Thanks, everyone, for the advice. Rather than a single possibility, it looks as if I need to start at the bottom and check everything. I no longer have the gauge and fitting to check cylinder compression, but I can get one. Pulling plugs on the even bank is tough with the A/C; would checking the odd bank be enough for these purposes? Spec on cylinder pressures with the 72cc heads is supposed to be 185-210 psi; is this realistic on a 35 year old engine?

Raining this afternoon, no road test today