RE: jett 50 problems
Your caution is wise, and thank you for writing....
A question first - did you perform the break-in proceedure on the test stand first? If so, what prop was used, what peak rpm did you observe after the 20 minute run time?
That jump you see from 15K to 18300K is the pipe staging. It is suppose to do that, and it will stage deeper onto the pipe another 1000 rpm once in flight with that prop. Find peak ground rpm (briefly), back out the needle so the engine drops about 800-1000 rpm to around 17,500, and that is where you fly it. If the needle is too sensitive to allow you to have that needle range, the prop is too big, or there is another issue to address.
Forget about "number of turns" they mean nothing. You have to go by the engine rpm and its response.
Once it gets over 18,000 ..... that is a whole new energy level. No other engine on the market comes close, that is why you have never seen other engines run into problems before. All sorts of problems can happen in the fuel system with that sort of power, and in fact the entire nose of the aircraft can react like a spring that will drive you and the engine nuts. The bubble tank will help with the fuel system. (stock tank installation is often way too tight). You also want to make sure the front end is solid.
Your next step - put the engine on a test stand. Run it there under controlled conditions. You can use the 8.5x7.5 prop that you have, and perhaps try an 8.5x7 as well. The needle should have some range after it stages up over 18000 you should be able to easily back it down 800-900 rpm without it falling off of the pipe. Find peak rpm (briefly), back off 800-1000 rpm and let it run. If the engine gets overheated and sags while finding peak rpm, back the needle out 1/2 a turn, shut it down, let it cool 15 minutes, and then run it again (no good data from a hot engine). Note that peak rpm and the flight-rich rpm. Note the appx needle position. Note the sound of the engine (should have an audible rich crackle to it. The break in is to help the engine seat in perfectly, and also so you can learn how the engine responds and what it sounds like when running properly This can only be performed and learned properly on a solid test stand.
If you see the engine perform and respond significantly better on the test stand, then you have isolated the problem to something in the aircraft.
The regular fuel tank can work, but no part of the tank may in any way touch the airframe. Sounds like it is padded ok, but double check. One point of contact, and it will buzz the fuel.
If there is a removable hatch on the Viper fuel compartment, it must be solidly glued in place to make a solid box structure on the nose of the plane. Screws or tape will not cut it. Also helps to add triangle stock inside of nose section of the plane too to make it more rigid.
If you can, post a few photos of the engine installation, and anything else you think might be helpful
Bob