Lawn tractor with 15.5hp Briggs & Stratton engine won't start

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  #1  
Old 11-11-17, 01:51 PM
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Lawn tractor with 15.5hp Briggs & Stratton engine won't start

In August I bought a battery, it still had trouble starting but eventually would. Today the engine would just turn for about a second, then stop. I'd turn the key off/on and the same thing would happen. Even tried charging the battery and jump starting the tractor, no luck. The starter itself emitted a bit of smoke and was very warm. Removed it, brought it to a reputable power equipment co. Was told it could be an engine valve problem. Tested the starter there, was told it turns fine. Did the same at home for the heck of it, seems to have plenty of torque. Reassembled everything for now. While attempting to start it today, it did backfire. It's a 2006 MTD or Murray tractor. a Lowes model. For about 8 years the transmission has been defective, if I re-engage forward I must 'double clutch' it is the best way to describe it, but it runs fine once it's engaged. At this point I'm thinking it's not worth putting money into it. The only thing making me still consider it is because I transferred the bagging system for the Agway tractor I had years ago and does a decent job with getting leaves up. Does it sound like a valve defect of some sort? Since the spring it would do the same, turn for about a second then stop. Usually after the second - fourth try it would start.
 
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  #2  
Old 11-11-17, 03:13 PM
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I had the exact same sounding issue and got rid of the mower, next owner adjusted the valves and it now runs and starts fine.
It's a cheap fix and worth a try.
 
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Old 11-11-17, 04:38 PM
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I'll need to see if I can find anything on how to adjust valves. The backfiring (actually twice today) when attempting to start it made that cause seem likely.

I have the engine manual, it specifies the valve clearances for intake and exhaust. Also saw a video of the adjusting procedure. Not sure if I'll attempt this myself, don't have a feeler gauge but it seems they don't cost too much. The specs are based on "with valve springs installed and piston 1/4 in. (6mm) past top dead center' Don't have any idea how to get that set.
 

Last edited by stevek66; 11-11-17 at 06:45 PM. Reason: Additional info after research
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Old 11-12-17, 05:31 AM
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When you have the valve cover off you will see the motion of the two valves. This procedure should be done right after the compression stroke ends and the power stroke begins. This is to ensure that both valves are fully closed. As you may know a 4 cycle engine has the following strokes. Intake, compression, power and exhaust and it then repeats those 4 strokes. So when turning the flywheel you will see the valve movement. Since the exhaust stroke and intake stroke are right next to each other it is the only time where one valve closes and one immediately opens. This not only can tell you which valve is which but now tells you what stroke the motor is on.

OK. So take out the spark plug and place a pencil in the spark plug whole and butt up the rubber end to the piston head as the piston rises up on the compression stroke. Turn the flywheel until the pencil stops rising out the spark plug whole. Make a mark on the pencil that lines up with the spark plug whole and make another mark 1/4" above it. Now turn the flywheel until the 2nd mark lines up with the spark plug whole.

Your engine is now 1/4" past TDC after the compression stroke.
 
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Old 11-12-17, 09:12 AM
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Thanks for the tips/instructions.
 
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Old 11-12-17, 12:42 PM
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Adjusting the valves will solve this problem. It's supposed to be done periodically and it sounds like you've never done it, so very likely to be out of specs. The backfire is from slow cranking more than valve adjustments.
 
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Old 11-12-17, 05:49 PM
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A neighbor who had a feeler gauge came over. We adjusted the valves accordingly. No luck starting at that point since the battery was weak. Charged it, tried again and got a backfire. About 40 minutes of charging again, first got a weaker backfire but it started. Tried again about a half hour later, started without a backfire. I never did adjust the valves, I'll probably buy a feeler gauge since it should be routine maintenance. I'm surprised how being loose such a small amount could cause this trouble. When I first took that valve cover off, only one valve seemed loose, but not much. The other was tight. I assume because the piston wasn't TDC. Once we got it to TDC, there was noticeable play with both. Thanks for the help. Leaves are finally coming down here so hopefully it will run OK next weekend. I transferred the bagging system from the previous lawn tractor I had to this. I does a decent job picking up leaves.

One other question - for this engine is the exhaust valve on top?

https://imgur.com/EhHn12V][/url][/IMG]
 

Last edited by stevek66; 11-12-17 at 07:04 PM.
  #8  
Old 11-13-17, 01:01 AM
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Yes, it's the top valve. I adjust both to .005".
 
  #9  
Old 11-13-17, 05:53 PM
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As you said, adjusting the valves solved the problem. Thank you for confirming the valve positions. After approximately how many hours would you recommend a valve adjustment?

It's amazing how being off such a small fraction of an inch could interfere with the 4-stroke process.
 
  #10  
Old 11-14-17, 05:20 AM
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I adjust mine when your problem starts to show its ugly head. Usually you can see it beginning before it leaves you high and dry by not starting at all. Maybe the motor hiccups once or twice. I find that when it is on the edge, if you move the motor JUST pass the compression stroke (that stroke you can feel, no need to view the valves) the motor will get enough momentum to start, even when the valves need adjusting. This all depends on how much out of spec they are.

Just to be sure you understand, these motors come with a compression release mechanism to allow for starting. Without this compression release, your battery will never be strong enough to turn over that motor. It will get stuck up on the compression stroke and stop turning over, during the starting process. When your valve cover was off you can observe this little bump in the intake valve, that allows enough compression to be released so that the battery can turn over the motor. You have to be looking for it, since it is not much of a bump and happens very quickly. This bump is only a few thousands of an inch big so if the valve clearance is larger then the bump, the bump only moves the rocker arm and NOT the valve itself, and no compression gets released. Lastly, if the valve clearance is too little the valve may not close completely, causing a bigger compression loss problem.

So that is why there is a spec. Not too much and not too little, makes it just right.
 
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Old 11-14-17, 10:44 AM
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I don't recall when Briggs advises it to be done but I'd do it at the first sign of problems at least, like OpsyEagle says. Probably every 200-300 hours should be a good point of reference.
 
  #12  
Old 11-16-17, 06:28 PM
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I could see then if the adjustment was off, the complete compression release process wouldn't occur. I assume when the battery was a bit stronger, or enough fuel/air was being released for the battery to keep the engine turning and then starting, the engine of course having ample torque is what kept it running.
 
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