Distributor misfiring
, It's an erratic miss and it pops. The valves seem fine and it's getting fuel to the carb. I gave it a tune up not a month ago.
An open plug wire or cracked plug can cause that, I assume it's popping out the carb? Or maybe the exhaust?
When does this occur? High RPM? High RPM under heavy load? ANY RPM, andy RPM under load?
Don't fixate on the distributor. Could be a lot of things.
Some easy things to do. Measure supply voltage to the ballast resistor from the IGN switch, key on, engine off
Pull the plug boots up from the dist. and figure a way to short them. You can either use insulated tongs/ pliers to lift them out, then pop a grounded probe (such as a 12V test light) into the dist. tower one cylinder at a time. Or, stick small brads down beside the wires into the tower terminals, then short them one at a time with a probe. At idle, you are looking for the same drop in RPM on each cylinder
I'd pull the plugs and look carfully, do they look like they are firing properly? Any cracks in the insulator?
Take your multimeter and measure the resistance of the plug wires. "Shake" them to look for intermittent connections, and don't forget the coil wire
Get a "feel" for the strength of the spark. Pull the coil wire out of the dist. with insulated pliers and try to get it out just enough to NOT kill the engine, Take a look at what the spark looks like
Pull the cap, look for cracks, damage in the cap and rotor
Look carefully at the reluctor / pickup. Apply side pressure on the shaft and see how far the shaft moves. Look for damage/ strikes/ debri on the pickup or reluctor
Unhook the dist. connector, and work it in/ out vigourously to "scrub" the contacts clean. Make sure it feels like it fits tightly. DO the same thing with the ECU connector.