Posted Saturday, March 17, 2012 @ 03:17 PM  

1. Carb icing
2. Ignition malfunction
3. Fuel starvation

At first, because of the weather conditions at the time, and the liquid in the manifold, we suspected carb icing. The first time the aircraft had been warming up on damp grass for some time. Venturi theory indicates that increasing the engine capacity (i.e. mass flow) without increasing the bore of the carb results in a larger pressure drop at the venturi and therefore makes susceptibility to icing more likely. However, all but one of the people I’ve corresponded with have said that carb icing on Fourniers is more or less unheard off, even with the Zenith 28 RZX carb (28mm throat) on bigger engines. Patrick says that the original Limbach 1700 used this carb with no declared icing problems before changing to the Stromberg 150CD.

The only person who had a strong opinion in favour of carb icing was Jean-Paul Dalegre, who operates from Ussel in the French Massive Centrale, at 2500ft altitude. He has two RF4’s, one 1200cc and the other 1400cc. On the 1400cc aircraft he suffered engine failure after takeoff causing a crash landing. His subsequent investigations with temperature probes downstream of the venturi indicate (if I read his english correctly) a temperature drop of 20 deg C greater on the 1400cc engine compared to the 1200cc.

However, the symptoms we experienced were not classic carb icing which is power reduction and rough running (which I admit I have never knowingly experienced) and not dead cuts and full restoral of power. So although possible, I’m not convinced we experienced carb icing.

An ignition problem was possible, but the mag had a 500 hour check at same time and one would think the problem would occur more frequently. I know Patrick’s investigations have revealed mag coil problems at high temperature that are not tested in the 500 hour check, but although still possible I think a mag problem was unlikely.

That leaves fuel starvation – in fact a series of lean cuts. I had never been entirely happy with the plug colour following the upgrade, the central electrodes were nearly white, suggesting a rather lean mixture, although I have CHT’s on the rear cylinders and it always runs very cool, normally about 150 deg C, and never more than 200 in the climb.

Discussion with Vincent Pesche, who also has an RF3 uprated to 1400cc revealed that he had experienced exactly the same problem. He increased the float jet size from 2.5mm to 3.0mm and has not had a recurrence of the problem since.

Bob told me that his investigations showed that the 1400cc Rectimo conversion needs a minimum of 16 litres/hour fuel flow at full power. I measured our fuel flow at the main jet with minimum fuel in the tank (remove the jet and time a measured amount) at 15 litre/hour – ah-ha – suspicious.

First following Bob’s recent advice, I opened up the main jet to 1.30mm. This immediately resulted in a much better plug colour – light brown. Needless to say the engine ran perfectly on airtest in similar conditions to those during Amanda’s engine failure. In addition, the rate of climb seemed better. I then did some experimenting with fuel flow and float jet size. I’m not sure where Bob gets the figure of 2.0mm as original. Both the jet I removed and a new one from Eugenio are stamped 2.5mm. Anyway I drilled it out to 3.0mm, as Vincent has done, and the main jet flow rate increased to 19 litres/hour, which sounds much more comfortable.

I’ve only been able to do one test flight since the float jet increase but all seems well. The rate of climb in still conditions from 3000-4000ft was 750ft/min with a ground temp of 12 deg c. We have a 36 inch pitch by 52 inch prop. It always performed well with the 1400cc engine but it really seems to climb even better since the main jet size increase. Time will tell whether the fuel flow increase has solved the problem, but I’m very hopeful.