a big fuel mixture or ignition problem, and I wouldn't fly it further without a mechanic's inspection. BTW, having the CHT's pegged out can destroy the valve guides even before the cylinder itself fails, so if this was a Lycoming engine, you might want to have a valve guide wear ("wobble") check IAW SB 388C performed.

 

[FastEddieB]

What kind of plane?

 

[ARFlyer]

Maule 180 tail wheel. Actually found a dude who has one around here!

 

[ARFlyer]

Your decision to turn back and land was wise. Sounds like a big fuel mixture or ignition problem, and I wouldn't fly it further without a mechanic's inspection. BTW, having the CHT's pegged out can destroy the valve guides even before the cylinder itself fails, so if this was a Lycoming engine, you might want to have a valve guide wear ("wobble") check IAW SB 388C performed.

MX looked at it. They said they could not find anything wrong and suspect it was just carb ice.

 

[poadeleted20]

MX looked at it. They said they could not find anything wrong and suspect it was just carb ice.

Doesn't sound like carb ice to me. I'm just not seeing carb ice happening like this with a 180 Lycoming engine, especially since pulling carb heat "basically killed the engine". That just doesn't make sense.

 

[bflynn]

Thinking it through logically - running rough supports a bad fuel mixture and high heat implies running too lean. Adding carb heat killing the engine supports the lean problem.

All cylinders or just a few? Induction leak?

What were temperature / humidity?

You're aware of Lycoming Service Instruction 1094D, which says to not lean the engine at full power?

B. LEANING THE NORMALLY ASPIRATED ENGINES.
1. Use full rich mixture during take-off or climb. Careful observation of engine temperature instruments
should be practiced to ensure limits specified in Textron Lycoming operator's manual are never
exceeded. Refer to the aircraft POH (pilot's operating handbook) or AFM (aircraft flight manual) for
more specific instructions.
2 For 5000 ft. density altitude and above or high ambient temperatures, roughness or reduction of power
may occur at full rich mixture. The mixture may be adjusted to obtain smooth engine operation. For
fixed pitch propeller, lean to maximum RPM at full throttle prior to take-off where airports are 5000 ft.
density altitude or higher. Limit operation at full throttle on the ground to a minimum. For direct drive
normally aspirated engine with a prop governor but without fuel flow or EGT, set throttle at full power
and lean mixture at maximum RPM with smooth operation of the engine as a deciding factor.
3. For cruise powers where best power mixture operation is allowed, slowly lean the mixture from full rich
to maximum power. Best power mixture operation provides the most miles per hour for a given power
setting. For engines equipped with fixed pitch propellers, gradually lean the mixture until either the
tachometer or the airspeed indicator reading peaks. For engines equipped with controllable pitch
propellers, lean until a slight increase of airspeed is noted.
4. For a given power setting, best economy mixture provides the most miles per gallon. Slowly lean the
mixture until engine operation becomes rough or until engine power rapidly diminishes as noted by an
undesirable decrease in airspeed. When either condition occurs, enrich the mixture sufficiently to
obtain an evenly firing engine or to regain most of the lost airspeed or engine RPM. Some engine
power and airspeed must be sacrificed to gain a best economy mixture setting.

NOTE
When leaned, engine roughness is caused by misfiring due to a lean fuel-air mixture which
will not support combustion. Roughness is eliminated by enriching slightly until the engine is
smooth.