Hi Leema
Firstly the fuel. Turning the fuel tap to PRIME just bypasses the diaphragm of the tap. This is operated by 2 pipes that come from the carbs (1 from each carb at the top) that go into a TEE piece that in turn connects to the fuel tap. This is the smaller of the 2 pipes on the tap. When the engine is running the carbs create a suction on these pipes which causes the diaphragm to open and let fuel through to the carbs. As the bike runs fine on prime then this implies your issue is NOT internal to the carbs. My guess is now you have change the diaphragm that there is an issue with the vacuum pipes just described.
It is safe to run on prime but I would strongly advise turning the petrol tap to on every time you turn the engine off to prevent flooding and fuel loss.
With regards to the MAC exhaust system I also have experience. They are built for the USA market which generally has much more bike friendly weather. My experience was bad. I first got black downpipes which were rusting in less than 2 weeks. Secondly within a month the baffle plate had torn out and I had to get it welded in.
I had several issues of damaged systems being sent to (3 in all) They got my address wrong so delivery took far longer than it should as the exhaust was sent to Uzbekistan instead of England and then when finally on its way it spent a couple of nights in Paris.
Yes you will have to pay import duties which in my case were about £40.
My advice would be to stay well clear. The best aftermarket system like original is the BSM system from Holland but these are quite hard to come by.
