Re: 90 hp timing
Get a manual or search past posts on synchronizing carbs and timing. This is important for proper performance and MUST be done correctly. If properly set and synchronized, if you set the timing to 28 or 30 degrees BTDC at WOT, then the idle timing should be close to but not necessarily at 0 degrees. It usually varies approximately between +2 and -2 degrees but it could be more depending upon engine condition.
A good running engine with properly set carbs, good compression, and properly synchronized will definitely fall between +2 and -2 at idle.
Unless it is excessive, gas in the airbox does not necessarily mean bad carbs. The reeds "spit" back fuel with each stroke and the airbox is designed to capture this spit back and recycle it to the bottom of the manifold. However, it would not hurt to remove the carbs and check the reeds for bent, broken, or missing petals.
There is also a condition of wet flooding due to a perforated or torn fuel pump diaphragm. This tends to happen more at low speed. SO: replacing the diaphragm and gasket is a task you should perform routinely. 5 bucks will save a lot of aggravation.
As a matter of course, you should also check the compression. This is the best indicator of engine condition. You said you did but did not give numbers.
4800 rpm is within the operating range of that engine, although on the low side of the recommendation. Unless it previously ran faster, indicating a recent problem, a prop change to 2 inches less pitch will get you to about 52-5300 and a faster plane time.
