6.75 B&S Quits running after 20 minutes, why? - Page 2 - Lawn Mower Forums : Lawnmower Reviews, Repair, Pricing and Discussion Forum
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post #11 of 25 (permalink) Old 11-22-2019, 09:22 PM
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At this late point, and all the common aliments tried, ...My suggestion would be to suspect engine "overheating"....Due to a partially sheered flywheel key, causing "retarded" ignition...Most likely from the a blade strike...When intake gets too hot, the fuel will vapor lock...engine will starve and stop...I would unbolt the flywheel and look real close at the key....or simply change it....
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post #12 of 25 (permalink) Old 11-26-2019, 08:06 PM
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I might have a solution for you. Here is some back ground on my problem and the fix. I had a push mower and it would start first pull and run great. It would run until I stopped it, but then if I tried to restart it, it would not start until it cooled down no matter what I did. Once it cooled, I could start it first pull and it would run until I stopped it again.

The fix finally was to regap the magneto. What was happening is that as the motor ran and warmed up, the magneto and flywheel would expand enough to throw the gap off enough to make the mower not start. Once I regapped it, it ran perfectly and started perfectly, hot or cold. Adjusting the gap is simple. Just look up the specs for your motor. I think I used a thick piece of paper or a piece of a cereal box to gap mine. It won't cost you anything other than a little time to try it. It could also be the magneto coil itself is going bad. But I would try regapping it first.

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post #13 of 25 (permalink) Old 12-18-2019, 11:13 AM Thread Starter
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Joe Kuhn - thank you for your replies - I am still at it. I replaced the O ring, same thing. Looked at flywheel key and I will be replacing that. I looked at it and I didn't think it was a problem but, took photo and showed to one of my go to guys and he said that if it only has a nick in it it could be my problem. I'll let you know. Thanks again. I will not be defeated!
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post #14 of 25 (permalink) Old 12-18-2019, 02:55 PM Thread Starter
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suspicionofignorance - I need to give you the credit for this one, the reason I looked at the flywheel key. Reason I didn't give you credit earlier is because of my iphone. I saw your reply on the iphone but when I got to my desk top I didn't realize your reply was on the second page. I'm old and still don't follow things like I should. So last night I tore into the mower and that is when I took a photo and shown it to my go to guy. Thanks for your input.



pepperpikker - been there done that, that was one of the first things I done. Many years pushing lawnboys so I'm very familiar with coil issues and spacing. Thanks though.
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post #15 of 25 (permalink) Old 01-02-2020, 05:47 PM Thread Starter
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I give up. I replaced the flywheel key and it now is worse, it now has a slight miss and even backfired slightly. When I find someone to take it off my hands it is theirs.
Thank you for all your help.
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post #16 of 25 (permalink) Old 01-02-2020, 11:01 PM
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Did you ever do a compression test and if so, what was the reading? If this is a OHV motor, it is easy to adjust the valves, if a flat head, a little more involved.
You said you replaced certain items, was the spark plug changed?

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post #17 of 25 (permalink) Old 01-09-2020, 04:12 PM
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I'd let it run until it quits. As soon it quits, pull the spark plug wire off, stick a brand new plug on the plug wire and turn the engine over to see if the spark plug sparks. If it doesn't, it's mag failing when it's hot.



My money is on a magneto coil that shorts winding to winding where the winding is made from a stretch of wire with poor insulation. When the mag gets hot, it expands. When it cools, the windings retreat enough to separate, act as coil, build a magnetic field and generate a spark when the field collapses. In the late 1980's, coil wire was rated and sold by how many feet of not-up to-spec insulation per hundred feet of wire. Cheap toys got the low end, aerospace and implant pacemaker charger manufacturers bought the high end. Engine mfgs were probably in the middle somewhere. It's probably the same nowadays.
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post #18 of 25 (permalink) Old 01-09-2020, 04:47 PM
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I already suggested the magneto, but OP says that has been ruled out.

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post #19 of 25 (permalink) Old 01-11-2020, 04:08 PM
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Interesting problem. When the engine "shuts down" does it do so immediately without hiccups?....that's electrical.....or does it run irregularly, sputter for a few seconds?....that's fuel.

Maybe I missed it, but when you try to restart right after a shutdown, what happens....Will it not restart until after a cooling down period?
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post #20 of 25 (permalink) Old 01-29-2020, 10:31 AM Thread Starter
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Robin said - Interesting problem. When the engine "shuts down" does it do so immediately without hiccups?....that's electrical.....or does it run irregularly, sputter for a few seconds?....that's fuel.

Maybe I missed it, but when you try to restart right after a shutdown, what happens....Will it not restart until after a cooling down period?



I probably was not clear - but here it goes.
This is my son-in-laws explanation - After running flawlessly for about 15-20 minutes it seems to start to sputter some but never shut down completely. Mind you this is in the heat of the summer and I didn't get this mower to fix until fall and the weather here is much cooler then. I personally have not had it sputter until my last supposed fix.
Now since I have had it, in the fall, I have done this on several occasions, and that is mulching leaves and cutting grass which it ran fine for 20+ minutes but, after I stop it then try to start again it doesn't want to re-start until a cool off period of about 1/2 hour. After that time it starts up, first pull and will run for another 20 minutes or more with no problem, again in the cool weather



NOTE: and I think I have covered this so please read through this thread for other items you may have missed - I know I must have a fuel delivery issue and here is why (this I think I covered earlier) At shut down, NOT on its own - I shut it down, I tried to pulling it 5 or 6 times then immediately pulled the plug and found no fuel on plug or in cylinder. Fuel delivery issue, vapor lock don't know why - cap changed, run it with cap loose, new carburetor.
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