Zephyr 750 spun bearing

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. 

 

 

 

Had the bearing turned and blocked the oil feel passage ?
Does this engine have an oil pressure relief valve that may of malfunctioned ?

To be honest if it is an oil pressure problem the cam bearings normally go first as they loose pressure first !

Yeah, the bearing spun and blocked the passage - went downhill pretty fast after that! Oil pressure was the first thing I thought as we were already chasing the problem with the warning light, but the cam bearings going first makes sense. 
It could have just been something blocking the passage to that journal, but the passageways in the crank were clear. I'm thinking it may have just been overheating. It wasn't that warm (2am) but given that the engine was running very lean and the bearings on conrod #2 were also stuck to the crank when we disassembled it (#3 was the one that siezed), it could have just been too hot. He wasn't exactly taking it easy - just slowed down from about 140km/h and wasn't exactly shifting early - also not used to keeping an eye on the temperature. An hour after he called the block was still hot. I could keep my hand on it, but only just. TBH I'm not really sure how long it takes my 750 to cool down after I've been thrashing it, but I also don't hit 140 too often on the old LTD

EDIT: I know a '93 Zephyr should be able to handle this sort of riding, but given how the plugs looked it was pretty hot. I'll have another good look at them tomorrow and take some pictures.

If it was lean anyway then the cold dense air at 2am would only make it leaner, is the ign timing possibly over advanced which would also cause it to run hot. The fact that it was still almost to hot to touch after a hour at that time of the morning (with an oil cooler) is unusual. It makes sense that the problem is more apparent with the inner two cylinders as they always run hotter anyway. Where you with him at the time ? I only ask as from experience 2am autobahn runs are normally for one thing only "MAX IT" might he have been doing a little more than 140 

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If it was lean anyway then the cold dense air at 2am would only make it leaner, is the ign timing possibly over advanced which would also cause it to run hot. The fact that it was still almost to hot to touch after a hour at that time of the morning (with an oil cooler) is unusual. It makes sense that the problem is more apparent with the inner two cylinders as they always run hotter anyway. Where you with him at the time ? I only ask as from experience 2am autobahn runs are normally for one thing only "MAX IT" might he have been doing a little more than 140 

I wasn't with him at the time and I figure 140 is a conservative estimate but he wouldn't have been able to go too much faster on that stretch of road. The knocking when cold made me think that the ignition was over advanced, didn't realise that would make it hot as well. Seeing the problem was on the middle two cylinders was what made me start thinking about overheating. That would mean the oil pressure warning would be a coincidence... usually don't like that reasoning but if the pump was on its way out and the engine was running hot it would be more likely to light up.

The pistons look beautiful.  No evidence of blow-by or over heating.  I seriously doubt heat caused that bearing to spin.  There is an oiling issue somewhere would be my guess.

It is possible that you either had a bad bearing or that it was "overrevved" on a "downshift" or "missed gear" possibly which could also have spun the bearing. It would have heated up fairly quickly as it was starting to seize 

Thanks guys. Hoping to get the bike back together next week. Decided to get a Zephyr crank instead of putting the spare 750 one in. The Zephyr crank was used in the 750's from about '84 on and is a lot lighter than the spare one I have. 
Will post again when it's up and running.

So.... Bike is back together. Checked everything when rebuilding the engine. 2nd hand crank was basically new - big end clearances were at the lower end of the standard range. Main bearing clearances were on the upper end of the standard range but well under the service limit. All the passageways were clear and really nothing that indicated what the problem could be. Oil pump seemed fine but I swapped it out with one from a 750 that had tighter clearances between the gears and the housing. 

Started her up and had about 20-25 psi on idle. After a trip around the block (youngster was riding so it the oil was pretty warm) there was no pressure on idle and revving to about 3000-4000rpm just registered on the gauge (about 2-3psi ).

It's pretty unlikely that both oil pumps were bad so at the moment I'm thinking oil cooler or relief valve. There is a bypass for the cooler in the oil pan but I'm not sure it's big enough to feed the top end. I'm thinking the next step is to put the oil pan from my 750 on the bike. The relief valve on my bike is good and the 750 pan would cut out the cooler completely..

To be honest I can't see the cooler circuit being an issue, lack of flow to the top end wouldn't cause the pressure to drop and as long as the cooler circuit isn't leaking it can't cause a pressure drop. I suspect the relief valve is the more likely culprit, it looks like when you give it some the valve relieves the excess pressure it is not seating back as it should (IMHO)

I'm picking the relief valve as well. Only have a KZ pan so I'll be cutting out the oil cooler as well. I guess we'll see how it goes. Once the Zephyr pan is off I can pull apart the relief valve as see how it looks.

The relief valve is pretty simple.  You should be able to take it apart to see if there is anything obviously wrong.

What about the O-rings on the sump cover.  I think there are three of them (according to the parts fishe.)  Easy to forget.