MG Car Repair: Fast Idle, vacuum leaks, vacuum leak

Question
QUESTION: Hi Howard:

Out of the blue my 1977 MGB started idling fast, like 2,000 RPMs.  I tried to tap the gas pedal and kick it down, no luck.  I inspected the gas pedal cable, seems fine, looked for vacuum leaks, none evident.  Could the auto choke be causing the problem?  Other causes? Other ideas?  Of course this issue comes up as the weather broke :-(

ANSWER: Hi Hugh,
First confirm that the throttle cable has a little slack at the carburetor. Then check the choke fast idle screw to see if it has tension on the throttle linkage (common problem) The automatic choke is a common failed item.

How did you test for vacuum leaks? Looking is useless. While it is idling fast slowly start restricting the air flow into the carburetor and watch the RPM closely and if at any time the RPM speeds up or gets smoother you have a vacuum leak somewhere. If you don't have a vacuum leak you will be able to slowly slow the RPM down by restricting the air intake.

Howard

---------- FOLLOW-UP ----------

QUESTION: Howard, when I cover the air intake with my hand the idle slows and engine starts to run rough.  When I remove my hand the engine idle increases (back to 1,500) and the engine smooths out.  Does that mean vacuum leak unlikely?  

What next?

Answer
Hugh, it is the SLOW progression of covering the intake not just "cover the intake" that gives you the indicator of a leak. It MUST be done in a very slow progression to look for the indicator. If you VERY SLOWLY cover the intake and at no time during the SLOW progression of covering the intake does it INCREASE in RPM then you don't have a leak.

This is the standard test on all engines for a vacuum leak.

Most common problem of this kind is throttle linkage or choke system failure.

Howard