>If you have an early bus and if the heater smells of exhaust fumes, the joint
>between the heat exchangers an muffler should be one of the first things to
>check.  

(Through '71) grind down the inside diameter of the clamps on the header
side a bit to compensate for the larger diameter tubes.  The fit and ease of
installation of the clamps improves.  These clamps, and the rest of the
exhaust system, should be tightened at each tune-up, they are always loose,
especially on the '68-'71 buses where they are hard to reach because of the
engine mount.   A 10mm deep socket on a 1/4 drive with a 6" extension, along
with a 10mm combo wrench, are my favorite tools for this job.

>In about half the cases where a customer complains of exhaust fumes coming
>from the heater, the problem is not the exhaust system or heater but failed
>engine compartment seals, particularily around the engine compartment hatch.

Indeed, now that my bus has accident induced ventilation through the rear
hatch, I can detect exhaust fumes. 

I'm not keen on driving with the heat on when aftermarket air cleaners are
in place.  The stock single carb air cleaner contains the fuel stand-off
very well.  Aftermarket paper air cleaners allow fuel stand-off vapors to
enter the engine compartment air stream.

Eug,
'71