Craftsman Chain Saw - Revs very, very high on idle

[GAVolunteer]

Craftsman Chain saw
Model 358.360360
SN 05040D302320
Zama Carb C1M (also has W26 4YA stamped on the carb)

Working on a friend's chain saw. He could not get it to start.

Removed the carb. No visible signs of dirt, bad gas, etc. Put in ultrasonic cleaner. With low pressure can of compressed air, carefully blew out the small openings. I did not have a carb rebuild kit (RB129), so I put the Zama gaskets back on. The metering diapghram was pliable and gasket looked in good shape. The Pump diaphragm had indentations from prior use and needs to be changed when I get the carb rebuild kit.

After reassembling the carb, I screwed the H and L in and then backed out 2 turns. would not start. backed out the low another 1/2 turn. Engine fired up.

Without the throttle engaged, the engine immediately rev's up extremely high to the point I have to shut the engine off. The throttle valve is closed and the linkages are correct. I checked them several times. All of the other gaskets around the carb are good. So far I've not found an area where there might be an air leak, but I may have missed something.

I removed the muffler and there was not carbon buildup. Piston looked ok. No scarring on the piston.

What would cause a two cycle engine in "idle" mode rev up so high?

Thanks.

Robert

 

[Mikel1]

Have you tried backing out the idle screw? May have to go back and forth between idle & L mixture screw. You can download zama's technical manual from zama's website.

 

[GAVolunteer]

Have you tried backing out the idle screw? May have to go back and forth between idle & L mixture screw. You can download Zama's technical manual from zama's website.

Thanks for the info. I've reviewed Zama's technical manual and I've followed their carb set procedure. Here are a couple of links:
http://zamacarb.com/pdfs/TechGuide_2007.pdf and http://zamacarb.com/assets/images/techsupport/DisassemblyC1U_140730.pdf

What is perplexing is that It runs so wide open with the throttle closed. It is somehow getting a lot of fuel and air to run so fast.

 

[GAVolunteer]

Problem fixed temporarily. I've never worked on a Zama C1M carb. There is a piece called an Airhead that fits on the outside of the carb. It has a throttle shaft and plate. When I first worked on the carb, I noticed The Airhead Throttle was moving back and forth without any resistance. Since it was at the top of carb and it own , I thought it was some kind of emissions part and that air would be coming out of the opening not air going in. Wrong assumption.

I looked closely at the Airhead shaft and noticed a spring underneath it. The problem was the spring was broken. I bent the end of the spring and got it to grab so the airhead throttle now springs back and forth. When you pull the trigger on the chainsaw, two throttle plates open one inside the carb and one on the Airhead. When I was starting the engine, the Airhead Throttle plate was open and the engine was getting a whole lot of air.

Although the chainsaw now runs great, I'm going to order a new carb because the spring could break at any time.

For those who might be interested, here is a link to a C1M Parts List:
http://www.m-and-d.com/pdfs/zama/C1M_PARTS_LIST.pdf

One question. When I removed the inlet fuel line from the carb (the one with the fuel pickup attached) and removed the fuel line attached to the primer bulb, gas bubbled out of the inlet fuel line. (The line was several inches above the fuel pickup.) So, I opened the gas cap to relieve the pressure to stop the fuel from flowing, which it did. However, when I screwed the gas cap back in, fuel started flowing out of the inlet fuel line like before. I replaced the fuel filter, and tried again, but as soon as I screwed the gas cap back on, it did the same thing. I finally had to drain the gas tank. Any idea why the above occurred?

Thanks

 

[ILENGINE]

the fuel cap contains a one way duck bill valve. By screwing the cap on you somewhat pressurize the tank.