Engine Troubleshooting

[robert l]

Update. We temporarily tried fine wire spark plugs, didn't help.
We also tried to swap the top and bottom ignition leads T1 to B1 and vice versa. And then it was all good!
Now this is very odd to us.

So you're saying that switching the top plug wires to the bottom plugs, and the bottom plug wires to the top plugs fixed the problem?