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