zx280 aged

[triziq1]

Hello,I had 1983 zx280 non turbo , engine L28E My engine doesn't go to its full performance, (engine in a good condition) but I think the ratio between air/ fuel need adjustments also I see my oil pressure gage fluctuate above zero and next lines never reach half of the scale.What to do? Also can I have help about these engine performances (measuring and clearance ratio)?

Thanks for your helpful