Craftsman Chain Saw - Revs very, very high on idle

[R4665]

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. Before he gave it to me, he said he "had a young man work on the chain saw" and that the saw has not worked since he got it back. So, I'm trying to fix what the other guy did.

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 and diaphragms 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? 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.

Thanks.

Robert

[KE4AVB]

Since someone else have had worked on it, I would check the carburetor gaskets, Crank seals, and Cylinder to crankcase sealing which is nothing but black, white, or grey RTV depending when it was assembled. Also make sure the mounting screws are torqued to spec on this 42cc as the cylinder screws especially tend to loosen otherwise.

This service manual will have your torque specs for this Poulan series.
http://www.mymowerparts.com/pdf/Poulan-Poulan-Pro-Service-Repair-Manuals/POULAN-POULAN-PRO-CHAIN-SAW-SERVICE-REPAIR-MANUAL-1900-1950-2050-2055-2075-2150-2175-2375-WILD-THING-2250-2450-2550-220-221-221LE-260-260LE.pdf

[DennyIN]

Idle speed screw turned in too much?

[bgsengine]

Just replace the carb - they run about 30 bucks shipped - about the same price as two carb rebuild kits and a lot cheaper than a rebuild kit + labor time.

Once you have that unknown out of the way you can properly diagnose any further issues such as a pinhole in a fuel line, or air leaks (crankcase pressure/vacuum test) - but a new carb may solve all the issues.

The fact that you have to turn out screws so far to get it to fire up, would point at an air leak somewhere, though... but then again a restriction or damage in the carb (ultrasonic won't fix that or bits of dirt or fuel line stuck in a jet) would cause the same problem - if you are not gonna do a crankcase pressure test, then the next best step would be eliminate carburetor problems by replacing the whole carb - Keep in mind the gaskets are not available individually any more and you have to buy a whole engine gasket kit just to get a freakin' carb intake gasket..

were that machine to come in to my shop today, it would promptly go back out un-fixed, or on the junk pile.

[R4665]

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.

Just replace the carb - they run about 30 bucks shipped

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 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

[bgsengine]

I thought it was some kind of emissions part

Correct. You have what is called a "stratified" 2-stroke - that air valve pumps in a charge of fresh filtered air ahead of the air/fuel mix to cushion in between the exhaust and incoming fuel charge - the fresh air helps push out the burned gasses rather that a fuel charge incoming pushing it out (and losing fuel charge in the exhaust) - reduces emissions and improves fuel economy without a catalytic muffler.

Just replace the carb - they run about 30 bucks shipped

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

Good move.

One question. When I removed the inlet fuel line from the carb (the one with the fuel pickup attached) and 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

That's normal operation for most properly working 2-strokes - Fuel is volatile all the time- in a sealed container it will build pressure in the air space available above the fuel - the open fuel line is the path of least resistance - if it DID NOT do that you would have a bad fuel tank vent - the vent does allow excess pressure to escape, but it does hold pressure slightly, and yet allows fresh air to come in easily as the fuel level drops.

[Arkie]

Now that you have her running and thinking about replacing the carb, take it too the wood first and run a cut performance test and then I would also take a look at the fuel lines and primer bulb. If the fuel lines are not clear and very flexible they should be ordered and replaced at same time as the carb and it's common for the primer bulb to crack or get hard if it has the primer bulb. (ethanol gas ruins the fuel lines and primer bulb rapidly)
Reason I mention such is shipping cost can be saved plus polan pains later. (go backs)

Look at some you tube videos for some good tips on replacing the fuel lines if you have not been their before on a little Poulan. Extra long reach needle nose pliers are also handy so as to get deep into the fuel tank for pulling on the fuel lines. Like bsengine says check it out good before ordering parts few at a time or you can easily end up with more $$ in the saw than it's worth. I only work on Polulans (craftsman saws for myself not customers because they are not rated for serious wood cutting operations common for them to fail while being stored or being used. (fuel lines and plastic parts on Poulans age rapidly and deteriorate especially if ever exposed to ethanol gas)

Summary: Poulans and Craftsman chainsaws can make a good mechanic look and feel bad, especially to customers.

[dougand3]

+1 on LONG needle pliers on these 42cc Poulans. The purge return hole is way up in the corner near the clutch. Harbor Freight has a set of 2 for $6. The fuel supply line is normally the tiny .08" ID (I hate that line - it deforms so easily. And hard to install on even the smallest neck fuel pickup/filter.) I've taken to drilling out the fuel supply hole to accept the more robust 3/32" ID line.

[KE4AVB]

It's easier to pull the lines from inside the tank out and you won't need those long pliers or forceps...

[Arkie]

It's easier to pull the lines from inside the tank out and you won't need those long pliers or forceps...

Yep, I've done that also by running a wire thru the hole and then thread the line over the wire using the wire as a guide for the new line and cut away about 1/2 of the new line for about 3 inches so as to reduce it's size and taper the very end to a sharp point so as the sharp point comes thru the hole first.

They Harbor freight long needle nose pliers and a set of curved nose locking type forceps are also very handy when you need more than two hands on surgical type procedures on china stuff.