One way is to pinch off the main vacuum hoses, one at a time, to see which one affects the idle quality. Follow that to its branches and keep doing that to narrow it down. Another method is to slowly run water over the hoses while the engine is still cold. You'll see and hear the water get sucked in. The best way, if you can find an auto parts store that borrows tools is to use a smoke machine. That allows you to inject white, non-toxic smoke into the intake manifold and watch where it sneaks out.
You will also hear a hissing sound if the float is set too low in the carburetor. That will cause a stumble and hesitation at any speed. A vacuum leak won't be noticed at higher speeds. Don't overlook a separated or cracked PCV valve.
Thursday, March 21st, 2013 AT 11:12 PM