Missing compression

After I did my engine I had a problem with the shims because one or two were really on the thin side but still ok (Just) ,,..I put this down to the head guys and seat inserts .....anyway when I got to the compression test I had five of them about 135 -145 and one of them at only 100 at best...engine in car and a pain to remove the heads or engine again....
I ran the engine and over a few runs of the car ( which seems to run great.purrs like a cat at ticker and no back firing on the runover (After fitting bigger Jets in the carb ) .....I rechecked my compression and all six pots were at 135 + I thought this was a bit low but as she runs like a dream.....Im leaving as is...
Just my experience....you have listed all the possibilities as far as I can think but I hope the experts will help you more ...

 

Put some WD40 down the plug holes in case the rings have stuck, turn the engine a little and leave for a day. Check the valve clearence and put wd 40 on the follower, see if the valves are free when piston is away from tdc. Push the valves down and check you get the same clearence each time it closes.

 

Stick with wd 40 as oil is too thick

 

Peter if you assembled the engine I would have little doubt that something is assembled incorrectly. Of course errors can happen to anyone but I would look for something 'stuck' before doing too much disassembly. In your shoes I would do a leak down test first to identify whether the leak is coming from the bottom end or from the heads. On the Dino removing the carburetors with the manifold is really easy so I would take them off before getting into cam cover removal. This will allow you to have a good listen and look to at least the intake valves to identify if one is leaking. Assuming the leak is coming from the heads I would then pull the cam covers and double check the entire valve train and timing.

My guess is that you have a slightly sticking ring. The problem is that you cannot be sure of where in the bore it is sticking so WD40 may do the trick but then again if the stick is near the top the WD40 may just pool at the bottom of the cylinder and do nothing. What I do find encouraging is that the compression increased dramatically with some turning.

After all these checks in your shoes I would crank the engine with the coil disconnected to confirm it makes oil pressure and then re-connect the coil and start the engine for 30 seconds - 1 minute and then re-check the compression. My theory is that with all of the initial checks done if nothing is found then you really do need to remove the engine to find your problem. At this stage I cannot see the harm in running an engine that makes oil pressure down on compression for 1 minute. You may get lucky and find that it is a sticking ring after all and it will work itself free with a little heat and vibration.

 

With this new development I would comfortably fire the engine up without even taking off the cam covers or carbs. Just confirm oil pressure, re-connect the coil and make some Italian noise.

 

Perfectly normal, your rings have not seated. You have to fire up the engine, put some time on it and then recheck the compression after cool down.
clay

 

+1

Also known as 'breaking in' your engine....

Regards, Alberto

 

Dont worry about sticking a good squirt of WD40 down the ppug hole as it will only run into the sump, you can change that.
If you wind the engine around until you have the piston at the top you should be able to bet the wd40 all arond the piston circumference and onto the full ring surface, do with each unit. Use the little plastic tube.. To free it up give it a good blast so you know its in there.