47' Super misfire/breaking up at 50mph

[agraz400]

I've got a 47' Super with the 248. I've been chasing this problem for as long as I've had the car running and driving. At right about 50mph and above, it'll start breaking up and possibly misfiring.. Doesn't seem to change if I give it more or less throttle. Once the speed drops below 50mph it'll run smooth again like nothing is wrong.
Just about everything I can think of has been rebuilt or replaced, with no change in the breaking up condition.
New points, new coil, pertronix ignition, pertronix coil, 2 sets of plug wires, 2 sets of plugs, all new wiring ran, electric fuel pump with confirmed good pressure to carb, carb regasketed and cleaned, distributor and vacuum advance rebuilt and confirmed advancing correctly, all new fuel tank, sender, and lines. Just got finished replacing all piston rings and 2 burnt exhaust valves.
The only part that hasn't been replaced or fully rebuilt is the carb, but that was taken apart completely, float set to correct height, cleaned and regasketed.
I've played with timing and plug gaps, doesn't seem to make much a difference for my problem.
Has anyone else experienced this issue before? My last resort might be to have to carb sent out and rebuilt or replace it with something else.. I'm just running out of ideas at this point

[Jenz38]

Your choke flapper ok ? ...may be it sucks to closed position under airflow runs throu the carb intake ?

[mavrick]

Your choke flapper ok ? ...may be it sucks to closed position under airflow runs throu the carb intake ?

You might try Marvel Mystery Oil in your engine oil, about a cupful, could be sticky valves

[322bnh]

Look at the power circuit of your carburetor.

[suntreemcanic]

My 263 straight eight gave me problem like that once. It was the wire going into the condenser was almost broken in two. Look at the coil, I have had two go bad after only about 5,000 miles each. Both were Eclin brand from NAPA.

[BluesBrothersBuicks]

Have you checked your distributor cap and rotor? How about the bushings in the distributor body - too much side to side play? How about cross firing? Easy way to check this - start your engine when pitch black outside - if you have cross firing around your spark plugs or wires - you'll easily see the arcs jumping to grounds. I had a car once where the engine looked like a Christmas tree!!!

[agraz400]

Your choke flapper ok ? ...may be it sucks to closed position under airflow runs throu the carb intake ?

Pulled the air cleaner off and confirmed choke flapper stays put while reved.

You might try Marvel Mystery Oil in your engine oil, about a cupful, could be sticky valves

Valves were all recently out and lapped (two exhaust valves replaced). All moved freely and springs looked to be in good shape. I tried Marvel previously as well with no change.

[agraz400]

Look at the power circuit of your carburetor.

I pulled the carb off and rechecked the float today. Fuel level sits right at the sight plug threads at idle. I rebuilt the carb with a full kit a couple years ago but I can't recall what all got replaced in that process. I might try to find a size or two larger jets. I'm running 2 1/4" exhaust with a smitthy glasspack, and small paper air cleaner assembly.. wondering if the engine is breathing much better than the factory jetting can cope with.

My 263 straight eight gave me problem like that once. It was the wire going into the condenser was almost broken in two. Look at the coil, I have had two go bad after only about 5,000 miles each. Both were Eclin brand from NAPA.

I believe the pertronix ignition system instructs you to remove the condenser, so that's no longer there. I swapped the original coil with the Napa Eclin, which has then been swapped with the Pertronix coil. No change either way.

[agraz400]

Have you checked your distributor cap and rotor? How about the bushings in the distributor body - too much side to side play? How about cross firing? Easy way to check this - start your engine when pitch black outside - if you have cross firing around your spark plugs or wires - you'll easily see the arcs jumping to grounds. I had a car once where the engine looked like a Christmas tree!!!

Cap and rotor has been replaced, no visible signs of cracks or cross travel in the cap that I can see. Distributor was sent out and rebuilt (as an attempt to fix this problem), no noticable shaft or bearing play. I've been through two sets of plug wires, but I'll have to do the nighttime trick and see if there's any visible arcing.

The engine will misfire by grounding out each separate plug/wire with a test light, so that seems to indicate plugs and wires should be good.. or at least they're firing off something.
Vacuum gauge jumps around from about 16-20lbs at idle.. not steady at all. I sprayed down any possible area for an intake leak, and all appears to be sealed up tight.
I might run a compression check to see if the new rings are seated.

[322bnh]

What carb do you have?