did the same on a 4.6 linc motor.  stunk up the neighborhood.  no mosquitos for a while.  motor had 250k miles.  no more pinging and about 5mpg more.  and next oil change-oil was a lot less black.  go figure.  well worth the 10 buck investment.

I've used Seafoam on other cars but not the Mini. It helped smooth out rough idle and hesitation in my old Camry and Wife's Subaru. Put recommended amount in the fuel, then suck it into the intake just to the point that the car starts to sputter but not stall, then shut it off immediately and let it sit for 20 minutes so the stuff can soak in. If you are around the general public, you might want to wait until night time to start it up as it will create the biggest smoke cloud you can imagine, and people might either call the fire department or give you a hard time. 

Once the smoke clears (this can take 5 minutes or longer), you should notice a difference if carbon was causing your pinging. 

There is a lot of data about this subject on the MX5 MIATA . NET forum you just have to look a little bit , alot of those guys use this stuff I would start with the NA part of the forum. Yes I have a 93 NA8 mx5 the perfect compliment to my replica cooper s .

Make sure your vacuum or mechanical advance is working on your distributor. I've had this stop working on two cars recently, and they became underivable. If it's stuck at full advance, you will never be able to retard the timing enough, and it will ping.

I have seen water used as a carbon remover. You just hold the revs at around 1500 - 1800 rpm and pour water down the carb intake slowly you will not believe the amount of carbon that comes outof the exhaust and i have heard the colder the water the better it works, in fact years ago it was referred to as a cheap mans tune up.

I used seafoam in my motorcycle and all of my lawn equipment with good results. I would add it to the engine all the ways you mentioned. Cant hurt. It tells all the ways you can use it on the bottle.

I have tried adjusting mix and timing every which way since swapping out my pertronix ignition, and I can't get my pinging to go away under load. I've reduced it alot, but it's still there. I have retarded the timing as far as I can without affecting performance, so I'm thinking it could well be carbon buildup.

Can anyone attest to the power of Seafoam? I've never used it before, but by all accounts it seems to be the best carbon cleaner. If it really works, should I spray it, pour it, suck it through a vacuum port, or add it to the tank? If not Seafoam, then is there a better way?