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I got my self a 1987 corvette convertible all original 36000 km i would like to change the spark plugs myself wondering if i should go to a garage they been there for 25 years
Sounds like it could use a good tune up. I'm assuming if it is the original plugs it probably has the original plug wires, cap, rotor, etc. Has it been driven regularly or has it ben sitting? If it's been sitting I'd take a look at the freshness of the gas as well. After the tune and some fresh gas it may need to just get a little exercise (push the pedal down).
You could pull the #1 spark plug to see if you think it needs changed. Driver side plugs are simple to change; passenger side will require a little more time. On my '88 I have to remove the little access panel in the wheel well to reach #2 & #4 plugs. Be sure to use a little anti-seize on the threads w/o getting any on the insulator...
If the wires look smooth with no cracks, they are likely ok.
You might pull the codes to see if there are any problems lurking.
Does it have good oil pressure when idling & when on the highway?
Does coolant temp run around 200-210 deg when warm?