1994 Isuzu pickup 2.3 carbureted base model. Just did a compression test 130 PSI on #1,3,4. 0 psi on #3. Adjusted #3 exhaust valve clearance and #3 is up to 90 PSI now. I am guessing valve is somewhat burnt. I am looking to order a valve to replace (because I still need to drive truck) but am noticing they have made different size exhaust valves. Head sizes and stem sizes. Any ideas on which one I should order ? This truck is stock and has never been opened up before.