I have a '62 235 with hydraulic lifters and a dual Offy/Fenton setup. I noticed that I was getting poor gas mileage, so I did some investigating. I figured that I was running rich since I spewed carbon out of one of the tailpipes when I gunned the engine. I pulled plugs and noticed that they were all covered in carbon, and the back 3 were wet. (The tailpipe that was spewing the carbon was attached to the front 3 cylinders.)

I adjusted the valves to ensure that they had 1 turn after the tapping stopped, and I adjusted the carb to a leaner mix. I took it for a drive, and it was running great. After about 60 miles, I decided to pull plugs again to check them out. They were still covered in carbon, and the back 3 were still wet.

I then checked the compression in one of the back 3 cylinders and one of the front 3. The back cylinder was reading about 75 lbs and the front was reading about 90 lbs. Both way too low. I only had time to fiddle with the front cylinder valves, backing them off 1/4 turn, checking compression, backing off another 1/4 turn, checking again. The compression improved, but not significantly. (The engine was cold by this time)

Now this is a recently rebuilt engine, so I wouldn't guess poor rings. Could the carbon be causing the valves to be unable to close all the way? Should I spray B-12 into the cylinders to clean up the carbon? What else can I try?

How did all this sawdust get under the hood?