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Jon K.
Jon K., Small Engine Diagnostic Expert
Category: Small Engine
Satisfied Customers: 5776
Experience:  Gold certified Stihl tech, Service Manager for a rental/repair shop.Certified in most small engines
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Chainsaw will idle but wont rev. Dies when throttle applied.

Resolved Question:

Chainsaw will idle but won't rev. Dies when throttle applied. Have had cary apart and cleaned. No difference. Apply choke and start.... will rev once but not again. Starving for fuel?
Submitted: 3 years ago.
Category: Small Engine
Expert:  Jon K. replied 3 years ago.

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Remove the muffler and check the exhaust port for carbon build up. clean it thoroughly and make sure the muffler is clean also including the spark arrestor screen at the output of the muffler

Customer: replied 3 years ago.
G'day Jon, The machine is only 10hours old so I really don't think there would be any carbon build up. Besides that; after you choke the machine(give it heaps of fuel) it will high idle, so there is no exhaust restriction. Thanks Robin
Expert:  Jon K. replied 3 years ago.
Carbon is built up because the saw is idled for more then a few seconds at a time. 2 cycle engines are not meant to sit at idle or they will not burn off all the oil. Because you can get it to full throttle though I agree with you. Because the machine runs better after you choke it that is telling me that you have a fuel restriction somewhere. The first place to check would be the fuel filter, then check the fuel lines for cracks or pin holes. If everything checks out then you will need to disassemble the carburetor and rebuild it. Make sure that all the welch plugs are sealed and that the secondary screen filter is not plugged up also.
Customer: replied 3 years ago.
G'day Jon, I have already had the carby off and apart. I blew thru all the ports and capillaries. It sounds to me like a fuel shortage but why? Could the compression be low so therefore the power stroke of the piston is not creating enough compression in the crankcase to make the fuel/air mixture rush to the combustion chamber?
Expert:  Jon K. replied 3 years ago.

This may be a compression problem but I doubt it. An easy check is to remove the carb and look at the piston. It should be clean and shiny with no makings or scratches on it. Also on the carb itself you need to inspect the welch plugs, they are sealed with an almost transparent sealer. When this deteriorates the welch plugs tend to leak and mix the high and low speed circuits when they are not supposed to.

One more thing, when the carb is apart and you can see the tab that pushes on the spring to open the inlet needle, you can bend that tab ever so slightly in order to allow more fuel to get into the carb.

Customer: replied 3 years ago.
G'day Jon, When I had the exhaust off I looked at the piston and rings and they were scraped like a dusted engine. Although the rings X2 were still free to rotate and not seized.This would cause compresion problems on the down stroke therefore not compressing the crankcase properly to force the fuel air mixture into the combustion chamber when the piston moves lower to open the inlet chamber. ?
Expert:  Jon K. replied 3 years ago.
You are 100% correct. Any scraping or scratches on the piston will cause the engine not to run. The only way to correct this is to replace the piston and cylinder. Sorry to be the bearer of bad news
Jon K., Small Engine Diagnostic Expert
Category: Small Engine
Satisfied Customers: 5776
Experience: Gold certified Stihl tech, Service Manager for a rental/repair shop.Certified in most small engines
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