patrol petrol and timings q's

I recently bought a 89 maverick 4.2 petrol with 200k on the clock
it seemed a bit sluggish and lacked any amount of torque down low so i did the usual - replaced air cleaner,fuel filter and plugs, then checked the timing. it was on 0 degrees. - I bumped it up to 10 degrees, which was on the sticker under the bonnet and in the gregory's manual and i start pinging. The low down torque is fantastic when i run it on premium. although costs are shocking. I returned it to a about 6-7 degrees and it stops pinging but i am gutless down low again. - it feels like a flat spot until about 2 grand then it picks up.

does anyone had this situation happen to their vehicle?
Is there anything else i should look to fix this problem.?

and also has anyone ever heard of the fuel popping from the fuel tank - it makes a noticeable popping sound just after i turn the engine off.

Any help would be greatly appreciated. thanks