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weed wacker will not start

Posted by kads (My Page) on
Mon, Sep 23, 13 at 18:12

craftsman weedwacker 17" 25 cc model # 358.791010.. it would start up and run for only a few seconds then die .. never could get it to run other than in the start position.. it has spark(checked with inline spark plug tesster) took off carb..blew air through it to get crud off the little mesh screen put back together .. it never fired up again.. took filter out of tank ,blew air through it .. made sure line are not ckacked.. still will not fire up.. any ides


Follow-Up Postings:

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RE: weed wacker will not start

D-I-a-p-h-r-a-g-m..... Maybe??? once you break the lines loose you induce air in the system..before the diaphragm will pump fuel the air has to be relieved and ALL the tiny passages have to be clean.

You can try priming with carb. cleaner (in the SPRAY CAN) see if it will take off on it...May have to do it several times to get all the air out of the fuel lines and carb. IF that don't work thorough carb. cleaning with new diaphragm maybe you only hope....IMO

"O" your getting spark through the spark plug right? (by grounding it against the head and pulling the rope and watching for pretty blue spark)


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RE: weed wacker will not start

i pluged in a inline spark tester that when i pull the rope the spark will show up.in that.. thanks.. okay so i will need a new diapham and then spray carb cleaner through the fuel lines thrrough the carb?? thanks


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RE: weed wacker will not start

i pluged in a inline spark tester that when i pull the rope the spark will show up.in that.. thanks.. okay so i will need a new diapham and then spray carb cleaner through the fuel lines thrrough the carb?? thanks


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RE: weed wacker will not start

Put a little 2-cycle mix in through the air intake - remove cover/filter and take it off 'choke' to open the airway. You can get the piston stuck sometimes using starter fluid or cleaning solvent because there's no oil in it. Then it sits out in a shed and rusts. If you can make it run with this method, it clears up the picture somewhat. It usually is a fuel restriction, like the diaphragm or blocked orifices. You probably will need a needle valve adjusting tool for it if it has the 'tamper-proof' needles. It may also need a primer bulb.


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RE: weed wacker will not start

thanks just ran a little mix through it still will not start..should i just get a new carb or just the carb. kit..or could it be something else??


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RE: weed wacker will not start

Blowing air through everything most times is not a cure all. High air pressures can sometimes do much more harm than good (damage internal carb check valves). The culprit is usually a malfunctioning carburetor or one out of adjustment. If there is any age on the trimmer at all; a gasket and diaphragm kit would be highly recommended as ethanol laced gas ruins the internal componets in time.. A full rebuild kit has the same componets and a few more parts (needle valve, lever pin and welch plugs) at a little higher cost.
You may also want to run a little compression test on the engine and make sure all gaskets are completely sealing the bottom end as it has to be air-tight to function properly and to start. Check the manufacturer and model number on the carb and maybe start with installing a new G & D kit. I suspect a failed fuel pump diaphragm as rc stated.


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RE: weed wacker will not start

Your carburetor is a Zama 530071811. I'd suggest a new one at about $25 - $30, rather than a rebuild kit for $15 - $20.

Although the kit has all the parts, you need to remove one or two welch plugs to clean some of the passages, and then press in and seal the replacement welch plugs. IMO, for $10 or $15, you are better off starting with a clean sheet and replacing the entire carburetor.


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RE: weed wacker will not start

If it won't fire at all by putting fuel in the intake, it is not getting ignition at the plug. Put a spark plug in it and try the same thing again. However, it is still possible, but unlikely, that cylinder compression is too low to support running. Crankcase compression is also critical. Even with good cylinder compression, it will not run without crankcase compression. A bad/broken reed valve would let fuel/air in and out, preventing the 'self-powered' cylinder charging of the 2-cycle.


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RE: weed wacker will not start

There is always talk of reed valves, but have you ever seen one installed on a Poulan/ MTD Yard Machine 25cc engine? The 26cc? I think most end users do not even know what one is. I have seen only a couple on trimmer engines and they were on 31cc and 32cc Troy Bilts. Even the Poulan 30cc doesn't usually have a reed valve on it.They usually just have a 3 or 4 screw cap with a Rubber ring crankcase seal on them as part of the rear engine cover.
If this guy is not getting ignition or any spark and he feels inclined, let him check the air gap between the coil and flywheel. The coils do get weak ; as I have found out.
People come on this forum to attempt do-it-yourself repairs. There is a wealth of info on here about weed whackers not starting or running if they would bother to do a search and READ the threads. Happy whackin' to all and keep fixin' um, not trashin' um.


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RE: weed wacker will not start

There has to be a valve where the intake enters the crank that allows air/fuel in but not out.


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RE: weed wacker will not start

The majority of small two stroke engines do not have an intake valve. They are piston ported. The piston passing the intake port functions as the valve.

During the compression stroke, the piston rises creating a partial vacuum in the crankcase. As piston continues to rise it uncovers the intake port, drawing air-fuel mix into the crankcase.

Above the piston, as it rises, the air-fuel mix is compressed, until it is ignited at or near TDC. The piston now starts down in power/scavenge stroke.

As the piston moves down, it covers the intake port, preventing backflow of air-fuel into the carburetor. Pressure is also building in the crankcase as the piston falls. Also the piston also uncovers the transfer port, which causes air-fuel mix to flow into the combustion chamber and forces the previously burned mix out the exhaust port and into the muffler.

Reed valves are used in higher performance engines (karts, some chain saws) to allow more aggressive porting and timing designs and eliminate backflow to the carburetor. Rotary valves are also used to achieve the same high performance. Rotary valves are more expensive and tend to be used in high performance motorcycles. Kawasaki was a big believer.


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RE: weed wacker will not start

thanks i am trying to find a new carb.. hope i am not throwing 30 bucks away on it ...and it still not start..


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