1949 8BA, 12 volt with alternator, Petronix electronic ignition.

Happens very consistently. Starts and runs perfectly for about 15-20 minutes. At that point something changes and it starts to run rough. Doesn't die or get worse after a time, just runs rough.

I discovered that two cylinders are not firing when the engine is warm, #3 and #7.

The only clue I can offer is that the two cylinders in question fire one right after the other on the distributer and are the second cylinders up from the firewall on each side, directly across from each other.

Replacing the two plug wires did not help.
Haven't replaced the plugs but they are pretty much brand new.
Distributer cap has no visible damage. Some pitting is present on the contacts but no worse than any of the others.
Battery charges while engine is running.
Engine runs very well when cold.

 

Which two cylinders are they? Are they right under the carb? Do the plugs on those cylinders look black? How about the others?

See if there is any wobble or play in the distributor shaft. What kind of electronic ignition is it?

 

Possibly a crack in the cap you can't see?

Only other thing I can think of is a couple of valves with too tight clearance at the lifter. Clearances tighten as it gets hot and your valves aren't closing all the way....

I'd try a new cap first

 

A lot of times you can see stray sparks by running it at night in the dark. (just watch the fan!!!)

 

Yes, now that you ask. One on each side, second up from the firewall, #3 and #7 if my numbers are right.

Petronix ignition


I'll have to check the rest of it this weekend.

Never done it but I can feel my fingers resist whenever I work with the timing.

 

Look down the carb when it's misbehaving and see if gas is running down the inside of the carb. I wouldn't think it would affect only those two, tho. Float level could be too high. Stock fuel pump?

 

No stray sparks visible in the dark.

Cylinders do not have spark. Spark plugs are a little black but not oily.

Electric fuel pump, but that reminded me that the flow regulator is set to 5 psi or something crazy like that, shouldn't it be closer to 2.5? (Holley 94)

 

2.0 would be ideal, but requires a good regulator. Is it a Holley reg or a Spectre (round chrome thing)? The Spectre are a POS IMO...

 

That describes it.