Bruce
I could't have done it without your suggestion to take the muffler off. Even after removing it and examining it it appeared fine - it was only when the compressed air wouldn't go thru that I smelt a rodent - or, more precisely, the insect's home - that the solution became obvious. I was about to start checking the magneto, which would have been a whole new can of worms.
So, thanks again. And the best 10 bucks I have spent in a long time.
Cheers
Doug
