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I think some are fixed, and if you go in there with hex key, can puncture the diaphragm if you are not careful.

adjusting the spring pressure on the diaphragm won't puncture the diaphragm. i do it all he time.

the real difference i've noticed in the canisters is the amount of degrees that are in them. the number of degrees in the canister is stamped on the arm that connects to the plate. they range from 7 degrees to 10 degrees (distributor degrees). i have noticed the lower degree canistrs having a slightly weaker spring.