Troublesome Stihl 017 cahinsaw

[Charleychainsaw]

HI, I have a 017 which refuses to start. 

I have fitted a new carburetor and spark plug. The spark is healthy
All filters are clean and the exhaust is clean too.
Can the ignition timing slip?

[Oldfeller]

47 minutes ago, Charleychainsaw said:

HI, I have a 017 which refuses to start. 

I have fitted a new carburetor and spark plug. The spark is healthy
All filters are clean and the exhaust is clean too.
Can the ignition timing slip?

how are you testing for a spark. a poor coil will often produce a spark on a test lamp or earthed plug but be insufficient under compression to ignite fuel/air mix. is the plug getting wet on successive attempts at starting, stihl carb or ting tong land carb? it can be a bit pot luck with ebay china carbs there's a reason why they are so cheap. have you tested compression.

[spudulike]

Check the compression or at least, pull the exhaust off and check the piston looks OK through the exhaust port.

The 017 is old and a typical homeowner saw with both these things meaning the saw will have been abused and/or run on old or near fuel.