Yardman//Briggs -Strat won't fire and run

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  #1  
Old 07-17-04, 09:10 PM
Bee Gee
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Red face Yardman//Briggs -Strat won't fire and run

Briggs mod 12W802 mounted on selfpropelled Yardman 3 in 1 mower seemed to bind up while pulling starter rope. Acted like broken rod or internals broken and jamming crank. Play with it a little and it would turn over. Tore engine apart, found nothing obivious other than lots of wear. was not good about changing oil first two years thus unit smoked periodically on side hills and "in and out" under trees. Ex Valve pitted/burned, moderate to heavy carbon in combustion area, replaced valve, carbon cleaned easily from head, replaced rings with stock issue (broke one when removing to inspece piston, groves, and old rings), new gasket set, new carb primer, cleaned carb, reassembeled, tried to start - no go. Spark week but present, primed manually directly into cyclinder and later into carb throat, Not even one pop. Tore a part 2 more times, still couldn't see or find anything I hadn't seen already. Noticed that under air pressure testing for valve integrety, that X-Valve did not close but figured this was due to compression release mech on cam assembly. Tried to check tappet clearance, but there is NO separation between tappet/valve on either X or In valves.
Frustated and am buying new mower but ticked off that the porblem(s) weren't evident during 4 tear-downs and re-assembly.
Any Ideas???
 
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  #2  
Old 07-18-04, 12:05 PM
Azis
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Cant find anything with that model # for briggs or yardman.
You say spark is present but weak? How are you testing for spark? Use a spark plug to test and make sure if you see spark that it is jumping the gap at the electrode and not just jumping to ground. If you have evidence of spark but not accross the gap of the plug, replace the plug.
Otherwise post back with some #'s both from the mower and engine (more the better) and we'll get out the big guns if need be
 
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Old 07-18-04, 08:16 PM
Bee Gee
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Yardman//Briggs -Strat won't fire and run

Aziz, thx for reply. New Info follows:
Briggs Mod # 12W802; Type 1174-01; Code # 95121359
Engine mounted on a Yardman (MTD) 3-in-1 mower, Mod # 126-849C-401 with Ser #1E166B60126

Spark observed was thru the plug while being grounded on the engine while pulling rope. Appears consistant. "Weak" was my term for visual appearance, expected more robust spark but did see a spark with pull over. Hesitate to try the wet finger test! also noticed approximate 1/8" end play in crank, thought crank was a little loose in the main bearing mounts, but seemed acceptable when covers were assembled and crank had no side to side play. Key on flywheel was good, no slippage or shear, coil gap was VERY close but not binding at any point. Carb has history of causing problems with the slightest water or dirty gas contamination. Have had to dissassemble once per year on average and clean/blow out, then ok. The injection of gas directly into the cylinder, and later into the carb throat should a least produce a "pop" even if the carb is dirty. Keep comming back to the X-valve and In-Valve with no tappet clearance or maybe a timing thing. Each reassembly the two dots on the crank gear and the cam gear were lined up notch to tooth.
 
  #4  
Old 07-19-04, 03:06 PM
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I once had a similar problem, in fact I removed the carburetor, hean, and valves about 10 times before I realised the problem. First off, yes one of the valves will be held up a little for easy starting, but the should have some clearance between the valve stem and tappet. Second, the propper way when replacing a valve is to lap the valve and valve seat together. This ensures a complete valve to seat seal. The exhaust valve would be letting compression leak past it. Did you clean the valve seat? The valve seat could be pitter or warped, which would require grinding the valve seat or replacing it. Also, make sure the flywheel key isn't sheared. Good luck with the problem, I know how agrivating it is to pull till your arm hurts and still not find any problem. It took me 6 weeks to figure out the valve was giving me trouble!!!!!
 
  #5  
Old 07-19-04, 10:33 PM
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You should get around .050" clearance between the valve stem and tappet. If not, you'll need to grind a bit off the stem. Lapping the valve face in would be a good idea too. Double check your clearance afterwards. Have you checked the flywheel key? Sounds like it is sheared to me.
 
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Old 07-20-04, 12:35 AM
sceptre
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Originally Posted by cheese
You should get around .050" clearance between the valve stem and tappet. If not, you'll need to grind a bit off the stem. Lapping the valve face in would be a good idea too. Double check your clearance afterwards. Have you checked the flywheel key? Sounds like it is sheared to me.
cheese,

the intake valve gap should be .005-.007. the exhaust should be .009-.011. no more than .010 on either. hope this helps you out.

sceptre
 
  #7  
Old 07-21-04, 01:33 AM
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Oops, I meant .005", not .050". BIG difference! LOL!
 
  #8  
Old 07-21-04, 07:21 AM
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This engine obviously ran without oil at some point and now has suffered the consequences. Two issues come to mind based on your posts. 1- The upper bearing is trashed, causing excessive side play and thereby giving an inconsistant coil to flywheel air gap. 2- The lack of lubrication has caused - among other trouble - valve-train issues. Sounds to me, though, that you now have alot of time and effort into this machine (not to mention, I'm sure, alot of money) and no sense giving up now. Go ahead and grind the valve stems to give proper clearances and set the armature clearance at .020" to compensate for the upper bearing wear, which hopefully isn't more than the ignition will allow. Good luck and let us know.
 
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