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Missing with a load

I have a sprint 200 6 cylinder 1966 Mustang. It misses some at idle but if I put a load on it the engine it misses a lot, in fact it is not drivable. I pulled each of the spark plug wires while in idle and all seem to distort the idle. So I know I am getting spark to the cylinders. I also changed the fuel filter and I am getting gas when I pump the gas with the car off.

Any ideas?

With a vehicle this old, I have to ask if you’re putting E10 gasoline in the gas tank? And if so, how long have you been using E10?

Tester

No i have not put E10 gas in. I am using unleaded gas. It was running just fine with 87 octane unleaded.

What about the points and condenser in the distributor? When were those last serviced?

Tester

A weak spark output will misfire more under load than idle. The reason is that the more that you compress air, the more voltage it takes to make electricity jump the spark gap. When the throttle is closed, there is so much vacuum in the intake manifold that the piston has to go most of the way up the cylinder before the air is back to atmospheric pressure and only then does the compression start.

Did you know that magnetos used in high altitude airplanes have to be pressurized so the distributor rotor doesn’t flash over when the plane is flying at 30,000 ft.?

Make sure your ignition system is healthy and that might fix your problem. Low voltage to the plugs, a carbon tracked distributor cap can work ok when the throttle is closed but will flash over when the spark plugs need more voltage to fire. You may have plug wires that flash to ground under those conditions.

And yes, it can also be a carb problem.

New points, distributer cap, spark plugs (gapped to specs), and spark plug wires. When I pull the spark plug wire and go to plug it back in the spark to the wire is close to an inch long, so I think I am getting good spark (this happens on all the wires 1-6). Oh also a new coil.
When I push on the brake (to keep the car from moving) and gas the car it shakes BAD. Now if I get the RMPs up hire it is runs fine.

Have you checked for a worn timing chain?

Tester