For the past few months when I shut down the warmed up the B18 (with a little over 100K on it) in my 122 I can count 1...2...3, then I hear a pop through the exhaust. It's not consistent, but rather happens about every five shutdowns. I haven't noticed any other symptoms but I am wondering if the popping is indicative of a more serious problem, like maybe the valves are starting to go.
During the winter I have been using "dry gas" and I use the lead substitute additive always. Recently I adjusted the valves, and even double checked them.
Any thoughts?

might be my guess. It might be a good thing to check your float levels to see if one's sinking, or if one of the jets has dropped a bit. Seems like a carb thing, really, and a minor one...
hth
t

My B21A does that occassionally. My guess, which will be confirmed today, is that the idle is far too rich or the deceleration diaphragm is hooped in my carb. I just recieved a rebuilt carb and will let you know if it fixes the popping and farting.

check ignition ands timing, use high octane fuel (98)or octane booster.
If this doesn't help your camshaft is toast, if the engine accelerates less than it used to and fuel consumption is bad, that's the diagnoses.