Engine Misfire

[TulsaFred]

my home rebuilt 1275 was doing fantastic, pulling smoothly through all gears and speeds without the least issue.
I was working on the generator and control box, and when I took it for a test drive it is not showing a pretty severe backfire issue that seems mostly at mid rpms on the lower gears under load. At higher speeds it is not apparent.
I wondered whether I might have bumped the dizzy cap or wires while wrenching, but nothing obvious on inspection.
I have an uninstalled points/breaker plate/red rotor from Jeff at Advanced Distributors so I suppose now might be a good time to install them.

Any thoughts on a diagnosis? The engine only has about 50 miles on it.

 

[SD Bugeye]

Check vacuum connection

 

[nomad]

Sure your valves are adjusted correctly? Most likely ignition but another base to cover.

Kurt

 

[TulsaFred]

So I got it fixed. I changed two things at once: pulled off the Moss rotor and replaced it with a red rotor from Jeff at Advanced Distributors (had one on the shelf waiting to go on). The other thing I changed was a faulty Lucas control box.
I am pretty sure it was the control box, which had not been working and did not allow charging of the battery. The voltage on the battery was only slightly low at about 12.4 volts, but without the generator adding some juice it seems like this was enough to make the spark too weak.
With the new control box the voltage goes up to around 13.8 volts with higher rpm.
I never would have thought this would cause a misfire problem, but it did.

 

[drooartz]

Well done!

 

[nomad]

Low voltage causing a misfire would make me suspect that the coil might not be up to the job. If you have more trouble I'd sure try swapping the coil.

Kurt.

 

[TulsaFred]

good thought on the coil, I have a bosch blue coil waiting to go in!