Kafin
I run an Evolution 36 on my P40 ARF w/ MAS 11-4 prop. The EVO 36 has roughly the same power as an OS 46LA. And I am running 60' 018 lines. And I typically run 10% nitro and 20% oil on my more modern engines. My ground rpm is around 11,500 on the MAS 11-4. (BTW, some have said the MAS props generally perform as if they had less pitch. In other words, a MAS 11-4 probably performs more like a typical 11-3 as an example. Not sure but maybe they flex under load relaxing the static pitch.)
I can't speak to the black oily residue or the health of your particular 46LA engine. But my description below assumes a 'good running' LA engine.
You also have an 'after-market' venturi and NVA. That setup, not being OS stock, might be contributing to some engine run issues, but not sure. If the venturi is too large for instance, that could contribute to an engine run or performance issue.
When I have a new model/engine combination, I don't start with a particular ground rpm. It is actually the last thing I just document for reference after I get the engine/model setup running how I like.
I have run all the various size OS LA engines. They all have worked great for me totally stock. I use the stock tube muffler with the internal baffle installed. And I run muffler pressure on the LA's.
I always start with a 4-pitch prop on a new model, usually an APC sport prop. And I start with a prop diameter that suits the model size. In the case of the Brodak P40 ARF with 46LA, I would start with a 11-4.
The LA engines like to run fast for best performance. I personally don't set the needle to try to get a 4-2-4 break on the LA's. Just like the OS owner's manual says, I find peak rpm on the ground (i.e.: if I go any leaner the speed starts to fall off). Then, just as the owner's manual says, I open (richen) maybe 30 deg or so just when I notice the rpm to just start to decrease a little. OS calls this the 'optimum' engine speed. Then I fly the model.
If the model is flying too fast for my liking, I might try a 3-pitch prop. If too slow, a 5-pitch, etc. Totally trial and error. I even try different manufacturers props. MAS, BYO, etc. until I find the prop that flies the model best. In the case of my P40, I ended up liking the MAS 11-4 best.
Then to adjust lap speed, I might play with the line length. (I might also tweak the rpm just a bit, but not more than maybe 200 rpm either way. I still like a relatively fast 2-cycle on the LA's where they seem to perform best based on my experience.
When trying different props, I still set the needle again to 30 deg or so off of peak rpm on the LA's. The ground rpm will be different for each prop I try.
Once I finalize the engine/prop combination I finally document the ground rpm so next time I fly I know were to set the needle.
There are always some tweaks on the above, but this is the basic procedure I have used on any new model/engine combination. It has worked well for me to date.
Also BTW, I run a 46LA on my heavy Vector 40 (62 oz). After numerous prop tries, I ended up with a wood BYO 12-3 which seems to pull the model best. The BYO design is a very wide prop. And I fly the model on 64' lines. My ground rpm is right at 10,000.