Engine runs good under light to moderate load. Idle vacuum is 21". Moderate load vacuum can steadily and predictably decline down to about 14" and run very nicely with great mileage. But when I open the throttle more, under heavier acceleration, the vacuum drops fast to about 2-4" and engine starts pinging. Going uphill, I now have to shift one gear lower, than I used to.
Installed new spark plugs and ignition cables. Base ignition timing is as per specs.
Next I wanted to try Sea Foam. But found a similar post here:
That guy claims cleaning carbon deposits and engine still pinging.
The abrupt drop of vacuum below 14" makes me think, it could be some vacuum leak or a faulty vacuum switch or sensor more than deposits. Or is it the other way around and the vacuum drop is a result of engine pinging?
Any advice on what I should be looking for?