today's quiz

[wzehring@cmb.biosci.wayne.edu (Will Zehring)]

Fellow fiends:

In conversation with my local auto electrics expert (see "Humble pie, 
revisited") I was told the following story.  I'll pose it as a quiz to the list:

One time a fellow with an XJ6 came into the shop with the following problem. 
 The car ran fine at idle, but would misfire badly under hard acceleration.  
Interestingly, it would run fine under hard acceleration, BUT ONLY IF IT WAS 
IN REVERSE, moving backwards.

What is the explanation?

Will "Watch this space" Zehring