Engine bogging out

[Harry]

I finally got tired of adjusting my Holly 600 Carb and not getting it right, so I took it to a good carb guy here in town. When I picked it up a few days ago, it ran great! I was really happy.

But now it has developed a problem. In third and fourth gears the engine bogs out really bad when I try to accelerate. It doesn't do it in first or second, and don't know about fifth because I haven't had the chance to take it out on the highway. I called the guy, and he said it was running really rich, and I should try swapping in new plugs first.

So, now my question. If it's running rich, and it has fouled the plugs, why wouldn't it also bog in first and second? And wouldn't running rich mean that I should go to a smaller jet?

Thanks

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See my 1985 Mustang GT