I had an old Pontiac that used to do the same thing.  It would hesitate 
before accelerating.  It turned out to be the accelerator pump on the 
carb.  You said your bus has a Weber(I accidently deleted the message so 
I'm not positive that's what you wrote).  Has it been rebuilt recently?  
You may want to check that out.  

While we're talking about engine problems...

My 74 will run fine, but when idling it bogs down every 15 seconds or 
so.  I'll pull up to a stop adn it will idle fine.  Then, it acts like 
its about to die, catches itself and idles fine again for another 15 
seconds or so.  Anyone have any ideas?  I haven't tuned it for 6 months 
and can't seem to find any obvious problems, like cracks in the 
distributor, so I'm lost.

Thanks,
Craig  

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Craig Standley				1974 Volkswagen Westfalia; The Cow Van
University of Washington
Physics Major				"Heaters?  We don't need no stinking
cstand@u.washington.edu			 heaters."
http://weber.u.washington.edu/~cstand    
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