Bench testing compression on partially assembled 720

[swannydoo]

I'm rebuilding a 720 and wondering if i can compression test it on the bench with a partially assembled engine... I'm getting 90 psi exactly for both cylinders. (new gauge)

I cranked it with just the core, core plus rotary valve, and core plus rv and carby - all come up at 90 psi both cylinders with no spark plug i the other cylinder.

Do I need to attach the intake, carbs, exhaust and oil lines to get a correct measurement?

The engine was an unknown with the back story that it overheated and ceased. A few pulls with a wrench free'd it up. I stripped it bare, crank is good, seals are good, pistons are .75 over, rings and pistons look good, nice snug fit when assembling, holds 5 psi on the rotary valve chamber for an hour. I sealed the case with 518, all new gaskets etc...

This is my first tear down and rebuild so any advice would be most welcome.

 

[swannydoo]

Update: I just tested my 94 XP with factory rebuilt 657x core and it came up to the same, 90 psi... and it starts and runs really well, with only an hour on the motor since rebuild.

What are the chances the new gauge is dodgy? Am I correct in assuming it should read >130 for an engine that starts and runs well?

 

[SIMPLEARKANSAN]

I've read that cheap gauges, especially Harbor Freight gauges will give 90 psi, when it actually reads 150 psi with a good gauge. I'd swap gauges and try again

 

[AKnarrowback]

90 psi is way to low. Find a good quality gauge that threads into the plug hole as deep as the spark plug does.

I have a snap-on gauge set with two different depth adaptors, one full length and one roughly half the thread length of a normal spark plug. The shorter reach adaptor gives me a reading 10 to 15 psi lower than the full length adaptor. A good quality gauge with the proper fitting is important.

You can crank the motor on the bench if you are feeling brave, it is best to have the entire intake system in place (manifolds, carb, airbox).