Motorcycle Repair: Two questions, fuel mix and rev at low idle, 1980 honda cb750c, compression spring

Question
I have a 1980 Honda CB750c runs quite well but not reliable yet.  

1st question: Do I need to mix lead into the mix for this bike? or am I fine running unleaded.  

Currently I run with a small bit of lead and it seems to give me better top end.

2nd... Bike has a bit of a problem coming down in idle at stops ... ie lights/stop signs (stopping in general, you get the idea)If I rev the engine it drops back down... My question is would replacing the compression spring on the vacuum cylinder possibly correct this.

Thank you!
Great web site thank you for all you do

Answer
No you do not need to put lead additive into it, but it can help against knocking.  I would check the air diaphragm, it is on the side of the carburettor and it perforates easily.  Replace that and your engine will return to idle much faster.

Hope this helps,  Jan