'82 KZ650 rebuild - style TBD

Wow. Just had a look at my "new member" posting and it was 13 months ago. Finally got around to starting this one, as I just sold my CB project that I had been riding for the last little while.
It's a basket case, and the more I dig into it I realize that there was probably a reason it was torn down to start with, and probably why the PO wanted to get rid of it. Oh well; if it was gonna be easy it wouldn't be nearly as much fun. Here are a few pics:

Mounted the engine this afternoon. Got the wiring temporarily hooked up so that I could test compression (I miss the kickstart like my CB had). No key for this thing, so I had to pull the headlight and hot wire it. Pretty big mess of wiring in the bucket, which is never a good sign.
Anyways - tested compression and on each cylinder, gauge went up pretty consistent until 60 then stopped. Exact same across all 4. I put bit of oil in each and it went up to 90. So unless these numbers are artificially low because motor is cold and doesn't have any oil in it and has been sitting for who knows how long, looks like I may need top end rebuild. Just over 40,000 km's on it so probably in need of it anyway.
From bit of research I've done I see that rings are pretty pricey. Will keep looking to see if there are any good deals out there.
Might consider just getting the motor running as-is so I can make sure everything else is okay.
Still deciding what to do with this bike. Tank is pretty nasty so will probably need to find alternate. I've got a GSXR fork that I've been saving for a project, but have yet to figure out if this is the one or not.

40,000 km (24,850 miles) is trivial mileage on these engines. I suggest you get it running before thinking about rebuilding it. Low compression could be simply stuck rings, mal-adjusted valves, etc. Adding oil to cylinders will always raise compression simply because the oil displaces space in the combustion chamber, so I would not bet on that as an indication of the engine needing a ring job.

Get it running, do a full tune-up, and run it a while which may break free any stuck rings. Then if compression still seems low do a leak-down test to identify the source of the compression leak. When you do a compression test the engine should be at normal operating temperature, the battery fully charged, all spark plugs out, and the throttle wide open.

Again, these engines are very tough and 40k km isn't much on them. Ed

Thanks. That's good info. I've got close to 80,000 km's on my '98 VTR1000 and it's got original rings, but it's a V-twin and spends most of its life around 3,500 rpm. Wasn't sure about this KZ. Am going to do exactly what you suggest.

The KZ650 is a really nice engine, so once yours is sorted out you may be pleasantly surprised at how well it runs, assuming the previous owner didn't do damage. I bought my bike new in '77 and it has more than 57,000 miles (about 92,000 km) on it. It runs very well and doesn't smoke at all. Some folks have logged more than 100,000 miles with original rings. I think Ronkz650 may hold the record. He has one with more than 167,000 miles (269,000 km) without a rebuild that was still running well! Granted, that's exceptional, but it shows what properly maintained KZ650 engines are capable of. Ed

not to hijack the thread, but are the 750s as reliable?

The 4 cylinder KZ750 should be since it's the same basic engine. Ed

Tore the carbs apart to see if I'd need any parts. Looks amazingly clean - I'm guessing PO did some work on them. Somewhere in the past there were some modifications made. I'm assuming he was running pods as the needles are on richest clips. Main jets are 115 but not sure if that's bigger than stock or not. I'll be using an airbox of some kind but will figure jetting out later.
One of the slide locking nuts was the wrong thread, making it pretty much non-adjustable. Swapped that out now so I don't forget later. One of the brass overflow tubes is broken off, so I'll need to repair or find another bowl.
In the design department, I've decided against doing the fork swap - too much money, time etc at this point. Will save it for the next bike (keeping my eyes open for older Guzzi Lemans).