Futures1

After reading about your problem.. a few thoughts!
Are you sure that your exhaust valve is not hanging up..  perhaps due to carbon on the stem or in the valve guide ??  Also, has the air temperature changed dramatically during the run and no run time ??
An engine which runs well in the summer will not always run well (if at all) in the winter without increasing the fuel flow. Did you say that you did replace th coil ?  I have had a few cases where mice, etc.
have chewed the coil wire causing a weak or no spark !!  Finally, some TEC's have a jet in the carb. which have an almost imposible to see hole in it which can plug up very easily !! Even seasoned mechanics have been known
to miss this one !!