Bad compression test on lz24 frankenstein

[kujocku]

I had a Datsun Frankenstein motor built for me. Bored Z24 block, W53 head, dual 40 mikunis. Approximately 1400 miles on engine.  I did a compression test,  #1  170psi, 2#  155psi, 3#  125psi, #4  115psi.  Gas mileage started at about 18.5 mpg dropped to 15 mpg. What should I be looking for?

[d.p]

A new engine?  

 

 

 

 

 

 

 

 

I kid. 

[datzenmike]

Try another compression tester.

If same results squirt couple of tablespoonfuls of engine oil in each cylinder and see if the compression increases dramatically. Record before and after numbers. If no change probably worn valves/seats or gasket.

Check valve clearance.

[Draker]

You can buy a cylinder leak down tester and find exactly where the air is escaping.

[distributorguy]

You can buy a cylinder leak down tester and find exactly where the air is escaping.

 

I second this.  Did something change, or did you just do a compression test now?  If nothing changed, it may just be a bunch of stacked up bad tolerances.  Start with valve lash and recheck compression, but if its still bad I'd buy a leakdown gauge set.  Jegs has a house brand setup that's pretty inexpensive and works well.  Normally my first assumption would be an improperly decked block (cut at an angle) to provide sloping compression front to rear.  Or if you forgot to disconnect the ignition coil, it could be that you simply damaged the gauge you were using.  

[hang_510]

Weak head gasket used...

Compression check will confirm.

When I pulled mine apart, blown piston, the HG was in bad shape.

Look into a Nismo gasket. No issues on my LZ22.