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TT .91F Pre-Ignition "Girgling sound while flying"

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TT .91F Pre-Ignition "Girgling sound while flying"

Old 11-09-2007, 03:27 PM
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Basil Yousif
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Default TT .91F Pre-Ignition "Girgling sound while flying"

I have a Thunder Tiger .91 Four Stroke engine which has a strainge sound while flying. It's a kind of Pre-ignition gurgling type sound. This engine has been gone through at the factory and the cylinder and piston have been replaced but still the sound whiile flying. Strainge thing is you don't hear it on the ground when you run the engine, the engine just hunts a bit on high throttle. Also it looses power when it makes the sound. The engine has only 5 gallons of fuel through it I can't see the cam being worn out but who knows.
Old 11-09-2007, 03:43 PM
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RhyanO
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Default RE: TT .91F Pre-Ignition "Girgling sound while flying"

Pre-Ignition is a sign that you are running your engine too lean.
As with all engine problems start with the fuel tank, make sure all fuel tubing (inside too) does not have any holes in it.
Make sure the carb. doesn't have any dirt in it. Check the carb o-ring. Re-Tighten all fasteners on the engine.

Basically your trying to eliminate all possibilities of possible problems with the engine.

Best of luck,
Ryan

Old 11-09-2007, 04:49 PM
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Default RE: TT .91F Pre-Ignition "Girgling sound while flying"

Are you using the original factory muffler for this engine or an aftermarket type?

A muffler without enough internal volume will cause high combustion temperatures and lead to detonation
Old 11-09-2007, 05:50 PM
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Basil Yousif
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Default RE: TT .91F Pre-Ignition "Girgling sound while flying"

The muffler might have got gummed up with caster it's all brown on the outside before I put some gasket seal around the threads but you can blow through the muffler with no problem, there's no restriction inside.
Old 11-09-2007, 10:00 PM
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Default RE: TT .91F Pre-Ignition "Girgling sound while flying"

I ran across one once that just about drove me nuts. It had the rattle sound and lost power in flight. It turned out to be a loose baffle inside the Thunder Tiger 91 four stroke muffler. The baffle would shift around inside the muffler and almost completely block off the exhaust. The cure was removal of the baffle with a needle nose plier out the end of the muffler which made the engine a little louder, but with more consistant power. The other fix would have been a new stock muffler.
Old 11-10-2007, 09:57 AM
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Default RE: TT .91F Pre-Ignition "Girgling sound while flying"

Are you running the choke adaptor? Without it the engine will not draw fuel well. This will make it lean out in flight. If not enough to make the engine quit, then possibly enought to make it detonate. The manual doesn't tell you this, but unless you are using a pump or regulator you need the choke adaptor.
Old 11-11-2007, 11:54 AM
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Basil Yousif
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Default RE: TT .91F Pre-Ignition "Girgling sound while flying"

I am running the choke adapter now when getting this sound but there was a period I was breaking in the engine I didn't use the choke and it ran perfectly no noise or nothing. I put the engine on a .60 size H9 Uglystick to break it in. Sticks are perfect for breaking in engines as the engine runs in the open air with no cowel. Now it's in a World Model .60 Zero. Could the fiberglass fuseloge be making a resinating sound.
Old 11-11-2007, 01:30 PM
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Default RE: TT .91F Pre-Ignition "Girgling sound while flying"

Yes, absolutely.

My first Senior Falcon used to go into mechanical resonance at mid throttle. It shook me up the first time that I heard it and it used to scare the pants off some of the guys in the R/C Club ("That's what you get for running a .58 size engine on a model designed for a .45 at the most!").

After a while, I used to fly it in that range on purpose just to make the sound. It used to steal away the attention of the crowd during our annual open house. Used to tick some of the hot shots off that were doing their best to wow the audience with their aerobatics. I used to do my best to time my flights when this one particular show-off was flying. He would have punched me if he could have gotten away with it. <G>

I was a whole 23 or 24 years old and just out of the military and in excellent physical condition. He wouldn't have stood a chance of getting away with it. I got a kick out of how burnt up he used to get. Looking back on it, had I to do it again, I would have stayed grounded and let the fellow bask in his glory. He was a good guy. I was a wiseass, or so I've been told.


Ed Cregger
Old 11-12-2007, 06:33 PM
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Basil Yousif
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Default RE: TT .91F Pre-Ignition "Girgling sound while flying"

I like planes with weard nioses too but when this happens the plane looses power. It can stay flying but at full throttle it only gives you the power you would get from around 1/4 open throttle if the engine was running correctly. It would be good to get some kind of Cam diagram which tells you the where the position of each valve should be as you rotate the prop all the way through the cycles. That way if the cam has a defective spot you can find it. All they give you is a point 1/4 turn after the compression cycle where you can make your valve adjustment but nothing more to check the liniarity of the cam movement.
Old 11-12-2007, 06:57 PM
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Default RE: TT .91F Pre-Ignition "Girgling sound while flying"

If the cams are worn it will be visible by inspecting the cam visually. What size prop are you running? What RPM?
Old 11-12-2007, 10:30 PM
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Basil Yousif
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Default RE: TT .91F Pre-Ignition "Girgling sound while flying"

I am running a 14x6 Master Airscrew prop it gets up to 1100 to 1200 rpm on the ground. i don't know what it runs at when it's in the air and starts making the pre-ignition sound. I am guessing around 4k to 5k rpm. Seems like it gets up in the air then starts to act up. The cowel is on over the engine.
Old 11-13-2007, 08:41 AM
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Default RE: TT .91F Pre-Ignition "Girgling sound while flying"

If the RPM is dropping from 11-12,000 to 4-5,000 at full throttle something is seriously wrong. Typically when you have detonation RPM will drop just a little. If detonation was causing it to drop that low the engine would have serious damage at this point.

