Ran a compression test tonight on the junkyard L33 with 799 heads I bought recently. Motor has 158k on it. Test was run on a stand since this is just a long block.

Numbers were as follows:
#1- 220
#2- 202
#3- 215
#4- 208
#5- 190
#6- 220
#7- 215
#8- 230

#5 is a good bit lower than most and has me a little worried. Plugs didn't have any buildup or anything on them, but definitely did not have much left on the electrodes. My mechanic friend suggested running a leakdown test- anything I should be looking out for on this motor? Seems pretty healthy other than #5.

 

Number 5 just isn't as carboned up as the others.

 

Carbon wouldn't result in a difference of 40 psi.
I wouldn't worry about it. Realistic causes are valve seats, cylinder out of round, small scratch in the cylinder wall from the ring ends, etc.

 

Did the leakdown on #5, everything came back normal. Going with the FIRI approach.

 

BS Those readings are all sky high from carbon! 158k I'll bet they are so caked the carbon hits the cylinder heads cold. Prolly knocks like hell on a cold start.

Decarbon the engine and they will come down to normal. Normal would be in 150-185 range. But since you have no idea how well your gauge is calibrated I wouldn't put too much stock in the readings regardless. The leak down tests tells all. Still needs decarboning though badly just from the difference in the readings.