tt f-91s backfiring
07-01-2003, 03:12 PM
#1
Member
Thread Starter
Join Date: Oct 2002
Location: RESTON/, VA
Posts: 65
Likes: 0
Received 0 Likes
on
0 Posts
tt f-91s backfiring
Yesterday I lost my prop 2 times on the ground, and finally one time during flight. Propeller, prop wash, prop nut, everything spitted out of the airplane which was luckily high in the sky, so I could manage to land safely. (the sight of these part flying away like a rocket was kind of fun, though!). Bad backfiring case. The engine run fine till 1/2 throttle, clunky noises after 1/2 throttle, the the prop that went bye bye.
Some specifications:
yesterday the weather was extremely humid and hot.. Never seen such a humid warm weather in a long time.
The main needle was 3 1/2 turns out. Before, it run perfectly like that.
Wooden prop 14 x 6. Used this size for the first time.
10% nitro fuel. I used this % for the first time.
Before to open the engine and check the timing, can someone tell me if the backfiring (and consequently too lean carb) was caused by one of these factors? High humidity in the air, 14x6 prop, fuel?
All together? :stupid:
Some specifications:
yesterday the weather was extremely humid and hot.. Never seen such a humid warm weather in a long time.
The main needle was 3 1/2 turns out. Before, it run perfectly like that.
Wooden prop 14 x 6. Used this size for the first time.
10% nitro fuel. I used this % for the first time.
Before to open the engine and check the timing, can someone tell me if the backfiring (and consequently too lean carb) was caused by one of these factors? High humidity in the air, 14x6 prop, fuel?
All together? :stupid:
07-01-2003, 03:43 PM
#2
Senior Member
Join Date: Dec 2002
Location: Jasper,
AL
Posts: 837
Likes: 0
Received 0 Likes
on
0 Posts
tt f-91s backfiring
"3-1/2 turns" may be meaningless; you are running lean. Using a tachometer, peak the rpm at full throttle and richen the mixture to give a 200-400 rpm drop from peak.
You should be using the factory-specified double jam nut on the prop, this is designed to not loosen in case of a backfire. Imagine how much damage the flying prop can do!
--Bill
You should be using the factory-specified double jam nut on the prop, this is designed to not loosen in case of a backfire. Imagine how much damage the flying prop can do!
--Bill
07-01-2003, 04:40 PM
#4
Senior Member
Join Date: Dec 2002
Location: Alpharetta,
GA
Posts: 792
Likes: 0
Received 0 Likes
on
0 Posts
tt f-91s backfiring
Like Bill says it not where the needle is, all motors are slightly different. My TT 91 is also at around 3 1/2 to 4 but last week when the weather warmed up to around 90 with high humidity it took a full 1/4 turn richer to compensate for the Temp change.
You were running lean that clunk(or cracking) noise you heard is a lean indicator YOU CAN also get this type of lean out over time when the valve clearances start to get to sloppy. Adjust the valve richen her up and you will be back in business.
You were running lean that clunk(or cracking) noise you heard is a lean indicator YOU CAN also get this type of lean out over time when the valve clearances start to get to sloppy. Adjust the valve richen her up and you will be back in business.
07-01-2003, 05:33 PM
#5
My Feedback: (8)
Join Date: Dec 2001
Location: Colorado Springs,
CO
Posts: 1,359
Likes: 0
Received 2 Likes
on
2 Posts
tt f-91s backfiring
RC Myth #1: You can set and forget your needle valves.
Not so. Our carbs do not compensate for changes in temperature, barometric pressure, and humidity. Any changes in these conditions require as a minimum the verification of your mixture setting and adjustment if necessary. This is done not only to ensure you don't accidently run your engine lean (as in this case) but also to ensure your engine is running at its peak performance. For example if you tune your engine on a hot summer day and the next time you fly it is much cooler and drier in the evening, you may not be realizing your engine's full potential to produce more power because you'd probably be too rich and vice versa.
Most people who do set and forget the needle valves are probably set richer than needed for the conditions and when temps, density altitude increase, they are still ok. However at the cooler temps their engine isn't running near what it could produce safely.
For the aforementioned reasons and situations, it is a disservice to tell someone what their optimum engine needle settings are. All you can do is give someone a ball park figure which they will need to adjust in order to optimize performance.
Andy
PS In this particular situation in response to the original post, the full needle valve for most engines takes effect at 1/2 throttle and above. The fact that your engine backfired (due to a lean condition) above 1/2 throttle indicates your high speed mixture setting was too lean. The best thing you can do is purchase a tach and for your first flight start your engine, but make sure needle valve is set about 5 turns out as a rich starting point first and than while taching engine slowly lean hs needle valve until engine is at maximum rpm at full throttle (called max lean rpm). Once at this point, note rpm and richen mixture so that rpm drops 400 rpm. This should get you buy for the rest of your day's flying unless you experience significant increases in temperature and barometric pressure while you are out there. Note: It costs nothing but 30 seconds of your time to check your rpm and ensure you are running slightly rich at least every other flight. A little prevention and safety could save you from an inopportune deadstick that could end up being expen$ive.
Not so. Our carbs do not compensate for changes in temperature, barometric pressure, and humidity. Any changes in these conditions require as a minimum the verification of your mixture setting and adjustment if necessary. This is done not only to ensure you don't accidently run your engine lean (as in this case) but also to ensure your engine is running at its peak performance. For example if you tune your engine on a hot summer day and the next time you fly it is much cooler and drier in the evening, you may not be realizing your engine's full potential to produce more power because you'd probably be too rich and vice versa.
Most people who do set and forget the needle valves are probably set richer than needed for the conditions and when temps, density altitude increase, they are still ok. However at the cooler temps their engine isn't running near what it could produce safely.
For the aforementioned reasons and situations, it is a disservice to tell someone what their optimum engine needle settings are. All you can do is give someone a ball park figure which they will need to adjust in order to optimize performance.
Andy
PS In this particular situation in response to the original post, the full needle valve for most engines takes effect at 1/2 throttle and above. The fact that your engine backfired (due to a lean condition) above 1/2 throttle indicates your high speed mixture setting was too lean. The best thing you can do is purchase a tach and for your first flight start your engine, but make sure needle valve is set about 5 turns out as a rich starting point first and than while taching engine slowly lean hs needle valve until engine is at maximum rpm at full throttle (called max lean rpm). Once at this point, note rpm and richen mixture so that rpm drops 400 rpm. This should get you buy for the rest of your day's flying unless you experience significant increases in temperature and barometric pressure while you are out there. Note: It costs nothing but 30 seconds of your time to check your rpm and ensure you are running slightly rich at least every other flight. A little prevention and safety could save you from an inopportune deadstick that could end up being expen$ive.
