HS 72 won't run

[rideon]

I have a Stihl HS 72 Hedge Trimmer. I bought it used. The carb needed a rebuild, so I found a new carb for $20 and installed it. The trimmer may run fine today, but the next time the carb needs adjusting. Today, it won't start. What could be the problem? Why is it not consistant? I am thinking it is leaking air through the crankshift seals. Could this be the problem, or am I overlooking something else?

Thanks

 

[pugaltitude]

Its usually fuel related.
Fuel pipes not sealing in the tank? Fuel filter blocked?

 

[pugaltitude]

Could also be carbon build up making blocked exhaust.
Need to heat the exhaust very hot to break carbon off.

 

[rideon]

I've cleaned the exhaust screen, replaced all fuel lines and tank filter, and installed a new air filter along with the new carb.

 

[rideon]

I tore into the cylinder today, and the rings seem to be worn and sticking. I ordered a set of rings and a cylinder gasket. I hope this fixes the problem. When I replace these parts and reassemble the trimmer, I'll post the results.

 

[rideon]

The new rings and base gasket came in on Monday. I replaced the rings and gasket yesterday. I lubed the crank bearings while I had it torn down. After reassembling everything, it fired up on the 3rd pull. I dialed the carb in and it is running like a top.:thumbsup: