Why Don't 2 stokes have Carby Heat?

[propfarmer]

Hi all,

 

just wondering why 2 stroke engines don't have/need Carby Heat. Went for a fly the other day in a Thruster with a 582 and having only done training in a Jab noticed the carby heat knob was not there. My instructor couldn't answer the question for me so i figure that someone here will know. I have a few ideas but will wait for someone with more knowledge to put me on the right track

 

Thanks for your help.

 

David

 

 

[farri]

Hi David, have a look at this....http://www.ultralightnews.com/enginetroublshooting/rotaxcarbheat.htm

 

Cheers,

 

Frank.

 

 

[facthunter]

It's a question of what you can get away with. The carbs are mounted quite close to the engine in most 2 strokes. Engines of any kind will have more icing trouble if the intake is long.

 

The later rotax's and some others don't use premix oil which is considered to aid in reducing ice formation (by some). Two strokes do not pull as high a manifold vaccuum as a four stroke does so that would help (a bit).

 

These are all factors, but in my opinion there is NO inherent reason why a two stroke would be immune from carburetter icing

 

 

[pudestcon]

What I have been told is that 582 Rotaxes are not prone to carbie icing because of the bing slide type carbies.

 

Certainly my instructor when I was getting 'endorsed' on Thrusters did not have an issue - not even warming the engine on a long decent when practising engine failures. I like to watch the temp gauges and warm the engine from time to time, but this is for a different reason than carby ice though.

 

Pud

 

 

[eastmeg2]

There was a recent article or thread on carby icing and 2 strike Rotax engines. Could have been in the Jan 2012 RAAus mag - maybe.

 

Basically the Bing carb's on the 2 stroke Rotax's use sliding barrels instead of butterfly valves which means there is very little for ice to build up on apart from the needle - something like that . . .

 

 

[fly_tornado]

jabirus also use bing carbies...

 

 

[alf jessup]

I had carby icing a number of times when I had my 2 stroke trike (oil injection), just depends on the dew point and temp on any particular day, if it's right you will get it 2 or 4 stroke depending on other factors of course.

 

Haven't come across it yet on the 912 trike in over 500 hrs, mind you it does have the permanent coolant fed heat rings, but it would not be immune either if it was serious carb icing conditions.

 

Can't really answer the question on why 2 strokes don't have it, but as someone else has said in a previous post I also heard the 2 stroke mix can help to some sorts in preventing it, whether that is true or not who knows.

 

Cheers

 

Alf

 

 

[Yenn]

Im had carby ice on a Thruster with premix fuel. It didn't stop but ran very rough. On another occasion I was flying under a low overcast into a Northerly wind and when I landed a friend pointed out the carbies were beautifully covered with white ice crystals, it had been running as normal on that occasion.

 

 

[Guest Crezzi]

2-strokes are certainly not immune from carb icing - I've had several silences caused by it with both 447 & 503 (but not 582 IIRC). To be fair all were in European winter time.

 

It is possible (though not common) to fit carb heaters - IIRC CPS (amongst others) sell them. Obviously they have to be electrical for the 447 & 503.

 

Cheers

 

John

 

 

[pudestcon]

So, would the consensus be to warm a Rotax engine on prolonged idle settings such as in a long descent? We are talking Australian conditions here, although we have a large range of climatic conditions anyway.

 

If so, would you always do this, or only when conditions are conducive to icing?

 

I remember farri telling me he has had carby icing on a Rotax - can't remember if it was a 503 or 582 though. I assume this was in the Cairns region, so quite tropical and humid conditions I expect.

 

Pud

 

 

[Guest Maj Millard]

Pud, I have had carb-ice on a dual-carb 503. Sat at cruise for a few minutes and then went to increase power but it wouldn't take it, just bogged down, had to be main-jet/needle icing...ran fine back at the cruise RPMs. Had done several flights prior on same day with no dramas, and same conditions, which were clear/no cloud, cool day around 4pm.

 

After landing no sign of any problem except for moisture on exterior of both carbs.........rare but it is possible !!....Never had any in eleven years of 582 DC /premix flying.......................................................Maj...

 

 

[Herm]

jabirus also use bing carbies...

Yes true, but not the same carbs. The Jab has a totaly diff unit than in a 582.

 

 

[Guest Maj Millard]

I believe it is possible with any carb, when the correct conditions line up. There are several types of icing IE: induction/venturi ice, fuel ice, main jet ice etc. Basically you need the right drop in temp and moisture to be present, for icing to occur. Often the evaporation of the fuel itself as it mists, causes the required drop in temps. (fuel ice)...or the pressure drop (and temps) as caused by a carb venturi, will do it also (induction ice). The moisture either comes from within the inducted air itself (humidity), or from water present in the fuel (suspended moisture). The closer the dew point is to the humidy, the more potential for icing. If your flying early in the morning and have to wipe a lot of dew off the aircraft...watch out !.

 

Problem with icing is that is is so infrequent to the point of being almost rare, and when it does occur it can creep in over several minutes, so indications are very gradual. It is often manifested by just a very slow reduction in RPMs, and it may dissapear as quickly as it arrived, causing even more confusion.

 

I've never had it yet on any 912, however I do use the carb heat often as supplied now on many ULs, when descending, prior to takeoff and landing, and when in conditions where I feel icing could occur...It is not a good idea to takeoff with carb heat selected, as carb-heat air is often unfiltered, plus the hotter air generally will also cause a reduction in available takeoff power................................Maj...

 

 

[pudestcon]

Interesting maj. It seems there is not a lot you can do in a 582 except not do long descents without warming the engine from time to time, and being aware when conditions are ripe for icing i.e. mainly humidity and cool conditions, although temperature would be a secondary consideration after humidity. I can see, around my area at least, these conditions would be present on cool winter mornings (when I like to fly) and also during that time before thunderstorms develop (sultry?), when I don't like to fly as the conditions are often unpleasant then. I also like to fly very early mornings in the middle of summer - dry as a wooden god, so very little risk then of carby icing I imagine.

 

Pud

 

 

[flyerme]

you can get carb heat kits for 582 ,bing 54 carbies..as above if the conditions are right you can have icing on any carbied engine,slide or butterfly,2st or 4,

 

I installed carbie silences to my old 447,(factory made),they are alloy tubes that goes on the intake side of carbie with air filter attatched to end of them,so looks like an extension for the airfilter,first test flight/taxi,was a cold dewy morning and after warm up I taxied fo sum run ups all good,gave it full throttle and just on round out she died slightly,I aborted take off and came back and removed air filters to find both ICE and Moiture inside the silencers.I removed the silences ,cleaned carbies,put airfilters back on and never happened again...stick your rotax silencers thanks..

 

 

[Keenaviator]

I had a Pegasus Q trike with Rotax 462. A factory option was electric carby heat. This was an aluminium collar heating element that surrounded the throat of the carby.

 

 

[Guest Maj Millard]

Shouldn't even have a problem when descending with a 582 being that it is water cooled /heated. There's only around 2 lts of coolant there remember, it gets circulated pretty often so should keep everything pretty warmish................Maj...