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Air Intakes
Air intake from outside the cowl should have smooth entry and the exit from the air box should be smooth inside. Scat hose is now 2 ¼˝ (55 mm) in diameter. Build up with suitable material on exit of air box to Carby Air vent tube from Carby goes to filtered side of air box. It is extremely important that there are no tight bends in scat hoses and that scat hose onto Carby has no build up, thus blocking the 2 sense holes at the back of the Carby . We use a small glass attachment here to stop this. A small sleeve of fibre glass would do the same thing in this area under the scat hose Air Induction System This relates to those who are fitting an engine to a non-Jabiru aircraft and who don't use the Jabiru airbox. By trial and error and at times intelligent reasoning we have refined the air filter box that usually is quite straight forward to install and provides trouble free running. During these investigations, we found out quite a few facts that the Bing Carby had been trying to tell us from day 1. All air ducting with scat hoses must be as straight as possible with the absence of 90° bends. The Carby bowl vent must be vented to a static supply of air. Air filter size must be adequate. The incoming air must arrive from outside the cowl and if too much ram air results we provide a simple flapper valve to unload some pressure. All glassed in fibreglass flanges should have no sharp corners but well rounded to enhance airflow. The Carby heat arrangement is taken from a muffler muff and controlled from inside the box by a simple flapper valve. Here the hot air is split with most going through the filter and some direct to venturi. Application of Carby heat will slightly lower RPM (by about 50rpm on the 4 cylinder engine). There have been many installations when the air box has caused problems. Another area of concern is the scat hose bunched up on the inlet flange of the Carby. Note that the equivalent length of SCAT hose is enormous! Air Filter When installed in a non-Jabiru airframe sometimes incorrect sized air filters or incorrect installations of the Jabiru air box has caused problems. If a non-Jabiru air filter system is used, the size of the filter needs to be carefully considered. When conducting 50 hourly inspections a cursory inspection should be made of the filter. Evidence of grass, damp, contamination etc means replacement of the filter. If operating on grass or dirt strips air filter life is shortened considerably. Blowing out a filter will only remove some material; small particles of dust will still be impregnated in the filter element. In a dusty environment 50 - 100 hours from a filter would be the maximum. Operating from sealed runway will give longer filter life. Remember a partially clogged filter will influence fuel burn and performance. Even though a filter may look ‘clean’, it can become clogged with microscopic particles, which greatly increase the back pressure across the filter. Be careful about using non Jabiru filters. The one Jabiru uses has a stainless steel mesh on the inside to stop the filter from being sucked in to the Carby in the event of the air filter becoming waterlogged or damaged. Altimeter Recently reported was a case where the instrument kept changing its displayed altitude when the pressure had remained static. Eventally the altimeter could not be zeroed by rotating sue sub-scale knob. The problem was most likely a leaking pressure capsule. The fix was to replace altimeter. Anodising A great idea for all aluminium parts, especially brake components. It looks good, and provides a degree of surface hardening. No dimensional change since anodising etches into the surface.
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