Yes, carbon can cause a horrible leak down...

[bpu699]

Figured I would put this in a separate thread...

My 930 motor is down to a long lock to replace studs...

Heads reportedly rebuilt 5000 miles ago. Great leak down when I bought it... Pretty much mid 90's... Ie 95 psi to a standard of 100 psi... Only loosing 5 psi.

Wanted to recheck before tearing down the block. Put the leak down tester on cylinder 1. Leak down was 30%. 70% loss. Awww foooey.... Was leaking like crazy out both valves...

Checked the other cylinders, all pretty much in the 90s... Great. That's good news.

Looked at the valves. Lots of carbon on all the exhaust valves. It has been running rich, that's another story in another thread...

Poured wd 40 into the closed exhaust valve outlet of the low cylinder. At 10 psi it fizzed all over the place. But fizzed equally. Not a bent valve...thank goodness...

Took a stainless paint gun cleaner brush, some wd40, and cleaned the valve mating surface. Just gently brushed around the surface. Took a couple minutes... I also loosened up the valve adjustment screw to totally loose in case it was tight.

Reran the leak down...now have 95psi/100psi!!! Helluva improvement... Repeated multiple times to verify... 5% loss.

I am always shocked how little carbon it takes, to royally screw up a leak down... Perhaps the valve was slightly tight, and carbon built up on the mating surface?

Closed the exhaust valve, poured in an ounce of fuel system cleaner. Checked an hour later, it was all still there... None leaked out...

Awesome result...

Figured I would post this, as you often have folks asking about lousy leak downs. It doesn't take much...