Service & Maintenance Question
Mower won't start, starter only spins half a turn
Service & Maintenance Question posted June 21, 2018 by Bryan Shumway
91 Views, 13 Comments
Question:
Mower won't start, starter only spins half a turn
Details:
I have a LTH 18538. When I turn the key, the starter only spins about half a turn, basically until the compression stroke on the piston. If I take out the spark plug, it spins and spins with no problem. I've replaced the battery, solenoid, and starter, but the problem remains exactly as it was before. What am I missing?
Answer
Either valves need adjusting to spec or the camshaft ACR is broken.
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Thanks for the quick reply, this sounds plausible. Help me improve my diagnostics, how did you get to valves or ACR from the symptoms? Just because of removing the spark plug?
Second, I've never dealt with an ACR before. Where is it and how do I check it?
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Hmmm, maybe it the 9 yrs of working on OHV engines. And I am fairly certain you have a Briggs on the mower.
On OHV engines when the valve clearances get too large the ACR (automatic Compression Release) becomes ineffective causing the engine to try start at full compression the starter is incapable of overcoming. And yes removing the spark plug removes the high compression that the starter can't overcome otherwise. These valve needs to be adjusted every 50-100 hrs of operation.
Now the Briggs 280000, 310000, and 330000 series the camshaft has a bad habit of ACR breaking thus no longer able to decompress the engine's cylinder. To check for this you manually rotate the thru at least two complete revs while the rocker arms for the ACR bump right at TDC compression stroke. On these Briggs it is the intake rocker that will have the ACR bump but valves must first be adjusted to spec otherwise the clearances might too large for this bump to occur.
BTW I have already seen two camshaft's ACR broken so far this year and it does require removing the oil pan to check for other damage if this has happen and to recover all the broken metal pieces. These 793880 camshafts lists for $85 plus you also need a crankcase gasket, RTV, and new engine oil.
It might be a little comfort that you are not the only one that have gone all this expense just to find out only the valves were needing adjusting though sometimes they do need a new camshaft.
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You nailed it. I've got a Briggs 310000 series. Thanks for the explain on the ACR. I'll have to inspect everything today or tomorrow. Any special valve clearance for this engine or is it typical .12 or .13 range?
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I believe you're referring to mm I hope and that would be fairly close. I am more acustom to using the SAE inch.
Valve setting are as follows:
Intake .003" -.005"
Exhaust .005"-.007"
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Okay, I checked the valves... I found intake = .004" and exhaust = .006", so right in the specs you gave me. Now on to this ACR bump. Am I looking for it on the intake rocker or on the camshaft? What does a good bump look like and what does a broken bump look like? I'll poke around elsewhere while I wait to educate myself further... Thanks!!
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And found it. YouTube is great. Getting to the cam is a fair amount of work, but as long as I can get the bottom pulley off, I think everything else is pretty straight forward. So, before I go down that road, I just want to make sure again that we have ID'd the correct problem. Is there any possibility that this could be something else? For the record, I don't think so, but that's why I'm asking the experts!
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Sorry for taking some time to reply but I am working in my own shop this morning trying to get a few jobs finished before the storms move in again. Unlike some shops nearly all my work has to be done outdoors exposed to the elements.
Glad you hopefully find the correct info. Just remember recover all the broken pieces include that hair spring. all the pieces should pretty even the same volume as the new ACR on the new camshaft.
That ACR on the 793880 camshaft bumps the intake rocker and is fairly noticeable at TDC compression stroke but the valve clearances correct I would definitely lean to the broken ACR theory. And yes fairly easy to change out just to torque things back to specs as you reassemble the engine.
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No worries on the delay, since it's my uncle's mower, I can't do any more work until he gets back tomorrow. I hope you get some things completed on the to-do list! Last question for now... would you mind helping me on the torques? I think the only bolts I'm dealing with are the engine-to-frame bolts and the ones on the engine cover.
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The crankcase sump (cover) are torque to 220 in/lbs currently, was 180 but due crankcase gasket problem Briggs increased it to the 220. I use stepped torque procedure of 1/3, 2/3, and then full torque to ensure even torque and not to warp the cover. Here is the torque pattern.
As for engine mounting bolts I just tighten to hold the engine. Engine pulley bolt is torqued to 55 ft-lbs.
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ACR is the problem! THANK YOU. Took everything apart this afternoon and the ACR was laying in the bottom of the pan. Found "most" of what I think are the parts. Now, just hope I can remember how it all goes back together. :P Just kidding, that should be easy enough. However, I did notice my timing marks on the cam and crank didn't line up. And no matter how many times I spun it either direction it wouldn't adjust. So, I just put the timing mark on the crank next to the cam and I plan to put the new cam in to line up with that mark. Will that work? Or should I attempt to offset the new cam the same number of teeth as the old one?
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Just be carefully as some cams do have two timing for various engines they are used in. Just put the piston and crankshaft at TDC (Top Dead Center) and the timing marks should line up. Or if the engine was mistimed it should always off by the same amount no matter how times you turn the engine in either direction. As for timing it is exactly a 2 to 1 ratio so if the timing is off it will never line up until manual retimed.
It does some sounds like someone had in inside the engine before and mistimed the engine. Now when I was a teenager and that was over 40 yrs ago I did time one engine off by one tooth and engine actually ran with more power but I think it was just sheer luck.
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