# Toro Ah520 Backfire problem



## ejluhn720 (Mar 25, 2005)

*Tecumseh AH520 Backfire problem*

This model AH520 Tecumseh motor is on a Toro Snow Thrower Model 38130, Ser # 0576831

This engine is difficult to start but will run with choke on. The engine has a habit of backfiring through the carb once or twice, causing the engine to stop. Any attempt to fully open the choke will cause the engine to quit, usually with the backfire problem. The original owner had removed the carb and had it professionally cleaned. A new spark plug has been installed, and fresh fuel is in the tank. I have removed the reed valves and one seems very slightly bent. Both were cleaned and reinstalled. The flywheel was removed to check the possibity of a timing problem. It was interesting to note that there was no shear key, per se, but a sleeve in the flywheel with detents to align with the slot in the crank shaft end. After reassembly, the engine seemed to be a bit easier to start but would still backfire and quit. It takes pumping of the primer bulb after each time the engine turns over a bit, in order to get the engine to start, practically to the point of flooding. The idle and run screws have been played with qite a bit with no success in stopping the backfire. Is it possible for reed valves failing to close to cause this backfiring problem? What else may be causing it?


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## bugman (Aug 12, 2004)

is it backfiring black smoke? may be the carb needs adjusting


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## ejluhn720 (Mar 25, 2005)

The smoke is more gray in color. The carb jets had initially been set to the recommended number of turns and adjusted all over the place from there without much affect on the backfire problem. You can adjust the settings (mix) to a point where you can hear revs increase and decrease, if you are lucky to keep it running for that long. No settings were found that could keep the engine running/backfiring when the choke was more than just partially open. Can defective reed valves cause backfiring? What other things could cause backfiring outside of a carb problem? I'd like some ideas on where I might look. 

Thanks, 
Ed

Ed


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## bbnissan (Nov 7, 2004)

It definately sounds like the reed valve to me. The motor should never backfire through the carb unless it A) has a timing problem or B) has a reed valve problem.


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## Bowman's (Dec 16, 2004)

Just a thought, check and make sure the muffler is not plugged. I have had them do that and found the owner mixed too much oil in the fuel and plugged the muffler. The exhaust has to go somewhere. Just another place to look. Mike


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## ejluhn720 (Mar 25, 2005)

bbnissan said:


> It definately sounds like the reed valve to me. The motor should never backfire through the carb unless it A) has a timing problem or B) has a reed valve problem.


 I guess its not really a reed volve, per se. Its a thin strip of metal similar to a reed but is used as a compression break to aid in electric starting. Do you think if this is sticking open that it could cause a similar problem?


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