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After reading your mechanic's report, I'll add this to Danny's comments.
Ask him (the mechanic, not Danny) what he measured the fuel pressure to be.
This is kinda a trick question, as it looks like he may not have done that, and it's another thing that needs to be done before you start poking around with the jets. Each type of carb needs a particular fuel pressure to work right, and your Webers should have pretty close to 3 psi. If that is off by much, they'll never be right. Not hard to check or adjust, but anyone who knows these carbs will start there.
Next, another trick question. What did he measure the float height and drop at? These are other key settings that need to be checked before the laying on of hands upon the jets. This will also tell you if he bothered to take the tops off the carbs and check the condition of the float bowls - often the root of all evil.
If he hesitates or waffles answering either of these two questions, you may need to seek help elsewhere.
Third, as is being discussed here in another thread right now by Danny and Stan, ask him if he checked the timing and saw any evidence of spark scatter. That is a third factor that needs to be right before you have any hope of having a smooth running engine.
If your man is worthy, he shouldn't have any problems with you asking about these things and should also have some direct answers for you.
Be self-effacing. Be gracious. Be polite.
But, as the Boy Scouts advise, be prepared.
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