I have a 51' 263 with hydraulic lifters running on 6 volts. Engine has 47,000 miles on it. Engine was "freshened up" at 37,000 miles. Lifters cleaned, bearing clearances checked, carb rebuilt, new coil, cap, rotor, points, wires, compression check etc... Engine was running really well for a couple of seasons until last fall during a 300 mile road trip where it developed the stumble. Prior to this it had a recent exhaust manifold leak (that has been fixed several times before) that I thought may have caused the intake to also leak. Both intake and exhaust have been fixed but the problem still exists. The engine also seemed to have a sticky lifter at the beginning of last season but freed up after a few longer trips. Engine runs well at idle and strong in park when giving it more throttle. The problem occurs only under load and particularly at the shift points from 1st to 2nd and 2nd to 3rd. It is a 3 speed manual. I have tried or done the following:
Replaced inline fuel filter between tank and (2 year old) mechanical fuel pump
Replaced inline filter between fuel pump and carb
Checked timing
Cleaned and checked point gap
Cleaned and checked plug gaps
Checked plug wires
Checked distributor lead wires
Cap and rotor look good
Adjusted existing carb throttle screws
Switched carbs with no change
Vacuum advance holds suction and moves freely
Bi-passed mechanical fuel pump with electrical pump
Performed a compression check not perfect but actually is slightly better than my original prior to engine install. (rings freed up?)
What am I missing?
Sticky lifter(s)?
Poor valve spring?
Burned valve?
Weak dizzy springs?
Sticky dizzy weights?
Need help!!
Replaced inline fuel filter between tank and (2 year old) mechanical fuel pump
Replaced inline filter between fuel pump and carb
Checked timing
Cleaned and checked point gap
Cleaned and checked plug gaps
Checked plug wires
Checked distributor lead wires
Cap and rotor look good
Adjusted existing carb throttle screws
Switched carbs with no change
Vacuum advance holds suction and moves freely
Bi-passed mechanical fuel pump with electrical pump
Performed a compression check not perfect but actually is slightly better than my original prior to engine install. (rings freed up?)
What am I missing?
Sticky lifter(s)?
Poor valve spring?
Burned valve?
Weak dizzy springs?
Sticky dizzy weights?
Need help!!