Strange response to carby heat

[sfGnome]

The situation: Cool clear day, 7500 ft. Engine starts to run a little rough.

 

The reaction: pull on carby heat (though not very much - I've since been told that carby heat should be all or nothing).

 

The response: Engine instantly picks up between 100 and 200 rpm.

 

I was expecting the engine to run more roughly to start with while the ice was consumed. Surprised, I turned the heat off; revs drop. Back on; revs rise.

 

Any idea what was happening? It just doesn't seem to fit in with any of my preconceived notions...

 

 

[Bruce Robbins]

Hi Gnome,

 

A side effect of carby heat is to richen the mixture. Sounds like that is what was happening. Your engine is on the lean side, especially at 7,500 and carby heat was richening the mixture enough to return it to smooth running.

 

Bruce

 

 

[Kiwi]

The situation: Cool clear day, 7500 ft. Engine starts to run a little rough.The reaction: pull on carby heat (though not very much - I've since been told that carby heat should be all or nothing).

The response: Engine instantly picks up between 100 and 200 rpm.

 

I was expecting the engine to run more roughly to start with while the ice was consumed. Surprised, I turned the heat off; revs drop. Back on; revs rise.

 

Any idea what was happening? It just doesn't seem to fit in with any of my preconceived notions...

The balance tube has come off between your air box and carby.

 

Kiwi

 

 

[old man emu]

I like Kiwi's explanation. It's simple, it allows for Murphy's Law, and I bet it comes from experience.

 

OME

 

 

[Yenn]

Kiwi's explanation sounds good to me. I had it happen on my last flight, slight roughness and carby heat gave me instant smooth power. The balance tube was adrift.

 

 

[Guest SAJabiruflyer]

That's interesting, I got told the balance tube on my Ford Wagon could be causing rough running.

 

 

[kaz3g]

An alternative thought.

 

Perhaps you had ice accumulating on your airfilter rather than in the carburettor? Could the filter have been damp from rain or a recent aeroplane wash? What were the atmospheric conditions at 7500' and what was the temperature on the ground?

 

Using carby heat means you have an alternative air source in which case the mixture would have leaned causing the engine to run at slightly higher rpm.

 

kaz

 

 

[old man emu]

Kaz,

 

One of the first things to learn about vehicles of all sorts is that it is rarely a big thing that causes a vehicle to misbehave. It's always something dead simple, and usually cheap to fix. Didn't Peter Brock fail to finish at Bathurst one year because of a twenty cent O-ring?

 

OME

 

 

[sfGnome]

Thanks for the suggestions, gents. The learning never stops... Much appreciated.

 

 

[kaz3g]

would have had to have been flying in 'visible moisture' (ie, cloud) for that to happen usually kaz.....0

Absolutely, CFI, cloud or freezing rain. But I had what I think were signs of induction icing around the filter after a wash. The filter on mine is mounted on the front of the nacelle under the prop and air intakes (like a Cub) and it obviously got quite wet from being hosed while I was washing down. If it's 12 deg on the ground and you climb to 6000, you are in air where the temp is going to be around 0 deg or less with wind chill... just a thought.

 

There is a good article on all forms of icing at http://www.aopa.org/asf/publications/sa11.pdf

 

...Didn't Peter Brock fail to finish at Bathurst one year because of a twenty cent O-ring?OME

Peter started his driving career in an Austin 7 special and so did I. Mine was a 1927 maggie ignition, 3 speed crash box, 2 seat open sports to start with. After a serious argument with a tree at a Park Orchards mud derby, it became a heavy 2 bearing, coil ignition, seriously modified 1936 model with a 4 speed box. Peter's dad owned the local garage here (now a panel shop) and his brother Lewis is our current municipal council rep.

 

When I grew up, I still messed with motor sports for a while and threw my mum's Morris Minor 1000 around hill climbs and occasional club meets at Calder. Mum never got to drive it, of course, because it was always having something done to it... carbies, head mods, valves, cam, brakes.... then I got married.

 

Speaking of brakes and your mention of little things... I was at Calder many, many years ago watching the Beech and Bob Jane terrorising a bunch of other Holden owners down the straight. Each circuit, as they got to the 75 yard mark before the big bend and the Holden drivers all began pumping their drums, Peter Manton went hurtling past them all in his Mini Cooper, only braking at the very last instant (disc brakes had just become available to the common man). Then one time his brake lights went on but the Mini didn't stop until it was buried in the grass way beyond the turn. Peter jumped out, lifted the bonnet, and put the vacuum hose back on his PBR unit! It's often the little things that count.

 

Now I'm unmarried and I have an aeroplane instead

 

kaz

 

 

Check for carburetor ice before takeoff

 

 

[Guest Howard Hughes]

It's always something dead simple, and usually cheap to fix. Didn't Peter Brock fail to finish at Bathurst one year because of a twenty cent O-ring?

The space shuttle Challenger, being another example!