Colin---While this could be ignition related, I'd suggest that you take a check
on the valve lash. Generally speaking, a 270 cam needs more lash, like around
.016-.018. Valves that open too soon (tight lash) tend to allow blowback into
the intake at low engine speeds.
If the lash is correct, then check the point gap, if this is what you have.
If the above is OK, try richening up the fuel mix to full rich, just to see if
it makes any difference. If this helps, then start leaning out the mix 1/4 at a
time to where the stumble/backfire happens again. Adjust accordingly.
Also, with the 10:1+ CR, I'm surpised that the engine is happy on 86 octane,
unless you are driving high in the mountains!
Dick
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