Chainsaw surges at full throttle


Old 09-08-09, 08:18 AM
Thread Starter
Join Date: Jun 2007
Location: Ludlow, MA
Posts: 172
Chainsaw surges at full throttle

I burnt my Husky 55 Rancher not to long ago. Yesterday I got the kit and put in a new piston, cylinder and ring. Everything went smooth and it didn't take long at all. I started the saw. It started right up, but it would die after 5 seconds. After tuning the carb, it idle greats, but at full throttle, the rpm keep going up, then down, then up, etc. Its certainly not the high idle adjustment, I have it set as good as it can get. I believe I replaced the fuel line recently, it looks new. I never gave the carb a rebuild kit, just blew it clean, as it ran perfect before I burnt the cylinder. Any ideas would be much appreciated.
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Old 09-08-09, 08:12 PM
Join Date: May 2006
Location: Iowa!!!!!
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The surging business is usually a lean mixture. It can be caused by a vacuum leak from carb to manifold to cylinder, or low fuel available to the carburetor, or leaking crank seals.

If the saw is running well except at full throttle, it's probably in the fuel available to the carb (ie lines, filter, tank cap venting) or in the carb itself (like the diaphragm setting, the pulse port, or the main jet circuit).

If it runs fast at idle or low throttle, look for a vacuum leak.

A word of caution - don't run the saw much before you get it taken care of = lean mixture/ high heat = scored piston and cylinder (again).
Old 09-08-09, 08:16 PM
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Join Date: Feb 2005
Location: canada
Posts: 1,563
Did you replace the crankcase seals & reseal the crankcase when you had it apart?? If you didn't see what caused the first failure, it almost sounds like it may be leaking air from the crankcase... I also noticed you didn't rebuild the carb.. The diaghragms get stiff & the meetering system won't work as designed.. I figure cleaning these carbs without puting a $10.00 kit in is kinda like washing you'r feet with you'r socks on... Kinda doesn't work... What about the fuel filter??? Recheck the fuel lines also & let us know..... Roger
Old 09-08-09, 08:53 PM
Thread Starter
Join Date: Jun 2007
Location: Ludlow, MA
Posts: 172
Ok, I figured I should start with a new carb kit and fuel line. I noticed these saws carbs mount to plastic which is a bad design in my opinion. I've stripped more then a few out, making new threads with epoxy. This could be a possiblility. It was evident last time I did it because if you tried tugging at the carb by hand, the idle would change. This time, the idle didn't change when I tried moving the carb. I hope its just a carb kit. And I definitly hope its not the crankcase seals. I did not change them.

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