When I had my shop decades ago in Merced, Ca., I had a LOT of Air Force people that rode the first GPZ600's. those engines had 4 valves per cylinder, DOHC, rockers under the cam lobes, two adjusters per rocker. I had a fair percentage of them come in "running poorly" or, "won't start". These bikes had either been to the local dealer, or, had no service done in about 5K miles. Literally, resetting the intake valves correctly would fix the no start and running bad. The exhausts were usually a lot more stable, but the soft intakes closed up when they weren't set every 4K miles, to the point of no clearance, loss of compression, no start.
Some engines today still have intake valves made of silly putty, not better valve materials, and the soft ones recede into the seats easily if not set regularly.
I had one town fellow have his bike do that, GPZ600. He had it to 5 different shops, all told him it needed a complete engine rebuild, with only 6K miles on the whole bike. Two told him to buy a different bike. ALL "looked at the valves", none reset them. I asked him if he would like to watch my whole tune and service, he said yes, but he was literally fed up with the bike not starting and after I fix it, IF I FIX IT, he was going to sell it, no alternative. The last two operations I do on those are adjust the valves, balance the carbs. He watched me reset the valves, and get ready to start it up to do the carbs. He to this day, says he does not know how I got the bike to fire right up for the carb synch by just pushing the start button. He finally sold it, years later, with over 45K miles on it.
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