dead B+S mower engine

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The mowers I've serviced. if the valve clearance is too low, the mower runs 15 minutes, fades out, and died. Restarts after it's cold. I learned that in a small engine repair course. One guy a couple streets over was going nuts on a mower repair, and turned out valve clearance was the problem.
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Christopher A. Young
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On Fri, 30 Apr 2010 13:19:10 +0000, Jules Richardson wrote:

I'm wondering about the compression, now. Can low compression result in an engine that won't even start/run under no load?
It's got some compression for sure (nice healthy hiss from the crankcase breather on the down-stroke if I turn by hand), but I tore into the 11HP engine from the donor mower I picked up at the weekend, and that engine's piston fits *far* more snugly in the bore than on the 10HP. It slides well (so no apparent issue with ears of it sitting there unusued), but fits better with no detectable side-to-side movement if I try wobbling it, unlike the 10HP.
(Although if it is a compression issue, I still don't get why it was running last season and magically broke over the winter...)
Oh, I cleaned up the carb on the 11HP and tried it on the 10HP. It will fire consistantly for maybe 4-5revs, then die, so not really any better than the 10HP's own carb was. No different swapping the plug, either. Magneto is unfortunately trashed on the 11HP, so I can't try swapping that.
cheers
Jules
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Jules Richardson wrote: ...

Certainly, but it has to be pretty doggone low to not run at all...

If you can use a thumb in the spark plug hole and keep it from blowing by, it's a good chance it is that bad...
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On Wed, 05 May 2010 13:27:36 -0500, dpb wrote:

Hmm, well it fails that test (or passes it, depending on how you look at it. I can keep my thumb there while it's turning over; I can feel it compressing, but it's not so much that I can't keep my thumb in place.
I've only worked with big multi-cylinder engines before, so messing with the small single-cylinder stuff is a bit of a (fun!) learning experience - I'm not sure how good compression *should* be on such a small engine, but it sounds like this engine doesn't exactly have a lot :-)
(unfortunately I do have a compression tester, but it's 4000 miles away, which isn't much use right now! Maybe I should just cave and buy another one...)
cheers
Jules
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http://www.harborfreight.com/flex-drive-compression-tester-92697.html Ten bucks. You can likely find a 20% off coupons, too.
As to piston rings loosening. I'd consider diesel or WD-40 which at least have some lubricating value.
Thanks for the correction: Spark plug gap .030 sounds right.
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On Wed, 5 May 2010 17:53:42 +0000 (UTC), Jules Richardson

One of the piston rings froze up?
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Yep, I thought the same thing.
OR A valve is sticking open.
Before ripping it all apart, pour some kerosene or diesel in the sparkplug hole. (bout a quarter cup) Put the plug back in finger tight, without the plug wire attached. Turn the engine over by hand. It will likely not turn all the way because of fluid lock when the piston comes to the top. Leave it sit like that for a day. If a ring is stuck, this may free it, as well as a stuck valve. You can always open a cover on most engines to see the valve stems move. You can always work them up and down with a plyers.
When you want to start it, remove the spark plug and spin it over to blow out the remaining kerosene, clean the plug and reinstall it, and try to start the engine.
Once you get it started, change the oil because the kerosene may have diluted it.
If the kerosene is gone after one day, you definately have a leaking piston ring.
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I just reread your original message. Since you have the head off,
lower the piston and pour some kerosene on it. Work it up and down by
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On Thu, 06 May 2010 03:06:48 -0500, mister_friendly wrote:

Yeah, I'll give that a go; as it's a horizontal cylinder I'll have to pull the engine, or just stand the whole mower up on its butt :-) It seems like a worthwhile test, though - I really don't like how loose that piston is feeling compared to the "new" 11HP engine I got.
cheers
Jules
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On Fri, 07 May 2010 12:25:36 +0000, Jules Richardson wrote:

OK, now sitting there with a gas/oil mixture (about 75% gas, 25% oil) in the bore - I'll see how that goes if I leave it overnight.
I checked the valve clearances - 0.005" on the inlet and 0.011" on the exhaust, so those are within spec.
I got the 11HP engine running earlier, borrowing the magneto and spark plug from the broken 10HP. That seems to rule those out as culprits on the 10HP. I still can't definitely rule out carb issues, although it's interesting that the 11HP ran with its carb, but that carb didn't get the 10HP going.
I've not checked the ignition timing yet as I didn't have a suitable wrench for the 1-1/4" bolt on the flywheel, but I suppose I can always compare the flywheel position on the 10HP at TDC relative to the 11HP.
cheers
Jules
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On Fri, 07 May 2010 23:39:04 +0000, Jules Richardson wrote:

And just as a final follow-up, that mix had all leaked through into the crankcase by the following morning... (oh, and I cross-checked flywheel position against the 11HP engine, and no problems there, so not sheared key)
Sometime I'll pull the piston out and check the rings, but I'm not in too much of a hurry now - I dropped the 11HP engine into the mower at the weekend and then cut the grass, and it worked far better than the 10HP ever had and didn't go through any more gas doing it.
I might try and fix that 10HP one day though (assuming it just needs new rings rather than a re-bore) just to keep a spare around... or I might find a use for another engine around the place.
cheers
Jules
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On Tue, 11 May 2010 15:07:05 +0000 (UTC), Jules Richardson

It is always (IMO) a good idea to hone the cylinder to remove any glaze and assist the new rings in seating.
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On May 5, 12:53pm, Jules Richardson

A compression gauge is cheap, maybe 15$ and will tell you all you need to know before wasting any time on a junker. I cant give accurate numbers but I will guess and say 130 is new, 110 its at half life and near 80-85 its not going to want to do anything. Try misting in gas with a hand spray bottle just as it starts.
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On Apr 30, 9:19�am, Jules Richardson

First.......put in a new plug. Just because it fires, doesn't mean the plug is good. I have seen engines run (poorly) with a bad plug.
You say it has compression, but does it have enough. Just because the valve seats look good, doesn't mean the are closing all the way. Check the gap, should be approx. .04
Hank
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Which gap? Spark plug? Valves? Piston rings? or.... or.....
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On May 5, 8:54�pm, "Stormin Mormon"

The valve clearance. Over time the valve seats wear and the gap closes, not allowing the vlave to close completely. If the vlave clearance is less than .04, the valves MUST be removed and ground down to the proper clearance.
Hank
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When I took a small engine course, we learned that the intake valve clearance is .010 and the exhaust is .020 inches. That from memory, and it's been a lot of years since then.
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It was very apparent what you meant...some people need to be hit with a 2X4!
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Spark plug gap for BS engines is 0.035, so 0.040 isn't that far off. Some idiots need to stop embarassing themselves with clueless flames.
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with clueless flames.
Now you're saying you were right comparing a spark gap with a valve clearance gap...and you're NOT the idiot here!!!!!!!!!! You have waaaay too much time here giving bad advice that you can't keep straight in your head (age related).
Bob Villa
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On Thu, 06 May 2010 12:58:07 -0400, Stormin Mormon wrote:

Just for the record, 0.030" for this B+S engine according to their operating manual (just in case someone stumbles across this thread at a later date! :-)
cheers
J.
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