I scratch built a .20/.25 size profile. 44 inch span 3/8 fuselage with a 1/4 tripler over 1/16 ply doublers. Set up for the FP .20 and .25 LA and a tongue muffler. I installed two .156 spray bars and needles and tried a remote nva with the stock .136 spray bar and a .156 spray bar. I have used 2 Du-Bro plastic 4 ounce tanks and I built a metal tank. 9 x 4 and 10 x 4 apc's and a 9 x 5 top flite and a 9x4 BY&O. 2/23 fuel 50/50 oil with Armor All added. From the first flight with the engine just below 2 cycle when I pull the launch rope the engine goes lean.
I note the use of the ST spraybar and I don't know what "2/23" fuel might be. But the emphasized bits above seems key - the stock muffler is mandatory to run properly, and you do not run it in a 4-stroke. I would suggest putting it back to stock, but unfortunately you can't easily do that, since it had to be drilled out to fit the oversize spraybar. You can't just put the stock spraybar back in without doing something to take up the extra space. You can't just put the stock spraybar back into the oversize hole, it will not seal up properly unless you do something else.
The stock venturi is .257. I am not sure where you got the other sizes unless someone had previously gone in and modified it. I can promise you that it will not needle properly with a *** .300 **** venturi even with an ST spraybar, it will be instant screeching lean the second it starts to move. It will be even more hopeless with a stock spraybar and oversize venturi.
If "2/23" fuel means 2% nitro and 23% oil, you are exacerbating the problem by also making it tougher to needle even though the choke area is greatly reduced. Using the ST spraybar you have knocked out about 30% of the power, but going to .300 overshot in the other direction.
Given that, I think this has nothing to do with vibration or the tank. I am not even sure what to tell you to do. But, you have what you have
IF you have an undamaged case - that is undamanged from drilling a hole in it for an ST spraybar - use that. Put in a STOCK .257 venturi and a STOCK spraybar or nozzle. Use the STOCK muffler. If that screws up the CG, then add lead to the tail to balance it. Put on a APC 9-4 prop. Use a tank that is no wider than 1 3/4" from the engineUse conventional 10% nitro, 17-20% synthetic blend fuel. If you have 15%, then use that. Start the engine, lean it out into a 2-stroke. Then continue to lean it out until the RPM peaks out, and one more click leaner causes it to "sag" lean. From the peaked out lean setting, back off 3-6 clicks until you get a distinct drop in the RPM but *still in a 2-stroke*. If it goes into a 4-stroke, that is too far, you have to launch it in a 2-stroke. Launch it, it may be too fast, but do the maneuvers anyway. If it goes over the top lean at any point, on the next flight, set it *one click* richer, fly again, repeat test. If it drops into a 4-stroke anywhere in the flight, set it *one click* leaner on the next flight, repeat test.
DON'T ANYONE ELSE DO THIS INTENTIONALLY - this is MAKING THE BEST OF A BAD SITUATION!! But... IF you only have drilled-out cases - Use the God-forsaken ST needle and the drilled out STOCK venturi, that is, .257 ID with a .153ish spraybar hole. USE A STOCK MUFFLER. Everything else is the same as above, the difference will be that it will do everything with far less power. You might be better off if you made a "25-sized" airplane, this may be to your benefit because the stock engine is likely far, far, too powerful for your airplane.
If there is any way to get any of the engines back to stock, please do so, in particular STOCK SPRAYBAR, STOCK VENTURI, and STOCK MUFFLER so at least we know what we are dealing with. As mentioned in every previous post on the topic, these are all key and any deviation almost always ends with problems like you are having.
I am sorry you are having problems, but your experience is all too common, if you do not use the engine STOCK out of the box with ALL THE STOCK PARTS, you absolutely will have these sorts of problems. You are exacerbating it by trying to run it in a 4-stroke, which is typically not sufficient stable (and for sure isn't if you modify the engine), and if I am understanding, virtually no nitro and probably excessive oil.
Brett