Our Toro mower with this engine started first pull after the winter. Ran fine for first lawn cut. Put mower on its side to clean under deck then as I usually do tried a restart. It wouldn't start so put away figuring probably just flooded. Got it out the other day and it wouldn't start. Pulled plug and it was wet with gas. Checked spark and that was good. After a few more tries I took off air cleaner and blocked the choke open and the engine fired up straight away. Ran perfectly for an hour of lawn mowing. Got it out the next day to finish and it wouldn't start. Blocked open choke and wouldn't start. Occasional loud bang out of the muffler as I pulled it over. Finally did a compression test and there was absolutely no compression. I pulled off the cylinder head suspecting a stuck valve but both are moving freely and I can rotate both valves with my fingers. They both seem to fully close. Is it possible that the valve timing is way off. I'm not familiar with the internals of this engine. Is it possible for that to happen very suddenly like this? Any other ideas?
Model 128M05-0026-F1 with no compression.
- Thread starter mikosh6
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If you can rotate the valves with your finger pressure while the valve is fully closed then the valves are not sealing against the seat or the seat has come loose. Will require further disassembly to remove the valves and then grind the stems to reset the proper clearances.
Thanks Ilengine. No I can only rotate the valves when they're up off the seat, gripping the valve edges with my fingers. I'm just really surprised that I can't see any movement on the compression tester needle. Same with putting my finger over the plug hole, can't feel any pressure at all. Even if valves were seating poorly or compression rings were damaged or worn I would still think there'd be a small amount of compression.
Might need to spin the engine with a drill to deactivate the compression release to get a true reading. Is the head gasket good? Blown will cause loss of compression. Also if you whacked something recently the flywheel key may be slightly sheared causing a timing issue. If you are getting a loud bang out of the muffler I would guess you have at least some compression.
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even with the ACR, you should still get at least 55-60 PSI compression on these model 12 briggs.
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It is possible that the valve timing could get thrown off as these engines use a plastic timing gear on the crankshaft (some metal) and camshaft gear, but i've worked on tons of these particular engines and have never seen one fail in that way, usually that happens if you strike a solid object with the blade.
You've verified the piston is moving in the cylinder, correct?
This engine is a flat head / side valve design. Does that type of Briggs & Stratton have compression release? I'm using the electric start to crank it over at a pretty good clip and not seeing compression. Is it possible for the compression release to fail and remain activated when the engine is turned over at higher speed? If so it could be the problem. The head gasket is good and yes the piston is moving.
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Yes the camshaft has an ACR.
Crankshaft gear is supposedly metal.
I would be checking the valve clearances as they do close up over time on L-head engines. When they have near zero clearance the engine loses most if not all it compression.
BTW most compression tester sold are for larger engine and not small engine like yours. Here I have Actron that is designed for small 23cc + engines. The compression check valve also must at the end of the hose at the cylinder end; otherwise the hose will expand and collaspe back to normal on every compression stroke throwing the readings.
Crankshaft gear is supposedly metal.
I would be checking the valve clearances as they do close up over time on L-head engines. When they have near zero clearance the engine loses most if not all it compression.
BTW most compression tester sold are for larger engine and not small engine like yours. Here I have Actron that is designed for small 23cc + engines. The compression check valve also must at the end of the hose at the cylinder end; otherwise the hose will expand and collaspe back to normal on every compression stroke throwing the readings.
bertsmobile1
Lawn Royalty
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Or a full metal hose like the one I use
I will check the clearances. I'm not familiar with how ACR works physically. Does it rely on the shape of the camshaft lobe or is there some camshaft movement due to centrifugal force. I better do some research on that. Thanks everyone for the helpful suggestions.