70HP Force carb

I have a 70HP Force (about '91-93 model, I believe, it was the 1st yr Merc took over) that has a Tillotson carb on it.

This motor was o-hauled by a local mech right before I got it. He insisted I use Bosch(?) plugs, the kind with no electrode or center post. The bottom plug would foul quickly (4-6 hrs). I finally switched over to a conventional plug and went up about 3 heat ranges until I have a plug with an acceptible life, and pretty fair performance, all things considered. It has always had a stutter or intermitant miss at speed and is a gas hog.

Several months ago, it started missing and wouldn't ever finally clear its throat and run up to speed. I'd describe the miss as intermittant. I finally thoroughly cleaned the carb (disassembled, cleaned and blown out). No change. I then decided to monkey with the timing, so I increased the advance with the linkage coming off the throttle to the plate under the f-wheel. I crept up incrementally until the motor runs pretty well. It still doesn't run like it should, compared to others like it that sometimes trade here at the marina, but it's a lot better.

Now, my question. The fuel that sprays in the venturi part of the carb when at full throttle is not a steady stream (spray, if you will), but wavers. I wonder if this could be the cause of the erractic missing. Do you know where I could get a replacement carb?
During the cleaning, I noticed that the small brass tube that runs from the bowl up to the carb top wasn't centered in the center post and its end was somewhat flattened.
I realize the whole setup is a poor design, with 3 cylinders fed by one not-too-good carb mounted between the top and middle cyl, but do you have any suggestions? I have enough trotline weights already, so please rule that one out.

Craig McDougald