My 26 began to run rough, like it was having an intermittent electrical problem or fouling a plug so I put in all new Champion 25's from Langs- still had the issue. I then changed the points, coil and condenser (I'm running a Texas T distributor) and still had the problem. Next I pulled the head and inspected the valves. The exhaust valves were uniformly light tan and the intakes were uniformly slightly sooty and black. The seating surfaces showed no signs of burning or pitting but had a light glaze on the valves, seats were clean. Even after cleaning and lapping the valves, it still misses and runs rough at low RPM. I set the valve clearance at 0.012" on the intake and 0.014" on the exhaust. The clearance may be more than some would use but I wanted to be sure that the valves had ample clearance so they wouldn't be held open after they heated up. I swapped carbs and still had the problem. I checked the distributor cap and found #4 and #1 cylinder plug wire connections had corroded and thought I found the problem but even after cleaning those it still has an issue. I checked the plug wires for bad connections to the brass ends but didn't find anything that would cause my problem. I ordered the materials to make new plug wires just the same in case the insulation was breaking down and shorting out periodically . I have new champion 25 spark plugs in it and the burn looks normal. If new plug wires don't fix it, I'm going to experiment with point gap. I have all new coil, points and condenser and ran a dedicated ground to the distributor head, all with no luck. I checked the gap between the rotor and distributor posts inside the cap and everything looks good. I checked the points gap and it's at 0.020" just like the set up directions call for. This is a real head scratcher. I disconnected the fuel line and made sure I have good flow and took the bowl of the carb and made sure I had good flow into it. The engine starts well but just doesn't have any power and runs rough at low rpms and at high RPMs. I grounded out each plug while running and every plug slowed the rpms down when grounded so it appears to be firing all cylinders. I'm stumped. Has anybody got any ideas on what to look at next?
Have you added a ground wire from the dist head?
Have you checked for a intake leak?
I think Lyndel is right on. It could also be a float level problem. Try adjusting the mixture needle at idle and see if it smooths out. Then drive that way. If it misses at higher speed try adjusting the mixture again. If the mixture has to be adjusted differently for a smooth idle and for a smooth high speed, you might very well have a vacuum leak or a float level problem.
I had similar problems.......added a small ground wire to the Texas T distributor head and the intermittent electrical problem went away. The Bosch 009 distributor is loose fitting in the housing where it rotates for advance/retard. Also had another similar problem and it turned out to be the original coil box switch contacts.
I have run a dedicated ground wire from the distributor head the chasis ground at the starter motor. I have cleaned and lapped the valves and I have played with the mixture to no avail. What would be an easy way to check for an intake leak?
With engine idling, spray something flammable like ether or brake parts cleaner around the intake gaskets and listen for the engine rpm to change. If it does, you found your leak.
If the plugs are all burning the same color you do not have a manifold leak. Quick check is to have the hot air pipe removed and spray WD 40 on and around the intake manifold gaskets, if a leak is present the engine will change it sound as it will be getting more fuel to burn in the offending leak.
Ground of the distributor can be a short wire to the front plate. What you are describing sure sounds like a ground problem of the Dizzy.
I type really slow LOL
Chester; about the only thing that I see you may have missed is the Spark plug holes. I had a similar problem and ran a battery post cleaner in the spark plug holes blew out the cylinders with compressed air coated the plug holes with WD40, sounds weird but it helped me.
I use a small propane torch not lit passing over possible leaks on intake parts. Heated fumes from brake clean are very dangerous with possible permanent health risk from mail I have received.
So what happens to an accumulation of propane gas in an enclosed area (Like under a hood) when it gets an inadvertent ignition source?
All things in moderation.
Plug up brake Kleen dangers on the web and read the feed back.
I have used the propane method for years learned from a very good factory rep trouble shooter. You don't need much propane to test and just the fan blade turning or remove the hood while checking works good. A hot exhaust manifold would produce lots of fumes with brake Kleen. A spark could ignite the torch but it can be shut off quickly. To each his own!!
Have you tried jumping the switch? Easy test. I had similar problems with the coil box switch on our 13. I now have a separate ignition switch....the original is for looks only.
Here's an easy test you can make Chester that I don't think anyone has suggested. Run the engine in the pitch black dark of night, starting with the engine idling and "missing" as you mentioned, and look for any errant sparks. Sometimes amazing what you might see,....FWIW,......harold
Here's a fun test - disconnect the exhaust pipe from the manifold, and see what that does. The neighbors will love you!
I'm suggesting a plugged-up exhaust system. Strange, but you've tried everything else.