It was one of the ebay $18 tapered venturi Mikuni's came with a 15 pilot and a main that said 56 on it. I bought a number of jets ranging from 100 up to 115. I measured the 115 and it was .046 so I drilled the original 56 which was very small to .051 and that ran OK initially but then started with the miss at wide open throttle getting worse as the ride went on Pulled the plug and it was black and a bit wet. I am now wondering if I have a ignition issue as I had a similar issue with the stock carb after reassembling the engine. I put a new plug in and the 115 jet but have not tried it yet. Meanwhile I ordered a new CDI to try as well. Idle is nice but I have to open the idle mixture screw at least 2.5 turns out so I am thinking of trying a 12.5 pilot as well. I have not removed the shroud to verify air gap of CDI iron to flywheel but I did set it at .0125 on assembly .
lol wheres that plug from, a top fuel car? :lol: 
I think I was not clear on my explaination, with the larger jet it took off ran real good and did not falter, then it started a little at first and then with less and less throttle. Remember when you open throttle engine sucks more air, more air to compress, higher voltage requirement to fire plug, when I got back plug was black and a bit wet. If ignition was ok and it was just rich would expect blubbering and dry sooty plug. Now the manifold is a bit long and I am also wondering if I could have a reversion issue when fuel/air is blown back and sucked in again causing it to run rich. I am going to go out and try with new plug and 115 jet.
I tried your advice since I was curious too and my plug looks like this. Does that look about right? :shrug:
But seriously If the plug black is wet, And it seemed to get worse the longer you ran it I would say your rich. Probably fouling the plug. Here is a link for jetting and tuning the mikuni carb, I have been using this method on my motocross bikes for many years, it takes patience but it works.
http://www.iwt.com.au/mikunicarb.htm
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Keep in mind it did the same thing with the stock carb which ran just fine before I took it apart and did the mods. Before I play any more with carb I am going to wait till new CDI comes and is replaced. If it still happens I will try it but I went from a 125 jet to a 115, to a 110 to a 105 and all behaved the same way. Usually rich on a 4 stroke is dry fluffy black not wet.
Finally sucess!! Got the aftermarket CDI made in Argentina, anyway it looked just like the original one although the resistance measurements on primary and secondary were each slightly lower. Ran just great never missed a beat. I think the current jetting may be a bit lean as it is a 105 in there now and there was not much difference between 3/4 to full throttle over 4K after it cools I will look at the plug although I could not do a real chop, had to drive it to the house from street.