What is the minimum compression? and what should a good engine read?

Wow I thought this was a pretty simple question but no responses. From what I can gather researching 100 psi is where they start getting difficult to start but will still run. 150 psi is what I think should be after a rebuild with new piston and rings. Does this sound close? My bike is presently giving me 135 psi so I guess I'm OK for now but heading for a new piston pretty soon

There are mixed feelings regarding compression readings with many of the experts on this forum. Your reading of 135 is about what I get on my DT400B and it shreds my lawn. I have yet to take a reading on my DT250B because it has always run fine. My "opinion" is that you'll be fine as long as it's fed a proper mix.

Well there is always the "your compression tester ability may vary" argument too. Yamaha's numbers are a controlled environment and standardized testing procedures & tools, because as a manufacturer they set the standard that we go by. 

Yamaha never published compression test numbers or specs used to obtain those numbers for the 70's Enduros as far as I know. 

And yes, one gauge versus another can vary considerably on these smaller engines...  the method (kick or push in gear, throttle open or closed, engine hot or cold, etc., etc.) and even the skill and strength of the person doing the kicking can make a difference.  Too many variables. 

Wouldn't surprise me if you had it bored and ended up with 135 psi after...  Now that you have a reading with your gauge and method you can monitor for any degradation.

I go by noise...  when the piston starts to rattle then it's time to see about replacing it and/or getting it bored. 

Just my personal opinions and experience, yours may vary.