weber downdraft

my friend has a 1971 mgb gt. he slapped a weber downdraft carburetor on it. when he takes off, it will stumble a little bit. what should he do?

Your friend has a couple of choices.

1) Live with it (I wouldn't)

2) Get a pro to jet the carb. Webers stay in tune very well, but they need to be fitted with the correct jets when you first install. They don't have a lot of adjustment screws, but are adjusted by changing the interchangable jets (and emulsion tubes) This will be a one time expense.

3) Get a Weber tuning book (Pat Braden's book by HPBooks is very good) and learn about the carbs. Then after he's educated, he can buy parts a little at a time, do 'plug runs' and get the most out of his Weber. A Colortune kit is a really good tool to help you tune a Weber. This will take some trial and error and a little tenacity. The good news is that when he's finished, he'll be an expert in something that many enthusiasts still know nothing about.

If he tries the third route, I suggest that he check out the progression circuit. It could be undersized idle jets. Or if it's a used carb, it could be a bad acclerator pump.