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cold starting
K2-K6:
As you note, its quite puzzling when things look all in order. Frustrating too, but at least you can get it out and running once over that initial part.
Further enquiry :) have the carb CV springs been changed or can you identify them as correct ? It's from your comments about thinking its generally rich overall by a small amount, which the spring will ultimately control in a CV carb and obviously in paired operation with jet and needle.
Another thought, have you tested compression to get a baseline psi figure for each cylinder ? That would be helpful to understand.
McCabe-Thiele (Ted):
As you have checked the timing, presumably also the mechanical advancer (if your electronic ignition system one).
I'm minded to agree with Nigel (K2-K6) check the compressions.
Many decades ago I rebuilt an engine that just would not start Circa 1978. I checked & re-checked everything, all seemed spot on. My late father suggested a tow start! The engine fired up and it drove great, when I returned home after a short drive I parked it on the driveway. I went back to it several times that day, it started on the button.
Next morning I flattened the battery trying to start it. A compression check revealed poor compression when cold, quite acceptable compression when hot. It was early December, engine stripped down and block re-bored. I collected the block on Christmas Eve, I spent part of Christmas Day rebuilding the engine (newly married aged 30) , it was fine after the second rebuild. I always blamed the loss of compression on over enthusiatic use of a glaze breaking tool, I had fitted new piston rings as well.
If your compressions are good, could poor spark quality be the culprit?
I might have dreamt it, thought I have read somewhere that plugs spark easier on a hot engine.
taysidedragon:
I would try another set of plugs to see if there's an improvement. And check that the plugs have a healthy spark.
McCabe-Thiele (Ted):
Or switch the plugs from the working cylinders with the other two.
Oddjob:
It's a bomber Ted, it's a twin cylinder bike ;) ;)
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