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graywolf1939

WH Model 518 will not start

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The magnet ring and ignition module as well as the coil and condenser was replaced in 2016. It ran the rest  of the 2016 mowing season with no problems. In the Spring of 2017 It started fine. A few minutes later I tried to start it again, it backfired once, would not start ever again. I got back to troubleshooting the ignition problem this Spring, it still would not start. It would turn over OK, it would backfire up through the carburetor once or twice while cranking over. I checked the coil (1 year old bought from AFC) and the primary was shorted and secondary read 70 ohms!! Bad coil. I replaced the coil today, I now have spark, but it still will not start, and it occasionally backfires while cranking. Battery is at full charge, gas is fresh, plugs are brand new. Removing one plug and checking for spark while turning the flywheel by hand yields a spark at near top dead center mark on flywheel, just about 1/2" to left of the TDC mark. So if timing is on the mark, gas is fresh, plugs have spark and are new, why will this Onan not start ??? Could the ignition module be intermittent? Could the flywheel key be damaged somewhat ( seems unlikely seeing the plug fires at near TDC) causing erratic timing? I am puzzled.

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A further update. I removed the top of the carburetor and both bowls were clean, full of fuel, floats worked well, jets clear of any crap. I used a can of Sea foam,  some in the carb., the rest in the fuel tank, no change, turns over well but will not start. I next checked the compression to see if I had a bad valve. Struck out here also, both cylinders read 120 lbs. My service tech. where I bought my 518 in 1988 says everything that I have checked says the engine start. The plugs are wet, so they are getting gas, the spark plugs are firing OK when I removed them and checked their firing. The only thing remaining is the key on the engine shaft. It is an elongated key on one end, that this longer extension is what ios also keyed into the ignition ring. If it is somehow slipping or not engaging properly, it will throw off the timing and cause a non start. What puzzles me is the fact that I have turned the flywheel by hand, and the plugs fire every time at about 12 degrees before TDC. So why won't the damn engine start ???? I guess I have everyone stumped on this forum, plenty of reads, no replies. Tomorrow I pull the flywheel again and check the key.

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Spark plugs will fire out of the engine but not fire under compression. I don't know why. Try another new set properly gapped. If it starts - great. If it does not start remove the new plugs and save for the next time you want to try new ones.

 

Garry

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You are correct Garry, that certainly can happen. I checked compression, it is 120 and 121 PSI. And, the plugs do fire installed, but the timing is off. I found a broken flywheel shaft key. That is causing the problem. I don't know how it broke however. Thank you for your reply Garry !

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I would guess the flywheel was not tight enough, but I might be wrong. Have had smaller engines do this. 

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You are probably correct  Howie, I can't think of any other thing that could cause the key to break. It ran a whole season however after I replaced the ignition trigger ring and module before this happened, so maybe it slowly loosened. I will use a torque wrench when I replace the key.

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