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Engine Runs Hot

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Hi, I am new here and, based on what the equipment I see other use on this site, I feel embarrassed with the little thing I have. I have to move to something larger in the future, but have no funds for this now, so I want to fix what I have, a Sears 6.5 Hp 22" Electric Start Power Propelled Rotary Lawn Mower (Model No. 917.377430, 1998). It has been running OK until last year when I noticed a reduced power output and, at the end of the season, it ran hot. It ran so hot that it melted a whole in the aluminum head, just 1/8” next to the spark plug (one side of the spark plug threading was gone as well). I replaced the aluminum head and the tested the mower; it ran, only again very hot and with reduced power output. To prevent melting a whole in it again, I had to stop it. And additional clue may be that it made infrequently loud exploding sounds as well. The engine has enough oil (I last replaced the oil after I melted a whole in the head) and I replaced the air filter recently a well. What could this problem be caused by and what part do I need to order and replace? I think that it could be explained by the ignition timing to be off. In that case, the engine will run less efficient; meaning that less energy will be converted into mechanical energy and will go to waste in the form of heat. But looking at the ignition system, with the magnet mounted on a fixed spot on the flywheel and the ignition coil mounted on the engine block, the timing of the ignition seems to be fixed as well. If it was right when new, it should be right now. I hope someone with a good practical knowledge of simple engines reads this and can educate me here concerning what part to replace. I went to the Sears Parts & Repair shop and they had no clue. Thanks for you advise, Vincent Dert

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MPH
I have yet had it happen but have read several posts on here, sometimes by dealers who work on many engines, that it might be a sheared flywheel key, thus changing your timeing. Sounds like it be a good thing for you to check out as advanced timing could be your problem..MPH

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Dutch
Hi Vincent, What kind of engine? Briggs, Tecumseh? I would do the following: 1) Check the cooling fins. Grass clippings or a mouse nest can cause overheating. 2) R&R head and remove carbon. 3) Replace the flywheel key. Slight wear can throw off timing. 4) Make sure you are using the correct spark plug. Check color of plug you remove. Should be brownish. 5) Check carb adjustment. Too lean mixture = hot engine. 6) Check muffler for blockage. Probably okay.

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BradT
The exploding sounds could have been backfiring which supports the idea of a sheared flywheel key. That, certainly would screw up your timing, reduce power, & increase heat. I bought a Sears snowblower with 5HP Techumsa that broke the con rod and sheared the key that way. Have you ever hit a large rock or something that suddenly stopped the blade while the engine was running at high RPM? There's enough inertia in the flywheel to shear the key even with a tight flywheel nut. I suggest yo pull the flywheel that will answer both the obstucted cooling and sheared key theories. Next time buy something made by Simplicity from one of our friendly for sale Ads.

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UCD
Thanks Dutch, MPH and BradT for your replies, All good points brought up! It did not cross my mind that the flywheel might have "advanced" a bit by partially shearing the key. To answer your questions: - The manual states "Craftsman 4 cycle engine", but I wonder whether they make these themselves. Is there a location on the engine where the "real" manufacturers name or code may be found? - Coolng fins are clean (it is a new head!) - Yes, I have hit quite a few large rocks, even to the extend that the machine "jumped up", so some damage could indeed have been done. I will indeed do all the things proposed and will let you know what I found. Thanks, VincentD

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Guest
Ok, Dutch, MPH and BradT you were all correct! I took the parts mounted above the flywheel off and below the bolt tightening the flyweel the key was still partly there, but for about 3/4 sheared away. The timing is now about 20 degrees off with respect to the original flyweel setting. The question now is how to get the flywheel off, without breaking it. I would normally use a flywheel pulley, but there is not much to hold on to (aluminum flyweel is wide and fairly thin). There are three holes, located at each 60 degree angles around the center of the flywheel. They have no thread tapped into them, but that could be done of course. Are these there to attach a part to allow the use of a pulley? Thanks for your advise, VincentD

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