I would try richening a few clicks at a time to see if the problem goes away. This might point to a fuel system problem. I think you will find your problem here.

Stop worrying about the cams. You don't get a cam diagram for a good reason, it's not needed. If it makes full power at times chances are the cams are fine. Have you checked the valve lash (clearance)?

After it happens in flight and you land, does the problem occur on the ground immediately after landing?

Does the cooling arrangement work?
Old 11-13-2007, 01:35 PM
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Default RE: TT .91F Pre-Ignition "Girgling sound while flying"

Hi!
First of all your running a bad prop!!
MA is the worst prop available when it comes to pruducing power. Get an APC 14x6!
Second it seams to me you are setting your engine too lean.
Use a 10-14 oz tank and never use any fuel valves!!! Preferably run a 2 clunk tank (Uniflow principle or Chicken-hopper tank to you Cl guys)
Run fuel containing 5-15% nitro.
Use an OS F plug.
Old 11-13-2007, 05:23 PM
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Default RE: TT .91F Pre-Ignition "Girgling sound while flying"

Basil:

I am currently running two (2) Thunder Tiger .91FS engines both in cowled installations. I have run one for 10 years and the other for six years. I agree, if you are running 11,000 - 12,000
on the ground then it does sound as if you are running too lean. I run both of mine in the 10,300 - 10,600 range on the ground so I would richen the mixture and try for the mid-10's on the
ground.

My specs are as follows:

I run the stock muffler on both.
I run MA 14x6 props on both and they work just fine.
I run Fox Miracle plugs in both and they work just fine.
I run 10% nitro, 20% oil (80/20 Synthetic/Castor) and it works just fine.
I run a single 14oz fuel tank.

These engines do not like high nitro so 5-15% as Jaka said is where you should be. I ran 15% for a year but could not tell any
significant difference so I switched back to 10% as that is what I run in all my engines (2 and 4 stroke).

The Fox Miracle plug is a fairly hot plug (their new 4-stroke is suppose to be hotter according to Charles at Fox). The OS-F plug a Mildly-hot
plug so if richening the mixture down to the mid-10's doesn't totally solve the problem you might switch to a medium or semi-hot plug and
try that. Some of those would be the OS-F, Enya #4, Fox RC-Long, or a Merlin Medium. There are others but I am familiar with these.

My experience with two TT-91's is that the ground (static) RPM should be kept below 11,000 RPM - but that is what works for me and I
have no problems with either engine. Always start, rarely dead-stick, don't "spit props".

You probably have done so already but do make sure that hour head/case bolts are tight as are the carb-to-intake bolts. They do tend
to loosen up from time to time. Also, as regular maintenance I check the valve clearances once a year (January).

I hope you get your problems sorted out as these engines do perform very well.

Traditional caveat here - "works for me - your mileage may vary".

Dan

Old 11-13-2007, 11:39 PM
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Default RE: TT .91F Pre-Ignition "Girgling sound while flying"

These engines do not like high nitro so 5-15% as Jaka said is where you should be.
They run great with 25% nitro and a K&B 4C plug.
Old 11-14-2007, 04:22 PM
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Basil Yousif
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Default RE: TT .91F Pre-Ignition "Girgling sound while flying"

Have you ever had prioblems with the Choke Going closed while flying. I was thinking about that as a cause to the problem. I did get around 10,500 - 11,000 rpm as you stated I just couldn't remeber exactly.
There is a spring that keeps it from returning to the closed position but could this be somehow closing. If it was would it be causing the loss of power and the noise.
Old 11-18-2007, 12:46 PM
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Basil Yousif
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Default RE: TT .91F Pre-Ignition "Girgling sound while flying"

Next time I fly it I'll add a rubber band pulling the choke arm the opposite direction and comming out the cowell with a stopper. At least this should put enough tention to keep the choke arm open without the possibility of going closed due to vibration.
Old 11-19-2007, 08:24 AM
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Default RE: TT .91F Pre-Ignition "Girgling sound while flying"

Basil:

I have never had a problem with the choke closing during flight - at least as far as I know.

Dan
Old 11-19-2007, 12:22 PM
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Default RE: TT .91F Pre-Ignition "Girgling sound while flying"

Nor I, the spring seems pretty strong. Bad spring?
Old 11-19-2007, 12:35 PM
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Basil Yousif
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Default RE: TT .91F Pre-Ignition "Girgling sound while flying"

The activating pushrod lightly pulls up but does return to the down position to keep the choke open position. It's most likely not the cause but an added rubber band will check. Vibration can do some strainge things. The choke has a very smooth operation making it more suseptable to vibration.

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