So we finally got my sons Zephyr roadworthy and he was over the moon for about 36 hours. He was on the highway doing about 120km/h a things got pretty loud. He pulled over straight away and the bike wouldn't start. We got it towed home and tore the engine down to find one of the big end bearings had burnt up and the conrod had siezed.
For a day or so before this we had been chasing down a problem with the oil pressure light. It kept coming on at low revs when the bike was warm. We changed the oil and checked the oil pump screen. Next step was to check the pressure with a gauge to make sure it wasn't just the switch (mine was at work otherwise I would have done this straight away) and after that swap out the oil pump. I guess we should have been in a bit more of a hurry
I am assuming low oil pressure caused this and we will be checking all the oil galleries and replacing the crank, conrods and oil pump with spares from an '84 KZ750. Do I need to worry about the oil cooler? There's a bypass in the oil pan so it shouldn't really cause any problems. I could also just stick the KZ750 pan without the oil cooler fittings if oil pressure is still low after reassembly.
I'd be interested in any other possible causes. We have an aftermarket ignition similar to the dyna s. I've read that knocking could cause this problem due to the force on the bearings. It sounded like there may have been a bit of knocking while idling when the engine was cold - but it was fine above 1500rpm and after a minute or two idled fine. Checking the ignition timing was also on the todo list but it was running well when warm so I just kept putting it off.
Another thing I noticed during the tear down was that the plugs looked really lean. The idle screws are out 3 turns but there are some plates in the intake to drop the power down to 48hp (A2 license requirements in Europe). I assumed reducing the size of the intake would make the mixture too rich but it ran fine with stock jets so we left it as-is. The plates will be out in March so didn't seem to be any point in rejetting if it ran well.