Motorcycle Repair: Intake Manifold Vacuum Line, vacuum pipe, manifold vacuum

Question
G'day Mark,

Recently had to replace the left side intake manifold on my 1984 Yamaha 250 SRX.  Come to find out the old one was actually the right side (obviously someone confused left and right in the past) and now I find that what appears to be a vacuum line from the intake manifold to the fuel petcock is missing.  Looks like the right side intake manifold should have this vacuum line connection so without replacing it I'm missing this connection.  

What, if any purpose, does this line serve?  I am able to start and ride the bike without it connected and haven't noticed any difference in riding with this line disconnected (I plugged the end off the fuel petcock with a screw).

Any info on this is much appreciated-haven't been able to find anything on my own on this one.

Many thanks,
Bill Murphy
Western Australia

Answer
Hi Bill,

The petcock on USA spec models is a vacuum operated type. A vacuum hose is coonected between one of the intake manifolds and the petcock.

If your SRX is working alright, no worries. Ensure that the manifold vacuum pipe and the petcock vacuum pipe are capped. A screw may allow a vacuum leak. Try using a rubber cap instead. The local car parts store should carry a variety of caps sizes.

Respectfully,
Mark Shively
Dayton, Ohio, USA