Thanks for the suggestions.
I am little perplexed about how it could be a timing issue if the mower runs ok until it gets hot but I am probably missing something. If the timing was off would the mower not keep backfiring on a regular basis from cold to full operating temperature?
The carby gaskets are all in very good condition so that shouldn�t be an issue.
Your comment about the muffler got me thinking so I swapped it with one from another Chonda that had a busted conrod. I ran the mower until hot and no backfiring. It very occasionally has the slightest surge but nothing to worry about.
The mufflers were exactly the same but the one that come off the engine was quite heavy which lead me to believe it may have in fact been blocked with carbon. I will give the engine another run and a good test to make sure the problem has gone completely. I am not 100% convinced just yet.
Do you have a good method for testing potentially blocked mufflers. I have had this happen a couple of times with 2 strokes but not with 4 strokes. My method was just to swap one from another engine and give it a try.
I am thinking the previous owner may have had in running on choke for an extended period because the quite severe carby blockage simply because it wouldn�t run without an enriched mixture hence the potential blockage.
Jaffa
