Craftsman LT1000, Briggs & Stratton issues

18hp B & S OHV 31P777 engine.
Before, I had issues with valve clearance (starter struggled to turn crankshaft), but adjusting the valves fixed that. Some time after, I had to adjust the valves every other 3 mowing periods due to the same issue.

Last year, same issue, but the remedy was to hold my hand over the intake while cranking and this caused it turn smoother. This has became the normal starting procedure.

Just pulled out the mower for the first time this year, checked oil, belts, etc and adjusted the valve clearances. Same starting procedure. It wouldn't start for a while, and had to clean the wet spark plug a few times. Got it started once, turned it off, and it wouldn't start again. Tried using a portable jumper as to not kill the battery and give the starter juice but it wasn't enough.

Connected it to the car, and without putting my hand over the intake this time, it started but instantly at a high rpm while the throttle never moved from it's lowest point. Even with the throttle at it's highest, it's never run such a high rpm so I turned it off after no more than 10 seconds. Started it again, same thing, but after 20 seconds it sputtered and started knocking. Took the head off and found that the piston rod broke at the crankshaft. Everything else looks fine.

I'm gonna order another rod, but was wondering what else do I need to check, clean, and/or replace before it breaks the new rod and while I have I'll have most of it disassembled. So far, I've only taken off the head. I've never replaced anything on this engine except the nuts for the setting the valve clearance. What would cause it to run at full rpm without throttle input? If the valve clearance is set right, what would cause it to turn smoothly only if the intake is covered when cranking?

 

Hello Crankshaft,


For this I would suggest checking the rocker arm studs and make sure their tight in the heads, They have been known to back out.



Just wondering how you seen the rod broke at the crankshaft with only removing the head ?

A broken rod near the crankshaft is a sign of lack of oil which causes aluminum transfer to the steel crank thus locking it down and snapping the rod.



I would suggest getting the crank out and check the rod journal condition before ordering any parts, Use a micrometer and make sure it's still with-in specs if it's not totaled from the aluminum transfer. If it's still usable look closely at the mechanical compression release on the camshaft, Excessive wear on it could be playing a part in your frequent valve adjustments along with the rocker studs. I personally don't go inside a engine with a plastic governor gear without replacing it as well, It's just to have piece of mind it will last for a while.



After a engine is shut down a certain amount of fuel is left sitting in the bottom throat area of the carb, After sitting a while the fuel gums up the throttle shaft and causes it to stick wide open which is where it stays when not running. Next start up the governor can't pull the throttle shaft back on start up so it revs wide open, Check and clean both the throttle and choke shafts with carb & choke spray cleaner should solve this problem.



What you are doing here is reducing the amount of air intake into the cylinder, With less than normal air intake there will be less air for the pistons to compress therefor lower compression so the engine spins faster.


Good Luck

 

I agree an all points and only would add that there are a couple other things that could make the engine run wide open like the throttle shaft breaking, or the screws coming out of the throttle plate, or the governor coming apart, but since it was sitting all winter before this happened, it's probably just stuck. Check the clearance between the balancers and the crankshaft too. Sometimes there is enough wear in enough places to warrant getting another engine rather than repairing.