Need Briggs troubleshooting info

I was mowing the lawn today with our MTD garden tractor when it suddenly and without warning started sputtering and died. The engine is a 14.5 HP Briggs & Stratton OHV job with electric start.
I checked for obvious stuff like clogged fuel and air filters, and cleaned the spark plug (it had some carbon deposits). Still no good, the engine barely catches even with the throttle wide open, backfires through the carb, and dies. On a car I'd say it sounded like it was trying to run with severely retarded ignition timing, or too lean a mixture, but I'm not quite sure what to test on this thing. (This is a 1996-vintage mower, so I'm pretty sure it has transistorized ignition, not points.) Maybe some dirt got past the fuel filter into the carb? Any "gotchas" to be aware of in taking the carb apart?
Any pointers on what to check, or to troubleshooting/tech info online would be greatly appreciated!
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Roger Blake
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An engine model/serial number might help with this. You could also try the Small Engine Technical Forum at http://www.perr.com/forum/ Some very knowledgeable people there.
GrtArtiste
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wrote:

Thanks for the lead on the tech forum, I'll check it out. The engine has a metal plate with the following info:
MODEL: 287707 TYPE: 0224 01 CODE: 9603044A
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Roger Blake
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Roger, If by chance you hit something, the key could have sheared (completely or partially) and then the timing will be off. You can pull the starter cover and look at the flywheel to check.
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jim.blakely wrote:

A compression test will tell if it is a burned valve.
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Compression is 100 psi, which would be low for an automotive engine but I'm going to guess is pretty much OK in this application.
I pulled the carb off, it is a side-draft Walbro that has a dome-shaped sheet metal fuel bowl with what looks like a solenoid (fuel cutoff?) on the bottom. How do I get the fuel bowl off to get inside? I don't see anything obvious and don't want to force/break anything.
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Roger Blake
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Roger Blake wrote:

Unscrew the solenoid.
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I have not had time to get back to it yet, the engine does run, but just barely and spits back through the carb.
I've purchased the B&S book "Small Engine Care & Repair" which has a fair amount of detail on the carb and ignition system. I need to scout up a thin wrench to get the solenoid off the bottom of the carb so I can at least clean the jets etc. in there. (I've already put in a new spark plug.)
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Roger Blake
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Today I pulled the carb apart to clean it, though it was pretty much spotless inside. Didn't make much of any difference in the way the engine runs, it's still weak and backfires through the carb on every crankshaft revolution. I also pulled the big bolt that holds the flywheel in place and the key looks OK, no sign that it has sheared, everything lines up. (Anyone know the torque spec for that bolt?)
So to summarize:
* Engine barely runs, backfires out carb on each crank revolution (puff of air accompanied by loud 'pop') * New spark plug * New gas & air filters * Carb cleaned * Compression is approx. 100 PSI * No sign of shearing on flywheel key. * Solenoid on carb works OK (clicks with ignition, also tested off carb)
Symptoms seem to be that of late ignition timing, but I don't see any obvious adjustment for that. Any ideas on the next logical troubleshooting step?
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Roger Blake
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On a suggestion in the perr.com forum, I removed that valve cover and found that the stud for the topmost valve had vibrated out and was partway unscrewed from the cylinder head! This of course royally screwed up the valve timing. I've tightened it down (fortunately it does not seem to be stripped) and will see how the engine runs after checking valve clearance...
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Roger Blake
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Good to hear that you are making progress. Keep asking those forum folks questions, and you'll keep getting answers. It makes for interesting reading, even if you don't need help with a problem.Another place I go to with lawnmower/snowblower info is www.opeonthenet.com
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