912UL - 500 RPM Vanished
in 912 / 914 Technical Questions
8 years ago
My 912 will only reach 4,400 RPM static, 4,900 in flight. It used to be around 4,900 static and 5,500 in flight. I wish I could identify exactly what changed before this happened, but I can't because several things happened about the same time, including:
- carb overhaul by a well-known Rotax shop (new needles, diaphragms, floats, rubber, etc.)
- coolant & coolant hose replacement
- replaced one choke cover on carb when I broke off one of the stops. New one installed with dimple toward the cable end.
- fuel shut-off knob failed and left valve in partially open position, resulting in rough engine at full throttle
- the problem could be coincidental to all the above
Prop pitch has not changed.
What we've tried so far...
- carbs balanced, starts easy, 1,800 idle, smooth at all throttle settings. Throttle arms go full forward, butterflies are horizontal (or very slightly past). Chokes travel full on/off.
- jets: 158, clear, no obstructions
- needles: replaced at overhaul, installed 2nd from bottom notch, slides operate smoothly
- diaphragms: replaced at overhaul, no tears or wrinkles, seated properly, tabs in correct positions
- floats: replaced at overhaul, slide smoothly, haven't checked the height
- no fuel overflow from carb vent lines
- .3 bar fuel pressure, electric pump provides 15 gph flow rate at carbs (tested with one line disconnected at a time) Engines runs smooth with or without electric pump on. Minor fuel pressure drop with electric off.
- replaced carb sockets, checked for leaks with can of starter fluid and brave friend behind the spinning prop
- gascolator clean
- 100LL clean
- plugs look normal to lean
- muffler checked for blockage, using both bore scope and vacuum cleaner
- air filters clean
- oil level normal
- coolant level normal
- 150 - 200 rpm mag drop at 4,000
- tach reading verified using optical tach
- EGT's 1,200 at 4,000 RPM, front two cylinders. I don't know if this is different than before, I've usually just monitored them for differences and they've always seems to read low, likely due to placement.
- compression seems normal (pulling prop through by hand)
- WOT RPM decreases 50-75 RPM as engine temps go up (but within normal ranges)
Any theories and advice would be greatly appreciated!
One thing that seems like a clue, but I haven't put a finger on it... the max 4,400 RPM is reached at about 3/4 throttle. Any additional throttle after that has no effect on the RPM.
Thanks
Brad Kramer
Allegro N221FA
Y19 - Mandan, ND
- carb overhaul by a well-known Rotax shop (new needles, diaphragms, floats, rubber, etc.)
- coolant & coolant hose replacement
- replaced one choke cover on carb when I broke off one of the stops. New one installed with dimple toward the cable end.
- fuel shut-off knob failed and left valve in partially open position, resulting in rough engine at full throttle
- the problem could be coincidental to all the above
Prop pitch has not changed.
What we've tried so far...
- carbs balanced, starts easy, 1,800 idle, smooth at all throttle settings. Throttle arms go full forward, butterflies are horizontal (or very slightly past). Chokes travel full on/off.
- jets: 158, clear, no obstructions
- needles: replaced at overhaul, installed 2nd from bottom notch, slides operate smoothly
- diaphragms: replaced at overhaul, no tears or wrinkles, seated properly, tabs in correct positions
- floats: replaced at overhaul, slide smoothly, haven't checked the height
- no fuel overflow from carb vent lines
- .3 bar fuel pressure, electric pump provides 15 gph flow rate at carbs (tested with one line disconnected at a time) Engines runs smooth with or without electric pump on. Minor fuel pressure drop with electric off.
- replaced carb sockets, checked for leaks with can of starter fluid and brave friend behind the spinning prop
- gascolator clean
- 100LL clean
- plugs look normal to lean
- muffler checked for blockage, using both bore scope and vacuum cleaner
- air filters clean
- oil level normal
- coolant level normal
- 150 - 200 rpm mag drop at 4,000
- tach reading verified using optical tach
- EGT's 1,200 at 4,000 RPM, front two cylinders. I don't know if this is different than before, I've usually just monitored them for differences and they've always seems to read low, likely due to placement.
- compression seems normal (pulling prop through by hand)
- WOT RPM decreases 50-75 RPM as engine temps go up (but within normal ranges)
Any theories and advice would be greatly appreciated!
One thing that seems like a clue, but I haven't put a finger on it... the max 4,400 RPM is reached at about 3/4 throttle. Any additional throttle after that has no effect on the RPM.
Thanks
Brad Kramer
Allegro N221FA
Y19 - Mandan, ND
