What should the compression be?

40 is super low...
 
Can you post an engine model #????
 
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https://www.jackssmallengines.com/jacks-parts-lookup/manufacturer/briggs-stratton/briggs-stratton-engine/100000-19z999-series/128m00-to-128m99/128m02-0017-f1
 
That model # is showing a "flat head" engine-NO overhead rockers. 
I don't believe they had a flat head engine that large..
 
IF it is a FLAT HEAD engine, check  / tighten the THREE HEAD bolts CLOSEST
to the exhaust valve.  They loosen up (very, very common) and will cause
no start issues...   
 
About 15' lbs on the head bolts.  
 
If you have an overhead valve engine, advise.  I own a 7 HP Briggs OHV
engine and can get you a pressure #...   Advise..
 
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It is a flat head. The  head bolts are tight.  It is acting different from what i know about engines.  I have replaced the coil, spark plug, checked the ground wire even though I have spark.  It starts when it's cold but is really hard to start after warmed up. The only smoking gun is the 40lbs compression. Why it would start when cold is beyond me. It won't even chug with starting fluid. That led me to check spark and it has plenty of spark.  I also check the keyway and it's fine. My next step is to take off the head and check valve clearance, lap the valves and install a new headgasket. I don't own a leak down detector. Are you able to find what the compression should be? Can the cam get stuck and be the cause of low compression or is it the opposite and there is too much compression to turn it over if it's broken?

I don't currently have a flat head engine to check but can give you a # on my OHV 7.25 Briggs.
 
Usually, the minimum is 100PSI.
 
I agree, that cam does have a compression release. The valve lash is checked
under the cover under the intake manifold.   To adjust wider, the valve must be removed and the tip slightly ground down to open the gap.
 
Now, with that said, when the engine gets hot, parts expand.  The valve clearances also tighten up
causing a possible , slightly open valve lowering compression even more.
 
I'll post a # later today.  I do have an old 2 hp flat head Briggs I'll check too...
 
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My flat head  2 HP Briggs is 65PSI ,
My 7HP, OHV is 85PSI, both cold..
 
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