The Stovebolt Site Forums The Shop Area Engine Shop Bad mileage, carbon, and poor compression

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?

I would guess valve seals. The first thing to try is a hotter plug.

I am currently running AC-45s now.

What carbs are you using on the offy? It sounds like the rear one has a float problem and is dripping raw gas into the intake, flooding out the rear cylinders and washing down the rings. Valve guide seals will not cause compression loss.

They are single bbl Rochesters.

Could be a jetting problem...??? maybe :confused:

What kind of air cleaners are you running on this beast? Those little "hot rod" Mr. Gasket air cleaners don't flow enough for even a 216, I lengthen the center rod and run 2 elements stacked for enough flow, otherwise you choke the carb and encourage leaks.

Squint down the back carb throat while it is running and see if you see gas or vapor dribbling down there. If so, you have a float set too high and you are dumping fuel into the intake. Rochesters can be real leakers.

O2 sensor are cheap ways of telling lean or rich
you sound like you are flooding some cyl
mount O2 sensor in the exhaust pipe and hook up a DVOM. The last sensor on a OBD 2 car is a O2 sensor if it reads no O2 you have given it the right mixture. Read up on them they are usefull for lots of diag.

duh. One problem was a miss on #4. I also leaned the mixture almost 3/4 of a turn on both carbs. Finally, (another duh) I noticed that the front carb choke could move however it wanted to since there was no linkage to it. Of course, it happened to be closed. I wired it open, and things are running MUCH smoother now. I haven't gone back to recheck compression yet, but I will soon.