2-cycle engine revs up and stalls


Old 12-30-07, 09:41 AM
Thread Starter
Join Date: Dec 2007
Posts: 2
Smile 2-cycle engine revs up and stalls

Please let me know if you have any ideas. I have a large Craftsman chainsaw. I used it yesterday for 30 minutes perfectly. It has always been easy to start and runs great. While using it yesterday it reved up and stalled. It is not the gas. I use the same clean 2-cycle gas in three other machines with no problems. Nevertheless, I drained the chainsaw gas tank, blew it out with compressed air, checked the filter (filter was spotless), and put fresh 2-cycle gas in it. I can start the engine each time (with choke on) and it revs up for about 15 seconds, leans out, and stalls. I tried adjusted the low, high, and idle adjustments and reurned them to their exact original positions when there was no change in the rev up and stall. I can keep the engine running by constantly putting choke on for 2 seconds and off for 5 seconds. Definitely not getting enough fuel.

I know it is not ignition, timing, or electrical. It is definitely carboration. I am leaning towards (1) a leak between the carburetor and the engine not permitting enugh fuel suction (2) or possibly some kind of a internal carboration flapper valve problem (3) or some kind of a fuel line cracked allowing air to suck in. There are no other symptoms, no visible leaks, no mechanical restrictions i.e. chain moves freely. I tried it again today with the exact same rev up and stall.

I appreciate any comments or suggestions.

Thank You,
Sponsored Links
Old 12-30-07, 01:48 PM
Join Date: Dec 2007
Posts: 8
Sounds to me like the needle valve in the carb is sticking on you. If you're any good with carbs just take it apart and clean it up. Some people clean the needle valve and rub it with waxed paper. This is supposed to let it move freely when you get it back together. Good luck!
Old 12-30-07, 06:13 PM
Join Date: Sep 2007
Location: Cottage Grove, MN
Posts: 239
I have a feeling because its 2-cycle it has a diaphragm carburetor which does not use a float bowl. My guess is the carburetor needs to be a) cleaned and b) a new diaphragm needs to be installed (would cure a "flapper" problem). You however did not post any model & serial numbers. If you could do so we might be able to look up a diagram of the engine and see if we can better help you. If the saw is an older unit (older than 5yrs) then it would not be a bad idea to replace the fuel lines (clamps are good failsafes against air leaks too). Also, check to make sure the bolts that hold the carburetor onto the engine are tight. I doubt there is an air leak between the engine and carburetor if the carb is tight.
Old 12-30-07, 08:09 PM
cheese's Avatar
Forum Topic Moderator
Join Date: Jul 2001
Location: USA
Posts: 16,821
I suggest starting by replacing the fuel lines. It sounds like the carb is sucking air through the fuel line via a crack or pinhole. If no change, go for the carb rebuild.
Old 12-31-07, 06:06 PM
Thread Starter
Join Date: Dec 2007
Posts: 2
Smile 2-cycle engine revs up and runs perfect! SOLVED

The problem with my chainsaw is resolved. Thanks to all of your suggestions I was encourage to solve this problem. Here is what happened:

I rebuilt the carburetor with the same old pars that were in perfect condition. I found no problems and when the chainsaw was re-assembled it still would not keep running unless I choked it. I kept digging deeper into the machine until I found that one of the small plastic tubes going between the carburetor and engine had the smallest of pinholes imperceptible to the eye. The hose was buried very deep into the cylinder behind all kinds of covers and the flywheel.

Turns out that the small tube between the engine and carburetor is used to operate a diaphragm in the carburetor as a fuel pump. One the hose with the pinhole was replaced, the machine was re-assembled again and now runs like a champ.

Thank again for all of your support.


Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread
Display Modes