Got any more ideas? Please?

[Phil Payne]

> It will run good for a few seconds, then I'll get a stumble, then good for a second or two,
and another stumble.  Then it will run good for a second, and stumble twice quickly.  After
this quick stumble, it will run good for a few seconds, then the cycle starts again (always
the same pattern).

Assuming an MC engine.

Two possible causes:

a) Is the rubber crankcase breather hose (side of the block, behind the warm-up regulator)
collapsed?

b) Crank rear oil seal letting air in when crankcase achieves vacuum and resealing when revs
drop and vacuum disappears?

Both would be diagnosed by constricting the hose that runs round the back of the head to the
metering head plenum chamber.  Progressively squeeze it with a clamp and see if the symptoms
change just before you close it completely.

--
  Phil Payne
  http://www.isham-research.com
  +44 7785 302 803