Specific carburetion issues
08
I had a suggestion from a pilot to write about this subject as he has used what I told him many times and thought it would be very informative for other pilots.
I am talking about two specific aircraft, a Cessna 177B Cardinal and a Beech C23 Sundowner. Both aircraft had Lycoming 0-360 engines and both have the HA-6 carburettor. The HA-6 is commonly known as a ‘pressure carburettor’ and is a side draught carburettor as opposed to the more common up draught carburettor found on most Lycoming engines.
The Cardinal had just been sold and the new owner reported that the engine misfired after take-off, but the previous owner had not experienced any similar problems. We duly investigated and ran the engine and found nothing wrong. Then went for a flight with the previous owner and found nothing wrong. The new owner went flying again and came back with the same problem, but we did not have the opportunity to fly with the new owner to actually experience the problem.
So, whilst we were scratching our heads about this, another customer came in and told us he had just acquired a Sundowner and the engine also misfired after take-off. Same situation as the Cardinal where we could not fault the plane on ground runs. The owner suggested I go flying with him to witness the problem, which I duly did.
So, here is what happened. At the holding point the owner did his pre-take-off checks, setting RPM for magneto checks and leaned the mixture. With the magneto checks complete and now ready for takeoff, the mixture was still very lean. On take-off roll the RPM started building up since this is a fixed pitch propeller, but the mixture was still very lean. At rotation speed the RPM is up around 2450 and as the plane lifted off the engine started misfiring. My immediate response is to shove mixture control to full rich and the misfiring stopped. “What did you do” asked the owner, but the answer was obvious.
Back at the hangar, I discussed this situation with the owner who told me that he was taught to lean the mixture to peak and then back in a bit, do undertake magneto checks and then take-off without changing mixture. “On this plane” I asked. “No, I learnt to fly on a Piper Cherokees and Cessna 172s” neither of which has HA-6 carburettor.
Now I am sure there are people who may disagree with me and all I am saying is, this is has been my experience of HA-6 carburettors. Over many years, I have probably ground run hundreds of light piston engine aircraft and in my opinion the Marvel Schebler up draught carburettor is very forgiving as far as the mixture is concerned. One can usually lean mixture way back before the
engine cuts. Typically, on a ‘pull to lean’ mixture control knob the shaft will come out a few inches without any response from the engine and then finally a response, with wanting to cut almost at the end of the travel. The same applies for quadrant type mixture levers which can be pulled back quite a way before the engine will want to cut.
However, the HA-6 carburettor can usually only be pulled back half to one inch, with immediate response from engine to leaning and then a sudden cut. So, the mixture leaning procedure being taught in a Cherokee 140/160/180 or Cessna 172 is completely different from the procedure required for a plane equipped with a HA -6 carburettor. I have spoken to many owners / pilots who have confirmed this phenomenon.
Typical of a Piper Cherokee or Cessna 172 to lean mixture to peak and then set the mixture slightly back, magneto check and take-off with mixture as set is standard procedure. However, the HA-6 carburettor will invariably not allow this. Therefore, if the RPM is set at 1800 and the mixture is set for best fuel / air ratio at that RPM, it would make sense that as the RPM increases the fuel / air ratio needs to increase as well. However, the mixture has been set for a lower RPM and the increased airflow into engine is not matched by an increase in fuel. Why the Marvel Schebler carburettors are more forgiving can probably be answered by a carburettor specialist, which I am not.
After the flight in the Sundowner I called the owner of the Cardinal and asked how he set mixture for pre take-off checks. His answer was the same as the Sundowner owner. I suggested he try full rich for take-off, which he did and reported that he did not encounter the same problem again.
In conclusion, not all aircraft carburettors are the same. Applying standard procedure taught during PPL training could be hazardous when operating an uncommon aircraft. The Pilot Operators Handbook (POH) should provide the correct information. Good practice would be to check with your maintenance organisation that will hopefully have product knowledge of your aircraft, or at least tell you that the carburettor is unusual.