1978 KZ400 Top End Rebuild Questions

I have a 1978 KZ400, it has failed a leak-down test on the left side. We are assuming the leaking petcock has washed down the cylinder over time. I am overbooked on projects so have a local guy that has been wrenching on motorcycles forever going over it for me. He is quoting $900 on parts and machining to rebuild the top end, plus a few hours labor. He's having a hard time finding all of the parts at a reasonable price since the 400 is less common at this point.
Another option is a donor 1981 KZ440 with a known good engine. I am hesitant to swap the whole thing over, I think there might be some snags that are not being accounted for. Is there any possibility of swapping just the head without too much heartache? If not, any suggestions on getting the 400 rebuild cost down a bit?

Can't add much in this topic, but I only wish my KZ-1000 engine rebuild would have been just $900.00. All things considered, if it's a nice motorcycle yet in otherwise good condition I would not hesitate spending the money. If it's a so-so motorcycle then much thought is needed. Remember too that even a donor engine that you think is good may not be so good later on. Unfortunately you can't see inside the donor engine. My rebuild costs for my KZ-1000 engine are well over $2,000.00 at this point without including little incidentals. $900 would have been a bargain for me.

Rick H.

Rick H. wrote: Can't add much in this topic, but I only wish my KZ-1000 engine rebuild would have been just $900.00. All things considered, if it's a nice motorcycle yet in otherwise good condition I would not hesitate spending the money. If it's a so-so motorcycle then much thought is needed. Remember too that even a donor engine that you think is good may not be so good later on. Unfortunately you can't see inside the donor engine. My rebuild costs for my KZ-1000 engine are well over $2,000.00 at this point without including little incidentals. $900 would have been a bargain for me.

Rick H.

Don't get too excited, $900 is just the parts and machining on the top end. That does not include anything else: labor, rebuilding the carbs, ignition, fluid and filters, etc. It would be nice to drop the 440 head on it though. I have seen overbore kits that go up close to 440 anyway, I would think it should be pretty straightforward.

I would, at the very least, make sure the valve adjustment is not the issue. If the valve is adjusted so it won't close, that will cause a leakdown issue.

Is the air for sure escaping into the crankcase? Or is it leaking into the exhaust/carbs? The latter would be a valve issue.

The Kz440 option will be cheaper and easier, as long as that motor is decent. The motor should swap in. Belt drive is easy to convert to chain drive if the 440 is belt. That basically leaves ignition, charging, and carbs.

The 440 ignition will be an upgrade, but it can probably downgrade back to points. If you stick with the 440 charging system, you will need the 440 reg/rec unless your 400 used the same charge system. I recall some overlap in the two model's charging system at some point.

Carbs would be the biggest question. Nebr_rex would be the guy to ask, probably.

JoshMcMadMac wrote: I have a 1978 KZ400, it has failed a leak-down test on the left side. We are assuming the leaking petcock has washed down the cylinder over time. I am overbooked on projects so have a local guy that has been wrenching on motorcycles forever going over it for me. He is quoting $900 on parts and machining to rebuild the top end, plus a few hours labor. He's having a hard time finding all of the parts at a reasonable price since the 400 is less common at this point.
Another option is a donor 1981 KZ440 with a known good engine. I am hesitant to swap the whole thing over, I think there might be some snags that are not being accounted for. Is there any possibility of swapping just the head without too much heartache? If not, any suggestions on getting the 400 rebuild cost down a bit?

Where is it leaking, valves? head gasket? rings?


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I responded previously, but it looks to not have posted.
To answer questions about the existing engine, it failed compression on the left cylinder (90PSI I think). Adding oil somewhat remedied the issue, indicating ring failure. The plug was also fouled.

As for the path forward, is it possible to drop an '82 440 block and head onto the late '78 400 (has the external oil line) without too much pain? Or is there still machine work involved?

If it only somewhat remedied the pressure, then I would still do the cursory inspection on the valve adjustment, as mentioned.

loudhvx wrote: If it only somewhat remedied the pressure, then I would still do the cursory inspection on the valve adjustment, as mentioned.

The pressure shot up and held with oil, it's the rings.

JoshMcMadMac wrote: "The pressure shot up and held with oil, it's the rings."

Perhaps, but not necessarily. Adding even a little oil to even the cylinder in a brand new engine will make the compression ratio go up unless the valves or some other area is leaking. See below for details. Ed

Ed is correct, adding oil to the bore is not a good test. I have had an engine with a huge gouge in the bore from the wrist pin backing out show 160 PSI only 5 PSI less than a healthy cylinder next to it. The only way to tell for sure where its leaking from is a leak-down test. In my experience if you are tearing down the top end its best to put new rings and valve seals anyway.

No eBay APP ID and/or Cert ID defined in Kunena configurationHow many miles on the engine? Has it sat and if so how long? Does it run now?

Reason for asking these questions is 1) if the bike has sat it's not unusual for the rings to stick a little and the compression will be low, 2) the compression will likely come up after some good hard miles in "italian tuneup" fashion. 3) if the engine has less than say 50k miles the only thing it may need is rings unless there is rust down the cylinders. You won't know until the engine is cracked open. Even if there is corrosion it's often possible to find a cheap usable cylinder off ebay for cheap. I've rebuild several engines this way.

Sorry this might be a bit of topic but where do you guys get new valves for kz440? I am having trouble with finding exhaust valves..