Topic: valve leak? will this be a problem?

[jessezm]

Hey folks, quick question, think I may know the answer already...

I just had my CB400f heads rebuild at a local machine shop.  They cleaned everything up real nice, checked the valves, guides, springs, replaced the seals, and lapped the valves.  I think he said something about restoring the 45 degree angles too.  He said the heads were in great shape (bike has 5800 babied miles on it)

Okay, so I just picked up the heads, was fooling around, and tried testing the air-tightness of the intake ports by creating a vacuum with my mouth (please spare me the obvious comments!).  Anyhow, 3 of the 4 ports held the vacuum, but one of them would not, meaning there is air escaping at the valve seat somewhere i assume.  What exactly does this mean?  Do I need to take the heads back to the shop and re-lap this one valve?  Worse?  Thanks

[MRieck]

 If they have a Serdi they can vacuum test it. Anyway someting is no good. It's not just the miles... age also effects the valve seats and valves (corrosion)

[jessezm]

Ok I just tried the much more preferred method of installing the spark plugs, inverting the heads and pouring penetrating oil into the chambers.  copious amounts of PB blaster bled through into the #1 intake port.  All others are dry.  Looks like it's back to the shop to fix that seat?

[Don R]

Yes, I'd go back.

[jessezm]

Problem solved!  Took it back to the machine shop and the guy beat on the valve stem a few times with a mallet, opening and closing the valve.  Retested and it seated perfectly.  He tested the pressure on the other valves for reference and they all came out the same.  I guess something had gotten in the way, preventing it from seating properly.