This is a question for Bill Baxter. I am posting it here based on recommendation from Gray Beard from the Beginners forum

I have a 75ax with a MAS K series 14x8 prop in a GP Reactor 46 for a couple of weeks now. The engine details are as follows

1) This is an old engine which was used on UCD60 a couple of years back. On that airplane, the tank centre was nearly 2" above the carb and I was never able to run the 75 properly.
2) We never actually did a full break-in for the 75. We ran it rich for a the first 3 tanks of flying. The engine wasnt loaded heavily in that period as that airplane was mainly 3D funfly and my first 3D airplane so I didnt do too much nose-up flying while I was learning the airplane. But we never could tune the engine properly, probably due to the tank height difference and I had to put the engine into storage. ( I have now lowered the tank and installed another engine with full break-in on that airplane and now it flies OK)
3) Before storing, I thoroughly washed the engine by immersing in plain alcohol for a couple of nights (with the o-ring parts removed) and then thoroughly oiling with light machine oil. I have never opened up the engine, only removed backplate, carb and plug befor immersion. Everything is tight
4) The fuel I had used then was 20% castor-methanol with no nitro.

Now coming back to the present

1) I had to install the 75 on the Reactor as my earlier 55ax did not have sufficient power for hovering this 2.8kg airplane
2) Started with a MAS K series 14x4 but we quickly realised that the engine was over-revving so put in a 14x8. Prop and spinner are balanced
3) Having opened both needles during the washing, I set the LSN at 1.7 turns out from fully closed carb and HSN at 3 turns. Then I tuned the HSN by ear at full throttle, it turned out to peak at about 2.5 turns, set it a quarter turn from peak for flying. (For the LSN I used the pinch test- when the fuel line is pinched it bains RPM for a few seconds before beginning to struggle)
4) When flying basic pattern without a cowl couple of weeks back it behaved OK but a bit rich. I didnt do too much hovering then, just some installation flights
5) This last Saturday, I fitted the cowl and flew (The cowl already has a lot of cooling openings, both inlet and exhaust. Any more and there wont be any cowl left). It kept losing power in the vertical and finally cut and I had to deadstick. When I tried to tune at full throttle and then came back to idle, it immediately cut. It was also extremely hot, you could feel the radiant heat just sitting in front of the engine and inspite of a breeze. I also got black oil in the exhaust and compression was very soft when hot, you could turn it over by hand. Compression returned as it cooled
6) Fearing overheating, therefore, on Sunday, I again flew without cowl and tuned the HSN by holding the model vertical. It tuned at about 3.2 turns out. It still lost power in vertical but didnt cut
7) Then I again ran the engine on the ground alternating between 4 turns out to 2.8 turns out for about 2 minutes. As before, it cut as soon as I idled, the compression was soft, got black oil in the exhaust and engine was very hot

The fuel I am using is Klotz 100 (80-20 synthetic castor mix) with 10% nitro (3.5l methanol + 500ml nitro + 1 bottle i.e. 1l Klotz 100). New OS #8 plug. Tank is at same height as carb

Note that when I put in the 75 it was because I was having exactly the same problems with the 55 on the same airplane with the same batch of fuel earlier. (On the 55 I had used all props- 12x4, 12x6, 12x8, 13x4, 13x6 with the same problems before I gave up on it)

Another small piece of Info- when I put the 75 ax together after washing, I accidentally over tigntened one of the carb screws and it has stripped. It wont fall out but wont stop turning when you try to tighten. I have currently put it with some teflon tape. The O-ring on the carb neck is OK and there shouldnt be any leakage as the engine cuts off when the baffle closes fully

Whew, that was quite a bit of information in one go!!!

Bill, can you help me understand and resolve my problem?

Ameyam