263 Stumbles under load


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!!


Active Member
I have a 263 52 straight eight. One time coming home I had a similar problem though I have a dynaflow. It would pull real well on acceleration under power but would not run at cruise speed. So I finally found the problem. The wire to the condenser was almost broken in two. I replaced the condenser and it ran great again. I know your engine problem is exactly opposite of mine but this was a long time ago and maybe I remember backwards. But it would be easy enough to check and cheap and easy to fix.
With the old straight eights a lot of people miss ignition cross firing because Buick put that nice plug cover on. Ignition wires must be kept apart, especially when run parallel to each other. Pull your cover off, separate the wires and take it for a drive.


Active Member
Lean>Starving for gas momentarily.

Sometimes you get a stumble/bog when transitioning from the (rich) idle jets below the throttle plate to the (leaner) jets above. Try setting the idle screws as lean as possible while still maintaining acceptable idle.