Dear Forumites
I have the exact same problem on my 912 UL (980 TTSN) installed on a Hawk Arrow 2 (0 hrs). Refer to my EGT records which clearly show that something is very wrong, but it doesn't say why. Note that the thermocouple on cylinder 2 reads high, I'm ASSUming that it closely follows the EGT of cylinder 4, but remains operational when combustion in cylinder 4 shuts down.
These are the symptoms:
1) It feels like the engine is missing that final kick on full throttle. I'm generally getting a rate of climb of about 600 fpm from 4,300 AGL at say 82°F OAT. Other Hawk pilots say I should be getting 1,000fpm. So I'm low on power.
2) The engine only revs to max 5,400 rpm at an IAS of 80 mph. I have a new 2 bladed wooden prop, 73", 39° pitch, which was designed and supplied by someone who has been in the game long enough. The design should put the revs at 5,600/5,700 at 80mph. Another indication that I'm low on power.
3) The spark plugs on cylinders 1 and 3 are sooty black, the spark plugs on cylinder 2 are more glossy black, the spark plugs on cylinder 4 look new. After 6 hours of mostly runway operation the insulator is as white as it left the factory (Very similar to Kevin's in post 4961)!? The spark plugs are all new and I'm running unleaded mogas.
4) With the spark plug cap removed, all 8 have a strong blue spark. I didn't check the timing.
5) The aircraft sometimes vibrates after throttling back from full power and it sounds as if the rpms aren't quite stable.
6) Ignition test rpm drops @ 4,000rpm: 1 = 250rpm, 2 = 150rpm.
7) Fuel flow maxes out at about 18l/h (4.8gph), pressure at 0.4bar, and 11l/h at slow cruise. I feel the fuel flow is low for full throttle, but I haven't calibrated the system yet, so the flow reading may be off. The fuel pressure went above normal when cylinder 4 first cut out, between 07h50 and 07h53.
8 ) Carburettor synchronization was done, but turning the idle mixture screw didn't really change the engine speed, so I left it at 1.5 turns out. I visually checked for air-leaks and can't find any.
I don't think it can be the ignition, since the two spark plugs on cylinder 4 are powered by different modules and different timing pick-ups, and any problem with either would also affect other cylinders. But if it is the carburettor, why is cylinder 2 behaving properly?
A local 912 guru is adamant that the diaphragms need replacing. On the one hand a carburettor overhaul wouldn't hurt, it would be high time anyway, and other forumites had good results with minor carb overhaulsbut the overhaul kit is quite expensive and I'd hate to fiddle with things that aren't broken.
EGT_Drop1.JPG (You do not have access to download this file.)
