briggs model 12M902-0451-E1 hard to start hot

[lugnut1]

engine is L-head 4.5hp

ok what I've done so far:
checked spark, cleaned carb, flywheel key, all checked ok.
took compression check 60psi.
pulled head off rotated flywheel and noticed that at about 3/4 up on compression stroke the exhaust valve will open slightly then closes.
(this is new to me) thinking maybe cam lobe may have been ground wrong.
or is this something they came up with to lower compression for some reason?

 

[mechanic mark]

http://www.ccdist.com/_serviceManuals/briggs/276781.pdf engine service manual compression is low at 60, check manual out.

https://www.briggsandstratton.com/us/en/support/manuals/results?NTT=12m902-0451* engine operators manual & parts list.

Compression System scroll down while reading

 

[lugnut1]

thanks for the links Mark

learn something new everyday

straight from the horses mouth
Compression Release System
Some small engines incorporate a compression release system to decrease operator effort when pulling a rewind starter. A compression release system is a system that relieves excess pressure during the compression event by allowing a small amount of compressed gas to be released through the muffler or carburetor.

The compression release system lifts either the exhaust or the intake valve slightly off its seat during the compression event (this is why the piston is positioned past top dead center to about 1/4" down from the top of the cylinder when adjusting valve clearances). This releases pressure and reduces the force required to pull the starter rope or load on the starter motor. A compression release is used on most Briggs & Stratton engines and does not affect engine performance above engine starting rpm.


Compression Release Problems
Too little or too much compression can damage pistons, rings, valves, valve guides, valve seats and the cylinder wall. If an exhaust valve leaks, exhaust can back up into the cylinder, causing premature wear. Too much compression can cause the air-fuel mixture to burn too fast, causing knocking or pinging. Excess compression can leave carbon deposits that further aggravate problems.


maybe useful:
http://www4.briggsandstratton.com/miscpdfs/RNT/Engine Specifications Chart_ms3992.pdf

 

[lugnut1]

update:
was going to put new rings and do valves on it. (being compression low)
but owner doesn't want to invest more money into it he's just going to get another mower. (says he's had to many problems with it, and wants to upgrade from side discharge mower anyways).

told me I could have it.

(I'm going to shelve it for now, and re-ring and do valves later on)