Hi Blaine,
Reading thru this thread, I think you need to leave the ignition system alone* and start looking elsewhere.
* Bill makes a good point however about whether each pick-up is connected to the correct cylinder. As the workshop manual says (and in bold, note!): "The red cable must be connected to the transducer of the vertical cylinder".
Just to review the situation:
1. You say,
"Getting pretty nice spark on the plugs as well. Blue and white". So we know the ignition system is doing what's it's meant to do, i.e. making sparks! So that just leaves the question: Is it doing it at the right time?
2. You say you've checked and re-set the static igniton timing. So, in theory at least: Yes, the sparks are happening at the right time. So there's no more to be done on the ignition system*. However:
.
3. You say that the compression is low: 128 psi +/-. While that isn't critically low, it's true that low compression will make any engine reluctant to start. From the 'oil-down-the-spark-plug hole' test you've done, it sounds like the rings aren't sealing. New rings should be tight. Yours aren't. But is there anything else it could be? You've checked and re-set the valve clearances and also the cam timing. So let's eliminate those. That leaves us with valve guide wear.
Conclusion : at this stage, I would be leaning towards a leak-down test to identify the cause of the low compression. But - and first :-)
4. Fuel supply. (As the old adage says, "99% of ignition problems are carburetor problems - and vice versa!").
You say:
"Carbs are rebuilt, with stock jetting, and I can confirm I'm getting fuel in them and into the cylinder. Pumpers are working as well".
These bikes are very sensitive to incorrect fuel supply - and ageing Dell 'Ortos are very good at delivering it! Too rich, too lean, unbalanced, air leaks - all these will stop your beast from roaring into life. So here's the question:
Who re-built the carbs and when? The 'Who?' is important because, if the carbs weren't thoroughly - and I mean: 'thoroughly' as in 'ultra-sonically' - cleaned, then you could be in for a very frustrating time. The carbs need to be really, really clean, especially all the little passages and drillings that are hidden to the naked eye!
I ask 'When' because, if the carbs have been left standing for any time with gas in them, this can very quickly block the small diameter passages that are used at start-up. Btw, the fact that gas seems to be arriving in the cylinders isn't particularly revealing. It has to arrive in the
right mix - or 'No Go'. And the pumpers are irrelevant at start-up. Except when an unknowing operator works the throttle, and floods everything ...
. DON'T work the throttle. Simply set the choke. It's all you need to enrich the mix at start-up.
So, if it were my bike, with the ignition system seeming to be working, I'd do what I could to eliminate the carbs as a possible cause. Take 'em off, dis-assemble, ultrasonically clean, check the components for wear and correctness, change the inlet manifold seals (they leak and stuff up your settings). If you're not confident of your abilty to do this, Steve and others offer the service.
HTH - and good luck!
Craig