Technical 250 with low compression on one cylinder.

Here is the story. Hard to crank and missed on the second cylinder. Does not smoke, overheat, no mixing of oil and water. Changed dist,rotor,plugs, and wires. Changed nothing so I changed the intake/exhaust gasket cause I had one here and changed nothing. Checked compression and had 150psi on all the cylinders exept that one and it had 30psi. Pulled the valve cover and nothing looked weird so pulled the head. Nothing looked odd so pulled the valves from that cylinder and two more. The keepers were stuck pretty good but hit them out and I don’t see anything that makes sense for it to have low compression. The only difference is that the low compression cylinder has a cleaner stem on the valve. It’s the one on the far right. Not sure what it could mean but hoping y’all can help me. Turned the fan and the pistons moved good and the cylinder walls looked great. I poured marvel mystery oil on top of the pistons to see if any leaks down. Not sure what I’m doing here cause it’s my first time. Please help me figure out why the low compression on the one cylinder.

 

Did you do a Leak Down test ?

 

No sir. This is my first time doing this and kind of going at it blind. Hindsight I should have done that but just didn’t know. Now that I have the head off I am not sure what to do next. I was really hoping to see some obvious signs on the valves or the head but nothing stands out to me.

 

You might try some light oil in that cylinder and see how quickly it runs through, Marvel Mystery Oil perhaps. You can re***emble the head and do the same with it on the bench.

 

The black faced exhaust valves aren't seating. Off to the machine shop !
Next time you decide to figure something out, don't start by replacing a bunch of parts ...

 

I poured marvel on top of all the pistons just to see, gonna try the head also.

 

I am a do it myself fella most of the time so I’m giving it a shot. I joined here to try and avoid paying someone to do something that I may be able to do myself. Money’s tight lol. Already had those parts so not out anything yet and sure do like learning. If the head checks out, it can’t be too hard to replace and lap the valves right?

 

Have the head rebuilt, period. It needs the guides checked, everything works off the valve guides, they are a critical component. How does the cylinder wall look in the dead-ish hole? You still need to rule out a possible wiped lobe on the cam, broken rocker arm, pulled rocker stud, bent push rod, bottomed out lifter. You can best diagnose a complete engine, it gets tougher being a pile of parts on the bench.

 

I see your point and I appreciate all the advice. Thank you

 

I figured it out fellas. Could have been a lot less involved and a lot less work. Whoever owned it before my little brother did a poor job on adjusting the valves and had those too tight to close all the way. I reckon we learn something everyday and thanks for all the help.