Hello,
Following Brett Buck's suggestions, I have done the following changes to my Intrepid XL having Jett 60 with Windy's carbon composite pipe:
( Please refer also to my earlier messages in the "Engine set up tips" Section ).

1. Removed the exhaust system extension ( 3/8" Al tube ) that was attached to the DuBro silicon exhaust deflector attached to the pipe short stinger.

Positives: 20 grams less and Jett seems to be much happier running without this restriction.
Negatives: castor mess is back.

Comment: next year I am going electric, primarily to avoid this oily mess, noise, pollution and endless tweaking of the IC engines.

2. Moved the pipe from 17.75" position to 18.75" after discovering that Jett's exhaust timing is 140 deg.

3. Removed my usual prop. having wider blades ( 12.5"x4", 2 blade Zinger cut from 14"x4" )

4. Installed 12.5"x4" 2 blades Zinger with normal or standard blade width and run the engine for 6 minutes. Max RPM were above 11,000 ( I did not pinch the fuel line so I do not know actual max RPM) but I set the RPM to 10,200 with the static thrust = 2,700 grams. Unfortunately 10,200 was 100% 2 cycle.

Effects: the model will fly too fast.

5. Removed the 12.5"x4" 2 blades Zinger and installed APC 12"x4" 2 blades that I had. Brett recommended APC 12.5"x3.75" but it would take a week to get this prop from some place in the US.

6. Run two full tanks with APC 12"x4" 2 blades on the ground an hour ago.

Results: max RPM ~ 14,000 ( no thrust measured at these RPM). RPM just below 2-4-2 ~ 10,200 with static thrust 2,200 grams. My model RTF, without fuel, weights now ~2040 grams ( APC prop is 9 grams heavier than 12.5"x4" Zinger) and these RPM look promising. 9,800 RPM gave true 2-4-2 sound but the static thrust was only 1,800 grams - probably not enough for my heavy model.

Comment: I can see clearly right now that building light is the absolute must. If my RTF Intrepid XL weighted 1,700 grams (60 oz.) I would launch it with Jett having 9,800 RPM and this would guarantee excellent lap times and good tension overhead. 2040 grams ( ~72 oz ) model MUST simply fly faster to stay in the air.

Will be flying on weekend. If I get 5.0-5.2s laps on 66' lines ( handle to the model COM ) and decent lines tension overhead, I will order APC 12.5x3.75 and fly again and again testing the pipe setup this time but with this model I will probably never get to 5.6 s lap and good overhead lines tension.

I am going to design and build something in the 60"+ wing span category using well known weight reduction techniques.
I will be aiming for the total RFT weight below 60 oz. with the following weight split: ~38 oz complete, covered structure with carbon landing gear and U-lite wheels and the rest Cobra electric motor, ESC, 4S LiPo, pusher prop and Eagle Tree logger.

Your comments and suggestions are most appreciated.

Regards,
Matt